// 入口函数
ReactDOM.render(<App />, document.getElementById('root'));

export type ReactElement = {|
  // 用于辨别ReactElement对象
  $$typeof: any,

  // 内部属性
  type: any, // 表明其种类
  key: any,
  ref: any,
  props: any,

  // ReactFiber 记录创建本对象的Fiber节点, 还未与Fiber树关联之前, 该属性为null
  _owner: any,

  // __DEV__ dev环境下的一些额外信息, 如文件路径, 文件名, 行列信息等
  _store: {validated: boolean, ...},
  _self: React$Element<any>,
  _shadowChildren: any,
  _source: Source,
|};

class App extends React.Component {
  render() {
    return (
      <div className="app">
        <header>header</header>
        <Content />
        <footer>footer</footer>
      </div>
    );
  }
}

class Content extends React.Component {
  render() {
    return (
      <React.Fragment>
        <p>1</p>
        <p>2</p>
        <p>3</p>
      </React.Fragment>
    );
  }
}

export default App;

class App_App extends react_default.a.Component {
  render() {
    return /*#__PURE__*/ react_default.a.createElement(
      'div',
      {
        className: 'app',
      } /*#__PURE__*/,
      react_default.a.createElement('header', null, 'header') /*#__PURE__*/,

      // 此处直接将Content传入, 是一个指针传递
      react_default.a.createElement(App_Content, null) /*#__PURE__*/,
      react_default.a.createElement('footer', null, 'footer'),
    );
  }
}
class App_Content extends react_default.a.Component {
  render() {
    return /*#__PURE__*/ react_default.a.createElement(
      react_default.a.Fragment,
      null /*#__PURE__*/,
      react_default.a.createElement('p', null, '1'),
      /*#__PURE__*/

      react_default.a.createElement('p', null, '2'),
      /*#__PURE__*/

      react_default.a.createElement('p', null, '3'),
    );
  }
}

// 一个Fiber对象代表一个即将渲染或者已经渲染的组件(ReactElement), 一个组件可能对应两个fiber(current和WorkInProgress)
// 单个属性的解释在后文(在注释中无法添加超链接)
export type Fiber = {|
  tag: WorkTag,
  key: null | string,
  elementType: any,
  type: any,
  stateNode: any,
  return: Fiber | null,
  child: Fiber | null,
  sibling: Fiber | null,
  index: number,
  ref:
    | null
    | (((handle: mixed) => void) & { _stringRef: ?string, ... })
    | RefObject,
  pendingProps: any, // 从`ReactElement`对象传入的 props. 用于和`fiber.memoizedProps`比较可以得出属性是否变动
  memoizedProps: any, // 上一次生成子节点时用到的属性, 生成子节点之后保持在内存中
  updateQueue: mixed, // 存储state更新的队列, 当前节点的state改动之后, 都会创建一个update对象添加到这个队列中.
  memoizedState: any, // 用于输出的state, 最终渲染所使用的state
  dependencies: Dependencies | null, // 该fiber节点所依赖的(contexts, events)等
  mode: TypeOfMode, // 二进制位Bitfield,继承至父节点,影响本fiber节点及其子树中所有节点. 与react应用的运行模式有关(有ConcurrentMode, BlockingMode, NoMode等选项).

  // Effect 副作用相关
  flags: Flags, // 标志位
  subtreeFlags: Flags, //替代16.x版本中的 firstEffect, nextEffect. 当设置了 enableNewReconciler=true才会启用
  deletions: Array<Fiber> | null, // 存储将要被删除的子节点. 当设置了 enableNewReconciler=true才会启用

  nextEffect: Fiber | null, // 单向链表, 指向下一个有副作用的fiber节点
  firstEffect: Fiber | null, // 指向副作用链表中的第一个fiber节点
  lastEffect: Fiber | null, // 指向副作用链表中的最后一个fiber节点

  // 优先级相关
  lanes: Lanes, // 本fiber节点的优先级
  childLanes: Lanes, // 子节点的优先级
  alternate: Fiber | null, // 指向内存中的另一个fiber, 每个被更新过fiber节点在内存中都是成对出现(current和workInProgress)

  // 性能统计相关(开启enableProfilerTimer后才会统计)
  // react-dev-tool会根据这些时间统计来评估性能
  actualDuration?: number, // 本次更新过程, 本节点以及子树所消耗的总时间
  actualStartTime?: number, // 标记本fiber节点开始构建的时间
  selfBaseDuration?: number, // 用于最近一次生成本fiber节点所消耗的时间
  treeBaseDuration?: number, // 生成子树所消耗的时间的总和
|};

export type Update<State> = {|
  eventTime: number, // 发起update事件的时间(17.0.2中作为临时字段, 即将移出)
  lane: Lane, // update所属的优先级

  tag: 0 | 1 | 2 | 3, //
  payload: any, // 载荷, 根据场景可以设置成一个回调函数或者对象
  callback: (() => mixed) | null, // 回调函数

  next: Update<State> | null, // 指向链表中的下一个, 由于UpdateQueue是一个环形链表, 最后一个update.next指向第一个update对象
|};

// =============== UpdateQueue ==============
type SharedQueue<State> = {|
  pending: Update<State> | null,
|};

export type UpdateQueue<State> = {|
  baseState: State,
  firstBaseUpdate: Update<State> | null,
  lastBaseUpdate: Update<State> | null,
  shared: SharedQueue<State>,
  effects: Array<Update<State>> | null,
|};

export type Hook = {|
  memoizedState: any,
  baseState: any,
  baseQueue: Update<any, any> | null,
  queue: UpdateQueue<any, any> | null,
  next: Hook | null,
|};

type Update<S, A> = {|
  lane: Lane,
  action: A,
  eagerReducer: ((S, A) => S) | null,
  eagerState: S | null,
  next: Update<S, A>,
  priority?: ReactPriorityLevel,
|};

type UpdateQueue<S, A> = {|
  pending: Update<S, A> | null,
  dispatch: ((A) => mixed) | null,
  lastRenderedReducer: ((S, A) => S) | null,
  lastRenderedState: S | null,
|};

var newTask = {
  id: taskIdCounter++,
  callback,
  priorityLevel,
  startTime,
  expirationTime,
  sortIndex: -1,
};

// LegacyRoot
ReactDOM.render(<App />, document.getElementById('root'), (dom) => {}); // 支持callback回调, 参数是一个dom对象

// BlockingRoot
// 1. 创建ReactDOMRoot对象
const reactDOMBlockingRoot = ReactDOM.createBlockingRoot(
  document.getElementById('root'),
);
// 2. 调用render
reactDOMBlockingRoot.render(<App />); // 不支持回调

// ConcurrentRoot
// 1. 创建ReactDOMRoot对象
const reactDOMRoot = ReactDOM.createRoot(document.getElementById('root'));
// 2. 调用render
reactDOMRoot.render(<App />); // 不支持回调

function legacyRenderSubtreeIntoContainer(
  parentComponent: ?React$Component<any, any>,
  children: ReactNodeList,
  container: Container,
  forceHydrate: boolean,
  callback: ?Function,
) {
  let root: RootType = (container._reactRootContainer: any);
  let fiberRoot;
  if (!root) {
    // 初次调用, root还未初始化, 会进入此分支
    //1. 创建ReactDOMRoot对象, 初始化react应用环境
    root = container._reactRootContainer = legacyCreateRootFromDOMContainer(
      container,
      forceHydrate,
    );
    fiberRoot = root._internalRoot;
    if (typeof callback === 'function') {
      const originalCallback = callback;
      callback = function () {
        // instance最终指向 children(入参: 如<App/>)生成的dom节点
        const instance = getPublicRootInstance(fiberRoot);
        originalCallback.call(instance);
      };
    }
    // 2. 更新容器
    unbatchedUpdates(() => {
      updateContainer(children, fiberRoot, parentComponent, callback);
    });
  } else {
    // root已经初始化, 二次调用render会进入
    // 1. 获取FiberRoot对象
    fiberRoot = root._internalRoot;
    if (typeof callback === 'function') {
      const originalCallback = callback;
      callback = function () {
        const instance = getPublicRootInstance(fiberRoot);
        originalCallback.call(instance);
      };
    }
    // 2. 调用更新
    updateContainer(children, fiberRoot, parentComponent, callback);
  }
  return getPublicRootInstance(fiberRoot);
}

function legacyCreateRootFromDOMContainer(
  container: Container,
  forceHydrate: boolean,
): RootType {
  const shouldHydrate =
    forceHydrate || shouldHydrateDueToLegacyHeuristic(container);
  return createLegacyRoot(
    container,
    shouldHydrate
      ? {
          hydrate: true,
        }
      : undefined,
  );
}

export function createLegacyRoot(
  container: Container,
  options?: RootOptions,
): RootType {
  return new ReactDOMBlockingRoot(container, LegacyRoot, options); // 注意这里的LegacyRoot是固定的, 并不是外界传入的
}

export function createRoot(
  container: Container,
  options?: RootOptions,
): RootType {
  return new ReactDOMRoot(container, options);
}

export function createBlockingRoot(
  container: Container,
  options?: RootOptions,
): RootType {
  return new ReactDOMBlockingRoot(container, BlockingRoot, options); // 注意第2个参数BlockingRoot是固定写死的
}

function ReactDOMRoot(container: Container, options: void | RootOptions) {
  // 创建一个fiberRoot对象, 并将其挂载到this._internalRoot之上
  this._internalRoot = createRootImpl(container, ConcurrentRoot, options);
}
function ReactDOMBlockingRoot(
  container: Container,
  tag: RootTag,
  options: void | RootOptions,
) {
  // 创建一个fiberRoot对象, 并将其挂载到this._internalRoot之上
  this._internalRoot = createRootImpl(container, tag, options);
}

ReactDOMRoot.prototype.render = ReactDOMBlockingRoot.prototype.render =
  function (children: ReactNodeList): void {
    const root = this._internalRoot;
    // 执行更新
    updateContainer(children, root, null, null);
  };

ReactDOMRoot.prototype.unmount = ReactDOMBlockingRoot.prototype.unmount =
  function (): void {
    const root = this._internalRoot;
    const container = root.containerInfo;
    // 执行更新
    updateContainer(null, root, null, () => {
      unmarkContainerAsRoot(container);
    });
  };

// 注意: 3种模式下的tag是各不相同(分别是ConcurrentRoot,BlockingRoot,LegacyRoot).
this._internalRoot = createRootImpl(container, tag, options);

function createRootImpl(
  container: Container,
  tag: RootTag,
  options: void | RootOptions,
) {
  // ... 省略部分源码(有关hydrate服务端渲染等, 暂时用不上)
  // 1. 创建fiberRoot
  const root = createContainer(container, tag, hydrate, hydrationCallbacks); // 注意RootTag的传递
  // 2. 标记dom对象, 把dom和fiber对象关联起来
  markContainerAsRoot(root.current, container);
  // ...省略部分无关代码
  return root;
}

export function createContainer(
  containerInfo: Container,
  tag: RootTag,
  hydrate: boolean,
  hydrationCallbacks: null | SuspenseHydrationCallbacks,
): OpaqueRoot {
  // 创建fiberRoot对象
  return createFiberRoot(containerInfo, tag, hydrate, hydrationCallbacks); // 注意RootTag的传递
}

export function createFiberRoot(
  containerInfo: any,
  tag: RootTag,
  hydrate: boolean,
  hydrationCallbacks: null | SuspenseHydrationCallbacks,
): FiberRoot {
  // 创建fiberRoot对象, 注意RootTag的传递
  const root: FiberRoot = (new FiberRootNode(containerInfo, tag, hydrate): any);

  // 1. 这里创建了`react`应用的首个`fiber`对象, 称为`HostRootFiber`
  const uninitializedFiber = createHostRootFiber(tag);
  root.current = uninitializedFiber;
  uninitializedFiber.stateNode = root;
  // 2. 初始化HostRootFiber的updateQueue
  initializeUpdateQueue(uninitializedFiber);

  return root;
}

export function createHostRootFiber(tag: RootTag): Fiber {
  let mode;
  if (tag === ConcurrentRoot) {
    mode = ConcurrentMode | BlockingMode | StrictMode;
  } else if (tag === BlockingRoot) {
    mode = BlockingMode | StrictMode;
  } else {
    mode = NoMode;
  }
  return createFiber(HostRoot, null, null, mode); // 注意这里设置的mode属性是由RootTag决定的
}

// 2. 更新容器
unbatchedUpdates(() => {
  updateContainer(children, fiberRoot, parentComponent, callback);
});

ReactDOMRoot.prototype.render = ReactDOMBlockingRoot.prototype.render =
  function (children: ReactNodeList): void {
    const root = this._internalRoot;
    // 执行更新
    updateContainer(children, root, null, null);
  };

export function updateContainer(
  element: ReactNodeList,
  container: OpaqueRoot,
  parentComponent: ?React$Component<any, any>,
  callback: ?Function,
): Lane {
  const current = container.current;
  // 1. 获取当前时间戳, 计算本次更新的优先级
  const eventTime = requestEventTime();
  const lane = requestUpdateLane(current);

  // 2. 设置fiber.updateQueue
  const update = createUpdate(eventTime, lane);
  update.payload = { element };
  callback = callback === undefined ? null : callback;
  if (callback !== null) {
    update.callback = callback;
  }
  enqueueUpdate(current, update);

  // 3. 进入reconciler运作流程中的`输入`环节
  scheduleUpdateOnFiber(current, lane, eventTime);
  return lane;
}

// 唯一接收输入信号的函数
export function scheduleUpdateOnFiber(
  fiber: Fiber,
  lane: Lane,
  eventTime: number,
) {
  // ... 省略部分无关代码
  const root = markUpdateLaneFromFiberToRoot(fiber, lane);
  if (lane === SyncLane) {
    if (
      (executionContext & LegacyUnbatchedContext) !== NoContext &&
      (executionContext & (RenderContext | CommitContext)) === NoContext
    ) {
      // 直接进行`fiber构造`
      performSyncWorkOnRoot(root);
    } else {
      // 注册调度任务, 经过`Scheduler`包的调度, 间接进行`fiber构造`
      ensureRootIsScheduled(root, eventTime);
    }
  } else {
    // 注册调度任务, 经过`Scheduler`包的调度, 间接进行`fiber构造`
    ensureRootIsScheduled(root, eventTime);
  }
}

// ... 省略部分无关代码
function ensureRootIsScheduled(root: FiberRoot, currentTime: number) {
  // 前半部分: 判断是否需要注册新的调度
  const existingCallbackNode = root.callbackNode;
  const nextLanes = getNextLanes(
    root,
    root === workInProgressRoot ? workInProgressRootRenderLanes : NoLanes,
  );
  const newCallbackPriority = returnNextLanesPriority();
  if (nextLanes === NoLanes) {
    return;
  }
  if (existingCallbackNode !== null) {
    const existingCallbackPriority = root.callbackPriority;
    if (existingCallbackPriority === newCallbackPriority) {
      return;
    }
    cancelCallback(existingCallbackNode);
  }

  // 后半部分: 注册调度任务
  let newCallbackNode;
  if (newCallbackPriority === SyncLanePriority) {
    newCallbackNode = scheduleSyncCallback(
      performSyncWorkOnRoot.bind(null, root),
    );
  } else if (newCallbackPriority === SyncBatchedLanePriority) {
    newCallbackNode = scheduleCallback(
      ImmediateSchedulerPriority,
      performSyncWorkOnRoot.bind(null, root),
    );
  } else {
    const schedulerPriorityLevel =
      lanePriorityToSchedulerPriority(newCallbackPriority);
    newCallbackNode = scheduleCallback(
      schedulerPriorityLevel,
      performConcurrentWorkOnRoot.bind(null, root),
    );
  }
  root.callbackPriority = newCallbackPriority;
  root.callbackNode = newCallbackNode;
}

// ... 省略部分无关代码
function performSyncWorkOnRoot(root) {
  let lanes;
  let exitStatus;

  lanes = getNextLanes(root, NoLanes);
  // 1. fiber树构造
  exitStatus = renderRootSync(root, lanes);

  // 2. 异常处理: 有可能fiber构造过程中出现异常
  if (root.tag !== LegacyRoot && exitStatus === RootErrored) {
    // ...
  }

  // 3. 输出: 渲染fiber树
  const finishedWork: Fiber = (root.current.alternate: any);
  root.finishedWork = finishedWork;
  root.finishedLanes = lanes;
  commitRoot(root);

  // 退出前再次检测, 是否还有其他更新, 是否需要发起新调度
  ensureRootIsScheduled(root, now());
  return null;
}

// ... 省略部分无关代码
function performConcurrentWorkOnRoot(root) {

  const originalCallbackNode = root.callbackNode;

  // 1. 刷新pending状态的effects, 有可能某些effect会取消本次任务
  const didFlushPassiveEffects = flushPassiveEffects();
  if (didFlushPassiveEffects) {
    if (root.callbackNode !== originalCallbackNode) {
      // 任务被取消, 退出调用
      return null;
    } else {
      // Current task was not canceled. Continue.
    }
  }
  // 2. 获取本次渲染的优先级
  let lanes = getNextLanes(
    root,
    root === workInProgressRoot ? workInProgressRootRenderLanes : NoLanes,
  );
  // 3. 构造fiber树
  let exitStatus = renderRootConcurrent(root, lanes);

  if (
    includesSomeLane(
      workInProgressRootIncludedLanes,
      workInProgressRootUpdatedLanes,
    )
  ) {
    // 如果在render过程中产生了新的update, 且新update的优先级与最初render的优先级有交集
    // 那么最初render无效, 丢弃最初render的结果, 等待下一次调度
    prepareFreshStack(root, NoLanes);
  } else if (exitStatus !== RootIncomplete) {
    // 4. 异常处理: 有可能fiber构造过程中出现异常
    if (exitStatus === RootErrored) {
      // ...
    }.
    const finishedWork: Fiber = (root.current.alternate: any);
    root.finishedWork = finishedWork;
    root.finishedLanes = lanes;
    // 5. 输出: 渲染fiber树
    finishConcurrentRender(root, exitStatus, lanes);
  }

  // 退出前再次检测, 是否还有其他更新, 是否需要发起新调度
  ensureRootIsScheduled(root, now());
  if (root.callbackNode === originalCallbackNode) {
    // 渲染被阻断, 返回一个新的performConcurrentWorkOnRoot函数, 等待下一次调用
    return performConcurrentWorkOnRoot.bind(null, root);
  }
  return null;
}

// ... 省略部分无关代码
function commitRootImpl(root, renderPriorityLevel) {
  // 设置局部变量
  const finishedWork = root.finishedWork;
  const lanes = root.finishedLanes;

  // 清空FiberRoot对象上的属性
  root.finishedWork = null;
  root.finishedLanes = NoLanes;
  root.callbackNode = null;

  // 提交阶段
  let firstEffect = finishedWork.firstEffect;
  if (firstEffect !== null) {
    const prevExecutionContext = executionContext;
    executionContext |= CommitContext;
    // 阶段1: dom突变之前
    nextEffect = firstEffect;
    do {
      commitBeforeMutationEffects();
    } while (nextEffect !== null);

    // 阶段2: dom突变, 界面发生改变
    nextEffect = firstEffect;
    do {
      commitMutationEffects(root, renderPriorityLevel);
    } while (nextEffect !== null);
    root.current = finishedWork;

    // 阶段3: layout阶段, 调用生命周期componentDidUpdate和回调函数等
    nextEffect = firstEffect;
    do {
      commitLayoutEffects(root, lanes);
    } while (nextEffect !== null);
    nextEffect = null;
    executionContext = prevExecutionContext;
  }
  ensureRootIsScheduled(root, now());
  return null;
}

renderExpirationtime -> renderLanes
  update.expirationTime -> update.lane
  fiber.expirationTime -> fiber.lanes
  fiber.childExpirationTime -> fiber.childLanes
  root.firstPendingTime and root.lastPendingTime -> fiber.pendingLanes

// 判断: 单task与batchTask的优先级是否重叠
//1. 通过expirationTime判断
const isTaskIncludedInBatch = priorityOfTask >= priorityOfBatch;
//2. 通过Lanes判断
const isTaskIncludedInBatch = (task & batchOfTasks) !== 0;

// 当同时处理一组任务, 该组内有多个任务, 且每个任务的优先级不一致
// 1. 如果通过expirationTime判断. 需要维护一个范围(在Lane重构之前, 源码中就是这样比较的)
const isTaskIncludedInBatch =
  taskPriority <= highestPriorityInRange &&
  taskPriority >= lowestPriorityInRange;
//2. 通过Lanes判断
const isTaskIncludedInBatch = (task & batchOfTasks) !== 0;

// 从group中删除或增加task

//1. 通过expirationTime实现
// 0) 维护一个链表, 按照单个task的优先级顺序进行插入
// 1) 删除单个task(从链表中删除一个元素)
task.prev.next = task.next;
// 2) 增加单个task(需要对比当前task的优先级, 插入到链表正确的位置上)
let current = queue;
while (task.expirationTime >= current.expirationTime) {
  current = current.next;
}
task.next = current.next;
current.next = task;
// 3) 比较task是否在group中
const isTaskIncludedInBatch =
  taskPriority <= highestPriorityInRange &&
  taskPriority >= lowestPriorityInRange;

// 2. 通过Lanes实现
// 1) 删除单个task
batchOfTasks &= ~task;
// 2) 增加单个task
batchOfTasks |= task;
// 3) 比较task是否在group中
const isTaskIncludedInBatch = (task & batchOfTasks) !== 0;

export const SyncLanePriority: LanePriority = 15;
export const SyncBatchedLanePriority: LanePriority = 14;

const InputDiscreteHydrationLanePriority: LanePriority = 13;
export const InputDiscreteLanePriority: LanePriority = 12;

// .....

const OffscreenLanePriority: LanePriority = 1;
export const NoLanePriority: LanePriority = 0;

export const NoPriority = 0;
export const ImmediatePriority = 1;
export const UserBlockingPriority = 2;
export const NormalPriority = 3;
export const LowPriority = 4;
export const IdlePriority = 5;

export const ImmediatePriority: ReactPriorityLevel = 99;
export const UserBlockingPriority: ReactPriorityLevel = 98;
export const NormalPriority: ReactPriorityLevel = 97;
export const LowPriority: ReactPriorityLevel = 96;
export const IdlePriority: ReactPriorityLevel = 95;
// NoPriority is the absence of priority. Also React-only.
export const NoPriority: ReactPriorityLevel = 90;

// 把 SchedulerPriority 转换成 ReactPriorityLevel
export function getCurrentPriorityLevel(): ReactPriorityLevel {
  switch (Scheduler_getCurrentPriorityLevel()) {
    case Scheduler_ImmediatePriority:
      return ImmediatePriority;
    case Scheduler_UserBlockingPriority:
      return UserBlockingPriority;
    case Scheduler_NormalPriority:
      return NormalPriority;
    case Scheduler_LowPriority:
      return LowPriority;
    case Scheduler_IdlePriority:
      return IdlePriority;
    default:
      invariant(false, 'Unknown priority level.');
  }
}

// 把 ReactPriorityLevel 转换成 SchedulerPriority
function reactPriorityToSchedulerPriority(reactPriorityLevel) {
  switch (reactPriorityLevel) {
    case ImmediatePriority:
      return Scheduler_ImmediatePriority;
    case UserBlockingPriority:
      return Scheduler_UserBlockingPriority;
    case NormalPriority:
      return Scheduler_NormalPriority;
    case LowPriority:
      return Scheduler_LowPriority;
    case IdlePriority:
      return Scheduler_IdlePriority;
    default:
      invariant(false, 'Unknown priority level.');
  }
}

export function schedulerPriorityToLanePriority(
  schedulerPriorityLevel: ReactPriorityLevel,
): LanePriority {
  switch (schedulerPriorityLevel) {
    case ImmediateSchedulerPriority:
      return SyncLanePriority;
    // ... 省略部分代码
    default:
      return NoLanePriority;
  }
}

export function lanePriorityToSchedulerPriority(
  lanePriority: LanePriority,
): ReactPriorityLevel {
  switch (lanePriority) {
    case SyncLanePriority:
    case SyncBatchedLanePriority:
      return ImmediateSchedulerPriority;
    // ... 省略部分代码
    default:
      invariant(
        false,
        'Invalid update priority: %s. This is a bug in React.',
        lanePriority,
      );
  }
}

export let requestHostCallback; // 请求及时回调: port.postMessage
export let cancelHostCallback; // 取消及时回调: scheduledHostCallback = null
export let requestHostTimeout; // 请求延时回调: setTimeout
export let cancelHostTimeout; // 取消延时回调: cancelTimeout
export let shouldYieldToHost; // 是否让出主线程(currentTime >= deadline && needsPaint): 让浏览器能够执行更高优先级的任务(如ui绘制, 用户输入等)
export let requestPaint; // 请求绘制: 设置 needsPaint = true
export let getCurrentTime; // 获取当前时间
export let forceFrameRate; // 强制设置 yieldInterval (让出主线程的周期). 这个函数虽然存在, 但是从源码来看, 几乎没有用到

// 接收 MessageChannel 消息
const performWorkUntilDeadline = () => {
  // ...省略无关代码
  if (scheduledHostCallback !== null) {
    const currentTime = getCurrentTime();
    // 更新deadline
    deadline = currentTime + yieldInterval;
    // 执行callback
    scheduledHostCallback(hasTimeRemaining, currentTime);
  } else {
    isMessageLoopRunning = false;
  }
};

const channel = new MessageChannel();
const port = channel.port2;
channel.port1.onmessage = performWorkUntilDeadline;

// 请求回调
requestHostCallback = function (callback) {
  // 1. 保存callback
  scheduledHostCallback = callback;
  if (!isMessageLoopRunning) {
    isMessageLoopRunning = true;
    // 2. 通过 MessageChannel 发送消息
    port.postMessage(null);
  }
};
// 取消回调
cancelHostCallback = function () {
  scheduledHostCallback = null;
};

const localPerformance = performance;
// 获取当前时间
getCurrentTime = () => localPerformance.now();

// 时间切片周期, 默认是5ms(如果一个task运行超过该周期, 下一个task执行之前, 会把控制权归还浏览器)
let yieldInterval = 5;

let deadline = 0;
const maxYieldInterval = 300;
let needsPaint = false;
const scheduling = navigator.scheduling;
// 是否让出主线程
shouldYieldToHost = function () {
  const currentTime = getCurrentTime();
  if (currentTime >= deadline) {
    if (needsPaint || scheduling.isInputPending()) {
      // There is either a pending paint or a pending input.
      return true;
    }
    // There's no pending input. Only yield if we've reached the max
    // yield interval.
    return currentTime >= maxYieldInterval; // 在持续运行的react应用中, currentTime肯定大于300ms, 这个判断只在初始化过程中才有可能返回false
  } else {
    // There's still time left in the frame.
    return false;
  }
};

// 请求绘制
requestPaint = function () {
  needsPaint = true;
};

// 设置时间切片的周期
forceFrameRate = function (fps) {
  if (fps < 0 || fps > 125) {
    // Using console['error'] to evade Babel and ESLint
    console['error'](
      'forceFrameRate takes a positive int between 0 and 125, ' +
        'forcing frame rates higher than 125 fps is not supported',
    );
    return;
  }
  if (fps > 0) {
    yieldInterval = Math.floor(1000 / fps);
  } else {
    // reset the framerate
    yieldInterval = 5;
  }
};

const performWorkUntilDeadline = () => {
  if (scheduledHostCallback !== null) {
    const currentTime = getCurrentTime(); // 1. 获取当前时间
    deadline = currentTime + yieldInterval; // 2. 设置deadline
    const hasTimeRemaining = true;
    try {
      // 3. 执行回调, 返回是否有还有剩余任务
      const hasMoreWork = scheduledHostCallback(hasTimeRemaining, currentTime);
      if (!hasMoreWork) {
        // 没有剩余任务, 退出
        isMessageLoopRunning = false;
        scheduledHostCallback = null;
      } else {
        port.postMessage(null); // 有剩余任务, 发起新的调度
      }
    } catch (error) {
      port.postMessage(null); // 如有异常, 重新发起调度
      throw error;
    }
  } else {
    isMessageLoopRunning = false;
  }
  needsPaint = false; // 重置开关
};

// Tasks are stored on a min heap
var taskQueue = [];
var timerQueue = [];

// 省略部分无关代码
function unstable_scheduleCallback(priorityLevel, callback, options) {
  // 1. 获取当前时间
  var currentTime = getCurrentTime();
  var startTime;
  if (typeof options === 'object' && options !== null) {
    // 从函数调用关系来看, 在v17.0.2中,所有调用 unstable_scheduleCallback 都未传入options
    // 所以省略延时任务相关的代码
  } else {
    startTime = currentTime;
  }
  // 2. 根据传入的优先级, 设置任务的过期时间 expirationTime
  var timeout;
  switch (priorityLevel) {
    case ImmediatePriority:
      timeout = IMMEDIATE_PRIORITY_TIMEOUT;
      break;
    case UserBlockingPriority:
      timeout = USER_BLOCKING_PRIORITY_TIMEOUT;
      break;
    case IdlePriority:
      timeout = IDLE_PRIORITY_TIMEOUT;
      break;
    case LowPriority:
      timeout = LOW_PRIORITY_TIMEOUT;
      break;
    case NormalPriority:
    default:
      timeout = NORMAL_PRIORITY_TIMEOUT;
      break;
  }
  var expirationTime = startTime + timeout;
  // 3. 创建新任务
  var newTask = {
    id: taskIdCounter++,
    callback,
    priorityLevel,
    startTime,
    expirationTime,
    sortIndex: -1,
  };
  if (startTime > currentTime) {
    // 省略无关代码 v17.0.2中不会使用
  } else {
    newTask.sortIndex = expirationTime;
    // 4. 加入任务队列
    push(taskQueue, newTask);
    // 5. 请求调度
    if (!isHostCallbackScheduled && !isPerformingWork) {
      isHostCallbackScheduled = true;
      requestHostCallback(flushWork);
    }
  }
  return newTask;
}

var newTask = {
  id: taskIdCounter++, // id: 一个自增编号
  callback, // callback: 传入的回调函数
  priorityLevel, // priorityLevel: 优先级等级
  startTime, // startTime: 创建task时的当前时间
  expirationTime, // expirationTime: task的过期时间, 优先级越高 expirationTime = startTime + timeout 越小
  sortIndex: -1,
};
newTask.sortIndex = expirationTime; // sortIndex: 排序索引, 全等于过期时间. 保证过期时间越小, 越紧急的任务排在最前面

// 省略无关代码
function flushWork(hasTimeRemaining, initialTime) {
  // 1. 做好全局标记, 表示现在已经进入调度阶段
  isHostCallbackScheduled = false;
  isPerformingWork = true;
  const previousPriorityLevel = currentPriorityLevel;
  try {
    // 2. 循环消费队列
    return workLoop(hasTimeRemaining, initialTime);
  } finally {
    // 3. 还原全局标记
    currentTask = null;
    currentPriorityLevel = previousPriorityLevel;
    isPerformingWork = false;
  }
}

// 省略部分无关代码
function workLoop(hasTimeRemaining, initialTime) {
  let currentTime = initialTime; // 保存当前时间, 用于判断任务是否过期
  currentTask = peek(taskQueue); // 获取队列中的第一个任务
  while (currentTask !== null) {
    if (
      currentTask.expirationTime > currentTime &&
      (!hasTimeRemaining || shouldYieldToHost())
    ) {
      // 虽然currentTask没有过期, 但是执行时间超过了限制(毕竟只有5ms, shouldYieldToHost()返回true). 停止继续执行, 让出主线程
      break;
    }
    const callback = currentTask.callback;
    if (typeof callback === 'function') {
      currentTask.callback = null;
      currentPriorityLevel = currentTask.priorityLevel;
      const didUserCallbackTimeout = currentTask.expirationTime <= currentTime;
      // 执行回调
      const continuationCallback = callback(didUserCallbackTimeout);
      currentTime = getCurrentTime();
      // 回调完成, 判断是否还有连续(派生)回调
      if (typeof continuationCallback === 'function') {
        // 产生了连续回调(如fiber树太大, 出现了中断渲染), 保留currentTask
        currentTask.callback = continuationCallback;
      } else {
        // 把currentTask移出队列
        if (currentTask === peek(taskQueue)) {
          pop(taskQueue);
        }
      }
    } else {
      // 如果任务被取消(这时currentTask.callback = null), 将其移出队列
      pop(taskQueue);
    }
    // 更新currentTask
    currentTask = peek(taskQueue);
  }
  if (currentTask !== null) {
    return true; // 如果task队列没有清空, 返回true. 等待调度中心下一次回调
  } else {
    return false; // task队列已经清空, 返回false.
  }
}

// ... 省略部分无关代码
function ensureRootIsScheduled(root: FiberRoot, currentTime: number) {
  // 前半部分: 判断是否需要注册新的调度
  const existingCallbackNode = root.callbackNode;
  const nextLanes = getNextLanes(
    root,
    root === workInProgressRoot ? workInProgressRootRenderLanes : NoLanes,
  );
  const newCallbackPriority = returnNextLanesPriority();
  if (nextLanes === NoLanes) {
    return;
  }
  // 节流防抖
  if (existingCallbackNode !== null) {
    const existingCallbackPriority = root.callbackPriority;
    if (existingCallbackPriority === newCallbackPriority) {
      return;
    }
    cancelCallback(existingCallbackNode);
  }
  // 后半部分: 注册调度任务 省略代码...

  // 更新标记
  root.callbackPriority = newCallbackPriority;
  root.callbackNode = newCallbackNode;
}

// 当前React的执行栈(执行上下文)
let executionContext: ExecutionContext = NoContext;

// 当前root节点
let workInProgressRoot: FiberRoot | null = null;
// 正在处理中的fiber节点
let workInProgress: Fiber | null = null;
// 正在渲染的车道(复数)
let workInProgressRootRenderLanes: Lanes = NoLanes;

// 包含所有子节点的优先级, 是workInProgressRootRenderLanes的超集
// 大多数情况下: 在工作循环整体层面会使用workInProgressRootRenderLanes, 在begin/complete阶段层面会使用 subtreeRenderLanes
let subtreeRenderLanes: Lanes = NoLanes;
// 一个栈结构: 专门存储当前节点的 subtreeRenderLanes
const subtreeRenderLanesCursor: StackCursor<Lanes> = createCursor(NoLanes);

// fiber构造完后, root节点的状态: completed, errored, suspended等
let workInProgressRootExitStatus: RootExitStatus = RootIncomplete;
// 重大错误
let workInProgressRootFatalError: mixed = null;
// 整个render期间所使用到的所有lanes
let workInProgressRootIncludedLanes: Lanes = NoLanes;
// 在render期间被跳过(由于优先级不够)的lanes: 只包括未处理的updates, 不包括被复用的fiber节点
let workInProgressRootSkippedLanes: Lanes = NoLanes;
// 在render期间被修改过的lanes
let workInProgressRootUpdatedLanes: Lanes = NoLanes;

// 防止无限循环和嵌套更新
const NESTED_UPDATE_LIMIT = 50;
let nestedUpdateCount: number = 0;
let rootWithNestedUpdates: FiberRoot | null = null;

const NESTED_PASSIVE_UPDATE_LIMIT = 50;
let nestedPassiveUpdateCount: number = 0;

// 发起更新的时间
let currentEventTime: number = NoTimestamp;
let currentEventWipLanes: Lanes = NoLanes;
let currentEventPendingLanes: Lanes = NoLanes;

type ExecutionContext = number;
export const NoContext = /*             */ 0b0000000;
const BatchedContext = /*               */ 0b0000001;
const EventContext = /*                 */ 0b0000010;
const DiscreteEventContext = /*         */ 0b0000100;
const LegacyUnbatchedContext = /*       */ 0b0001000;
const RenderContext = /*                */ 0b0010000;
const CommitContext = /*                */ 0b0100000;

export function scheduleUpdateOnFiber(
  fiber: Fiber,
  lane: Lane,
  eventTime: number,
) {
  if (lane === SyncLane) {
    // legacy或blocking模式
    if (
      (executionContext & LegacyUnbatchedContext) !== NoContext &&
      (executionContext & (RenderContext | CommitContext)) === NoContext
    ) {
      performSyncWorkOnRoot(root);
    } else {
      // 后续的更新
      // 进入第2阶段, 注册调度任务
      ensureRootIsScheduled(root, eventTime);
      if (executionContext === NoContext) {
        // 如果执行上下文为空, 会取消调度任务, 手动执行回调
        // 进入第3阶段, 进行fiber树构造
        flushSyncCallbackQueue();
      }
    }
  } else {
    // concurrent模式
    // 无论是否初次更新, 都正常进入第2阶段, 注册调度任务
    ensureRootIsScheduled(root, eventTime);
  }
}

export function createUpdate(eventTime: number, lane: Lane): Update<*> {
  const update: Update<*> = {
    eventTime,
    lane,
    tag: UpdateState,
    payload: null,
    callback: null,
    next: null,
  };
  return update;
}

export function updateContainer(
  element: ReactNodeList,
  container: OpaqueRoot,
  parentComponent: ?React$Component<any, any>,
  callback: ?Function,
): Lane {
  const current = container.current;
  const eventTime = requestEventTime();
  const lane = requestUpdateLane(current); // 根据当前时间, 创建一个update优先级
  const update = createUpdate(eventTime, lane); // lane被用于创建update对象
  update.payload = { element };
  enqueueUpdate(current, update);
  scheduleUpdateOnFiber(current, lane, eventTime);
  return lane;
}

const classComponentUpdater = {
  isMounted,
  enqueueSetState(inst, payload, callback) {
    const fiber = getInstance(inst);
    const eventTime = requestEventTime(); // 根据当前时间, 创建一个update优先级
    const lane = requestUpdateLane(fiber); // lane被用于创建update对象
    const update = createUpdate(eventTime, lane);
    update.payload = payload;
    enqueueUpdate(fiber, update);
    scheduleUpdateOnFiber(fiber, lane, eventTime);
  },
};

export function requestUpdateLane(fiber: Fiber): Lane {
  // Special cases
  const mode = fiber.mode;
  if ((mode & BlockingMode) === NoMode) {
    // legacy 模式
    return (SyncLane: Lane);
  } else if ((mode & ConcurrentMode) === NoMode) {
    // blocking模式
    return getCurrentPriorityLevel() === ImmediateSchedulerPriority
      ? (SyncLane: Lane)
      : (SyncBatchedLane: Lane);
  }
  // concurrent模式
  if (currentEventWipLanes === NoLanes) {
    currentEventWipLanes = workInProgressRootIncludedLanes;
  }
  const isTransition = requestCurrentTransition() !== NoTransition;
  if (isTransition) {
    // 特殊情况, 处于suspense过程中
    if (currentEventPendingLanes !== NoLanes) {
      currentEventPendingLanes =
        mostRecentlyUpdatedRoot !== null
          ? mostRecentlyUpdatedRoot.pendingLanes
          : NoLanes;
    }
    return findTransitionLane(currentEventWipLanes, currentEventPendingLanes);
  }
  // 正常情况, 获取调度优先级
  const schedulerPriority = getCurrentPriorityLevel();
  let lane;
  if (
    (executionContext & DiscreteEventContext) !== NoContext &&
    schedulerPriority === UserBlockingSchedulerPriority
  ) {
    // executionContext 存在输入事件. 且调度优先级是用户阻塞性质
    lane = findUpdateLane(InputDiscreteLanePriority, currentEventWipLanes);
  } else {
    // 调度优先级转换为车道模型
    const schedulerLanePriority =
      schedulerPriorityToLanePriority(schedulerPriority);
    lane = findUpdateLane(schedulerLanePriority, currentEventWipLanes);
  }
  return lane;
}

export function scheduleUpdateOnFiber(
  fiber: Fiber,
  lane: Lane,
  eventTime: number,
) {
  if (lane === SyncLane) {
    // legacy或blocking模式
    if (
      (executionContext & LegacyUnbatchedContext) !== NoContext &&
      (executionContext & (RenderContext | CommitContext)) === NoContext
    ) {
      performSyncWorkOnRoot(root);
    } else {
      ensureRootIsScheduled(root, eventTime); // 注册回调任务
      if (executionContext === NoContext) {
        flushSyncCallbackQueue(); // 取消schedule调度 ,主动刷新回调队列,
      }
    }
  } else {
    // concurrent模式
    ensureRootIsScheduled(root, eventTime);
  }
}

// ...省略无关代码
function performSyncWorkOnRoot(root) {
  let lanes;
  let exitStatus;
  // 获取本次`render`的优先级
  lanes = getNextLanes(root, lanes);
  exitStatus = renderRootSync(root, lanes);
}
// ...省略无关代码
function performConcurrentWorkOnRoot(root) {
  // 获取本次`render`的优先级
  let lanes = getNextLanes(
    root,
    root === workInProgressRoot ? workInProgressRootRenderLanes : NoLanes,
  );
  if (lanes === NoLanes) {
    return null;
  }
  let exitStatus = renderRootConcurrent(root, lanes);
}

// ...省略部分代码
export function getNextLanes(root: FiberRoot, wipLanes: Lanes): Lanes {
  // 1. check是否有等待中的lanes
  const pendingLanes = root.pendingLanes;
  if (pendingLanes === NoLanes) {
    return_highestLanePriority = NoLanePriority;
    return NoLanes;
  }
  let nextLanes = NoLanes;
  let nextLanePriority = NoLanePriority;
  const expiredLanes = root.expiredLanes;
  const suspendedLanes = root.suspendedLanes;
  const pingedLanes = root.pingedLanes;
  // 2. check是否有已过期的lanes
  if (expiredLanes !== NoLanes) {
    nextLanes = expiredLanes;
    nextLanePriority = return_highestLanePriority = SyncLanePriority;
  } else {
    const nonIdlePendingLanes = pendingLanes & NonIdleLanes;
    if (nonIdlePendingLanes !== NoLanes) {
      // 非Idle任务 ...
    } else {
      // Idle任务 ...
    }
  }
  if (nextLanes === NoLanes) {
    return NoLanes;
  }
  return nextLanes;
}

function beginWork(
  current: Fiber | null,
  workInProgress: Fiber,
  renderLanes: Lanes,
): Fiber | null {
  const updateLanes = workInProgress.lanes;
  if (current !== null) {
    const oldProps = current.memoizedProps;
    const newProps = workInProgress.pendingProps;
    if (
      oldProps !== newProps ||
      hasLegacyContextChanged() ||
      // Force a re-render if the implementation changed due to hot reload:
      (__DEV__ ? workInProgress.type !== current.type : false)
    ) {
      didReceiveUpdate = true;
    } else if (!includesSomeLane(renderLanes, updateLanes)) {
      didReceiveUpdate = false;
      // 本`fiber`节点的没有更新, 可以复用, 进入bailout逻辑
      return bailoutOnAlreadyFinishedWork(current, workInProgress, renderLanes);
    }
  }
  // 不能复用, 创建新的fiber节点
  workInProgress.lanes = NoLanes; // 重置优先级为 NoLanes
  switch (workInProgress.tag) {
    case ClassComponent: {
      const Component = workInProgress.type;
      const unresolvedProps = workInProgress.pendingProps;
      const resolvedProps =
        workInProgress.elementType === Component
          ? unresolvedProps
          : resolveDefaultProps(Component, unresolvedProps);

      return updateClassComponent(
        current,
        workInProgress,
        Component,
        resolvedProps,
        // 正常情况下渲染优先级会被用于fiber树的构造过程
        renderLanes,
      );
    }
  }
}

function renderRootConcurrent(root: FiberRoot, lanes: Lanes) {
  const prevExecutionContext = executionContext;
  executionContext |= RenderContext;
  const prevDispatcher = pushDispatcher();
  // 如果fiberRoot变动, 或者update.lane变动, 都会刷新栈帧, 丢弃上一次渲染进度
  if (workInProgressRoot !== root || workInProgressRootRenderLanes !== lanes) {
    resetRenderTimer();
    // 刷新栈帧
    prepareFreshStack(root, lanes);
    startWorkOnPendingInteractions(root, lanes);
  }
}

/**
刷新栈帧: 重置 FiberRoot上的全局属性 和 `fiber树构造`循环过程中的全局变量
*/
function prepareFreshStack(root: FiberRoot, lanes: Lanes) {
  // 重置FiberRoot对象上的属性
  root.finishedWork = null;
  root.finishedLanes = NoLanes;
  const timeoutHandle = root.timeoutHandle;
  if (timeoutHandle !== noTimeout) {
    root.timeoutHandle = noTimeout;
    cancelTimeout(timeoutHandle);
  }
  if (workInProgress !== null) {
    let interruptedWork = workInProgress.return;
    while (interruptedWork !== null) {
      unwindInterruptedWork(interruptedWork);
      interruptedWork = interruptedWork.return;
    }
  }
  // 重置全局变量
  workInProgressRoot = root;
  workInProgress = createWorkInProgress(root.current, null); // 给HostRootFiber对象创建一个alternate, 并将其设置成全局 workInProgress
  workInProgressRootRenderLanes =
    subtreeRenderLanes =
    workInProgressRootIncludedLanes =
      lanes;
  workInProgressRootExitStatus = RootIncomplete;
  workInProgressRootFatalError = null;
  workInProgressRootSkippedLanes = NoLanes;
  workInProgressRootUpdatedLanes = NoLanes;
  workInProgressRootPingedLanes = NoLanes;
}

class App extends React.Component {
  componentDidMount() {
    console.log(`App Mount`);
    console.log(`App 组件对应的fiber节点: `, this._reactInternals);
  }
  render() {
    return (
      <div className="app">
        <header>header</header>
        <Content />
      </div>
    );
  }
}

class Content extends React.Component {
  componentDidMount() {
    console.log(`Content Mount`);
    console.log(`Content 组件对应的fiber节点: `, this._reactInternals);
  }
  render() {
    return (
      <React.Fragment>
        <p>1</p>
        <p>2</p>
      </React.Fragment>
    );
  }
}
export default App;

document.getElementById('root')._reactRootContainer._internalRoot.current;

// ... 省略了部分代码
export function updateContainer(
  element: ReactNodeList,
  container: OpaqueRoot,
  parentComponent: ?React$Component<any, any>,
  callback: ?Function,
): Lane {
  // 获取当前时间戳
  const current = container.current;
  const eventTime = requestEventTime();
  // 1. 创建一个优先级变量(车道模型)
  const lane = requestUpdateLane(current);

  // 2. 根据车道优先级, 创建update对象, 并加入fiber.updateQueue.pending队列
  const update = createUpdate(eventTime, lane);
  update.payload = { element };
  callback = callback === undefined ? null : callback;
  if (callback !== null) {
    update.callback = callback;
  }
  enqueueUpdate(current, update);

  // 3. 进入reconciler运作流程中的`输入`环节
  scheduleUpdateOnFiber(current, lane, eventTime);
  return lane;
}

// ...省略部分代码
export function scheduleUpdateOnFiber(
  fiber: Fiber,
  lane: Lane,
  eventTime: number,
) {
  // 标记优先级
  const root = markUpdateLaneFromFiberToRoot(fiber, lane);
  if (lane === SyncLane) {
    if (
      (executionContext & LegacyUnbatchedContext) !== NoContext &&
      (executionContext & (RenderContext | CommitContext)) === NoContext
    ) {
      // 首次渲染, 直接进行`fiber构造`
      performSyncWorkOnRoot(root);
    }
    // ...
  }
}

function performSyncWorkOnRoot(root) {
  let lanes;
  let exitStatus;
  if (
    root === workInProgressRoot &&
    includesSomeLane(root.expiredLanes, workInProgressRootRenderLanes)
  ) {
    // 初次构造时(因为root=fiberRoot, workInProgressRoot=null), 所以不会进入
  } else {
    // 1. 获取本次render的优先级, 初次构造返回 NoLanes
    lanes = getNextLanes(root, NoLanes);
    // 2. 从root节点开始, 至上而下更新
    exitStatus = renderRootSync(root, lanes);
  }

  // 将最新的fiber树挂载到root.finishedWork节点上
  const finishedWork: Fiber = (root.current.alternate: any);
  root.finishedWork = finishedWork;
  root.finishedLanes = lanes;
  // 进入commit阶段
  commitRoot(root);

  // ...后面的内容本节不讨论
}

function renderRootSync(root: FiberRoot, lanes: Lanes) {
  const prevExecutionContext = executionContext;
  executionContext |= RenderContext;
  // 如果fiberRoot变动, 或者update.lane变动, 都会刷新栈帧, 丢弃上一次渲染进度
  if (workInProgressRoot !== root || workInProgressRootRenderLanes !== lanes) {
    // 刷新栈帧, legacy模式下都会进入
    prepareFreshStack(root, lanes);
  }
  do {
    try {
      workLoopSync();
      break;
    } catch (thrownValue) {
      handleError(root, thrownValue);
    }
  } while (true);
  executionContext = prevExecutionContext;
  // 重置全局变量, 表明render结束
  workInProgressRoot = null;
  workInProgressRootRenderLanes = NoLanes;
  return workInProgressRootExitStatus;
}

function workLoopSync() {
  while (workInProgress !== null) {
    performUnitOfWork(workInProgress);
  }
}

function workLoopConcurrent() {
  // Perform work until Scheduler asks us to yield
  while (workInProgress !== null && !shouldYield()) {
    performUnitOfWork(workInProgress);
  }
}

// ... 省略部分无关代码
function performUnitOfWork(unitOfWork: Fiber): void {
  // unitOfWork就是被传入的workInProgress
  const current = unitOfWork.alternate;
  let next;
  next = beginWork(current, unitOfWork, subtreeRenderLanes);
  unitOfWork.memoizedProps = unitOfWork.pendingProps;
  if (next === null) {
    // 如果没有派生出新的节点, 则进入completeWork阶段, 传入的是当前unitOfWork
    completeUnitOfWork(unitOfWork);
  } else {
    workInProgress = next;
  }
}

function beginWork(
  current: Fiber | null,
  workInProgress: Fiber,
  renderLanes: Lanes,
): Fiber | null {
  const updateLanes = workInProgress.lanes;
  if (current !== null) {
    // update逻辑, 首次render不会进入
  } else {
    didReceiveUpdate = false;
  }
  // 1. 设置workInProgress优先级为NoLanes(最高优先级)
  workInProgress.lanes = NoLanes;
  // 2. 根据workInProgress节点的类型, 用不同的方法派生出子节点
  switch (
    workInProgress.tag // 只保留了本例使用到的case
  ) {
    case ClassComponent: {
      const Component = workInProgress.type;
      const unresolvedProps = workInProgress.pendingProps;
      const resolvedProps =
        workInProgress.elementType === Component
          ? unresolvedProps
          : resolveDefaultProps(Component, unresolvedProps);
      return updateClassComponent(
        current,
        workInProgress,
        Component,
        resolvedProps,
        renderLanes,
      );
    }
    case HostRoot:
      return updateHostRoot(current, workInProgress, renderLanes);
    case HostComponent:
      return updateHostComponent(current, workInProgress, renderLanes);
    case HostText:
      return updateHostText(current, workInProgress);
    case Fragment:
      return updateFragment(current, workInProgress, renderLanes);
  }
}

// 省略与本节无关代码
function updateHostRoot(current, workInProgress, renderLanes) {
  // 1. 状态计算, 更新整合到 workInProgress.memoizedState中来
  const updateQueue = workInProgress.updateQueue;
  const nextProps = workInProgress.pendingProps;
  const prevState = workInProgress.memoizedState;
  const prevChildren = prevState !== null ? prevState.element : null;
  cloneUpdateQueue(current, workInProgress);
  // 遍历updateQueue.shared.pending, 提取有足够优先级的update对象, 计算出最终的状态 workInProgress.memoizedState
  processUpdateQueue(workInProgress, nextProps, null, renderLanes);
  const nextState = workInProgress.memoizedState;
  // 2. 获取下级`ReactElement`对象
  const nextChildren = nextState.element;
  const root: FiberRoot = workInProgress.stateNode;
  if (root.hydrate && enterHydrationState(workInProgress)) {
    // ...服务端渲染相关, 此处省略
  } else {
    // 3. 根据`ReactElement`对象, 调用`reconcileChildren`生成`Fiber`子节点(只生成`次级子节点`)
    reconcileChildren(current, workInProgress, nextChildren, renderLanes);
  }
  return workInProgress.child;
}

// ...省略部分无关代码
function updateHostComponent(
  current: Fiber | null,
  workInProgress: Fiber,
  renderLanes: Lanes,
) {
  // 1. 状态计算, 由于HostComponent是无状态组件, 所以只需要收集 nextProps即可, 它没有 memoizedState
  const type = workInProgress.type;
  const nextProps = workInProgress.pendingProps;
  const prevProps = current !== null ? current.memoizedProps : null;
  // 2. 获取下级`ReactElement`对象
  let nextChildren = nextProps.children;
  const isDirectTextChild = shouldSetTextContent(type, nextProps);

  if (isDirectTextChild) {
    // 如果子节点只有一个文本节点, 不用再创建一个HostText类型的fiber
    nextChildren = null;
  } else if (prevProps !== null && shouldSetTextContent(type, prevProps)) {
    // 特殊操作需要设置fiber.flags
    workInProgress.flags |= ContentReset;
  }
  // 特殊操作需要设置fiber.flags
  markRef(current, workInProgress);
  // 3. 根据`ReactElement`对象, 调用`reconcileChildren`生成`Fiber`子节点(只生成`次级子节点`)
  reconcileChildren(current, workInProgress, nextChildren, renderLanes);
  return workInProgress.child;
}

function completeUnitOfWork(unitOfWork: Fiber): void {
  let completedWork = unitOfWork;
  // 外层循环控制并移动指针(`workInProgress`,`completedWork`等)
  do {
    const current = completedWork.alternate;
    const returnFiber = completedWork.return;
    if ((completedWork.flags & Incomplete) === NoFlags) {
      let next;
      // 1. 处理Fiber节点, 会调用渲染器(调用react-dom包, 关联Fiber节点和dom对象, 绑定事件等)
      next = completeWork(current, completedWork, subtreeRenderLanes); // 处理单个节点
      if (next !== null) {
        // 如果派生出其他的子节点, 则回到`beginWork`阶段进行处理
        workInProgress = next;
        return;
      }
      // 重置子节点的优先级
      resetChildLanes(completedWork);
      if (
        returnFiber !== null &&
        (returnFiber.flags & Incomplete) === NoFlags
      ) {
        // 2. 收集当前Fiber节点以及其子树的副作用effects
        // 2.1 把子节点的副作用队列添加到父节点上
        if (returnFiber.firstEffect === null) {
          returnFiber.firstEffect = completedWork.firstEffect;
        }
        if (completedWork.lastEffect !== null) {
          if (returnFiber.lastEffect !== null) {
            returnFiber.lastEffect.nextEffect = completedWork.firstEffect;
          }
          returnFiber.lastEffect = completedWork.lastEffect;
        }
        // 2.2 如果当前fiber节点有副作用, 将其添加到子节点的副作用队列之后.
        const flags = completedWork.flags;
        if (flags > PerformedWork) {
          // PerformedWork是提供给 React DevTools读取的, 所以略过PerformedWork
          if (returnFiber.lastEffect !== null) {
            returnFiber.lastEffect.nextEffect = completedWork;
          } else {
            returnFiber.firstEffect = completedWork;
          }
          returnFiber.lastEffect = completedWork;
        }
      }
    } else {
      // 异常处理, 本节不讨论
    }

    const siblingFiber = completedWork.sibling;
    if (siblingFiber !== null) {
      // 如果有兄弟节点, 返回之后再次进入`beginWork`阶段
      workInProgress = siblingFiber;
      return;
    }
    // 移动指针, 指向下一个节点
    completedWork = returnFiber;
    workInProgress = completedWork;
  } while (completedWork !== null);
  // 已回溯到根节点, 设置workInProgressRootExitStatus = RootCompleted
  if (workInProgressRootExitStatus === RootIncomplete) {
    workInProgressRootExitStatus = RootCompleted;
  }
}

function completeWork(
  current: Fiber | null,
  workInProgress: Fiber,
  renderLanes: Lanes,
): Fiber | null {
  const newProps = workInProgress.pendingProps;
  switch (workInProgress.tag) {
    case ClassComponent: {
      // Class类型不做处理
      return null;
    }
    case HostRoot: {
      const fiberRoot = (workInProgress.stateNode: FiberRoot);
      if (fiberRoot.pendingContext) {
        fiberRoot.context = fiberRoot.pendingContext;
        fiberRoot.pendingContext = null;
      }
      if (current === null || current.child === null) {
         // 设置fiber.flags标记
         workInProgress.flags |= Snapshot;
      }
      return null;
    }
    case HostComponent: {
      popHostContext(workInProgress);
      const rootContainerInstance = getRootHostContainer();
      const type = workInProgress.type;
      if (current !== null && workInProgress.stateNode != null) {
        // update逻辑, 初次render不会进入
      } else {
        const currentHostContext = getHostContext();
        // 1. 创建DOM对象
        const instance = createInstance(
          type,
          newProps,
          rootContainerInstance,
          currentHostContext,
          workInProgress,
        );
        // 2. 把子树中的DOM对象append到本节点的DOM对象之后
        appendAllChildren(instance, workInProgress, false, false);
        // 设置stateNode属性, 指向DOM对象
        workInProgress.stateNode = instance;
        if (
          // 3. 设置DOM对象的属性, 绑定事件等
          finalizeInitialChildren(
            instance,
            type,
            newProps,
            rootContainerInstance,
            currentHostContext,
          )
        ) {
          // 设置fiber.flags标记(Update)
          markUpdate(workInProgress);
        }
        if (workInProgress.ref !== null) {
          // 设置fiber.flags标记(Ref)
          markRef(workInProgress);
        }
        return null;
    }
  }
}

import React from 'react';

class App extends React.Component {
  state = {
    list: ['A', 'B', 'C'],
  };
  onChange = () => {
    this.setState({ list: ['C', 'A', 'X'] });
  };
  componentDidMount() {
    console.log(`App Mount`);
  }
  render() {
    return (
      <>
        <Header />
        <button onClick={this.onChange}>change</button>
        <div className="content">
          {this.state.list.map((item) => (
            <p key={item}>{item}</p>
          ))}
        </div>
      </>
    );
  }
}

class Header extends React.PureComponent {
  render() {
    return (
      <>
        <h1>title</h1>
        <h2>title2</h2>
      </>
    );
  }
}
export default App;

Component.prototype.setState = function (partialState, callback) {
  this.updater.enqueueSetState(this, partialState, callback, 'setState');
};

const classComponentUpdater = {
  isMounted,
  enqueueSetState(inst, payload, callback) {
    // 1. 获取class实例对应的fiber节点
    const fiber = getInstance(inst);
    // 2. 创建update对象
    const eventTime = requestEventTime();
    const lane = requestUpdateLane(fiber); // 确定当前update对象的优先级
    const update = createUpdate(eventTime, lane);
    update.payload = payload;
    if (callback !== undefined && callback !== null) {
      update.callback = callback;
    }
    // 3. 将update对象添加到当前Fiber节点的updateQueue队列当中
    enqueueUpdate(fiber, update);
    // 4. 进入reconciler运作流程中的`输入`环节
    scheduleUpdateOnFiber(fiber, lane, eventTime); // 传入的lane是update优先级
  },
};

function dispatchAction<S, A>(
  fiber: Fiber,
  queue: UpdateQueue<S, A>,
  action: A,
) {
  // 1. 创建update对象
  const eventTime = requestEventTime();
  const lane = requestUpdateLane(fiber); // 确定当前update对象的优先级
  const update: Update<S, A> = {
    lane,
    action,
    eagerReducer: null,
    eagerState: null,
    next: (null: any),
  };
  // 2. 将update对象添加到当前Hook对象的updateQueue队列当中
  const pending = queue.pending;
  if (pending === null) {
    update.next = update;
  } else {
    update.next = pending.next;
    pending.next = update;
  }
  queue.pending = update;
  // 3. 请求调度, 进入reconciler运作流程中的`输入`环节.
  scheduleUpdateOnFiber(fiber, lane, eventTime); // 传入的lane是update优先级
}

import ReactDOM from 'react-dom';
function tick() {
  const element = (
    <div>
      <h1>Hello, world!</h1>
      <h2>It is {new Date().toLocaleTimeString()}.</h2>
    </div>
  );
  ReactDOM.render(element, document.getElementById('root'));
}
setInterval(tick, 1000);

// ...省略部分代码
export function scheduleUpdateOnFiber(
  fiber: Fiber, // fiber表示被更新的节点
  lane: Lane, // lane表示update优先级
  eventTime: number,
) {
  const root = markUpdateLaneFromFiberToRoot(fiber, lane);
  if (lane === SyncLane) {
    if (
      (executionContext & LegacyUnbatchedContext) !== NoContext &&
      (executionContext & (RenderContext | CommitContext)) === NoContext
    ) {
      // 初次渲染
      performSyncWorkOnRoot(root);
    } else {
      // 对比更新
      ensureRootIsScheduled(root, eventTime);
    }
  }
  mostRecentlyUpdatedRoot = root;
}

function markUpdateLaneFromFiberToRoot(
  sourceFiber: Fiber, // sourceFiber表示被更新的节点
  lane: Lane, // lane表示update优先级
): FiberRoot | null {
  // 1. 将update优先级设置到sourceFiber.lanes
  sourceFiber.lanes = mergeLanes(sourceFiber.lanes, lane);
  let alternate = sourceFiber.alternate;
  if (alternate !== null) {
    // 同时设置sourceFiber.alternate的优先级
    alternate.lanes = mergeLanes(alternate.lanes, lane);
  }
  // 2. 从sourceFiber开始, 向上遍历所有节点, 直到HostRoot. 设置沿途所有节点(包括alternate)的childLanes
  let node = sourceFiber;
  let parent = sourceFiber.return;
  while (parent !== null) {
    parent.childLanes = mergeLanes(parent.childLanes, lane);
    alternate = parent.alternate;
    if (alternate !== null) {
      alternate.childLanes = mergeLanes(alternate.childLanes, lane);
    }
    node = parent;
    parent = parent.return;
  }
  if (node.tag === HostRoot) {
    const root: FiberRoot = node.stateNode;
    return root;
  } else {
    return null;
  }
}

function renderRootSync(root: FiberRoot, lanes: Lanes) {
  const prevExecutionContext = executionContext;
  executionContext |= RenderContext;
  // 如果fiberRoot变动, 或者update.lane变动, 都会刷新栈帧, 丢弃上一次渲染进度
  if (workInProgressRoot !== root || workInProgressRootRenderLanes !== lanes) {
    // 刷新栈帧, legacy模式下都会进入
    prepareFreshStack(root, lanes);
  }
  do {
    try {
      workLoopSync();
      break;
    } catch (thrownValue) {
      handleError(root, thrownValue);
    }
  } while (true);
  executionContext = prevExecutionContext;
  // 重置全局变量, 表明render结束
  workInProgressRoot = null;
  workInProgressRootRenderLanes = NoLanes;
  return workInProgressRootExitStatus;
}

function workLoopSync() {
  while (workInProgress !== null) {
    performUnitOfWork(workInProgress);
  }
}

// ... 省略部分无关代码
function performUnitOfWork(unitOfWork: Fiber): void {
  // unitOfWork就是被传入的workInProgress
  const current = unitOfWork.alternate;
  let next;
  next = beginWork(current, unitOfWork, subtreeRenderLanes);
  unitOfWork.memoizedProps = unitOfWork.pendingProps;
  if (next === null) {
    // 如果没有派生出新的节点, 则进入completeWork阶段, 传入的是当前unitOfWork
    completeUnitOfWork(unitOfWork);
  } else {
    workInProgress = next;
  }
}

function beginWork(
  current: Fiber | null,
  workInProgress: Fiber,
  renderLanes: Lanes,
): Fiber | null {
  const updateLanes = workInProgress.lanes;
  if (current !== null) {
    // 进入对比
    const oldProps = current.memoizedProps;
    const newProps = workInProgress.pendingProps;
    if (
      oldProps !== newProps ||
      hasLegacyContextChanged() ||
      (__DEV__ ? workInProgress.type !== current.type : false)
    ) {
      didReceiveUpdate = true;
    } else if (!includesSomeLane(renderLanes, updateLanes)) {
      // 当前渲染优先级renderLanes不包括fiber.lanes, 表明当前fiber节点无需更新
      didReceiveUpdate = false;
      switch (
        workInProgress.tag
        // switch 语句中包括 context相关逻辑, 本节暂不讨论(不影响分析fiber树构造)
      ) {
      }
      // 当前fiber节点无需更新, 调用bailoutOnAlreadyFinishedWork循环检测子节点是否需要更新
      return bailoutOnAlreadyFinishedWork(current, workInProgress, renderLanes);
    }
  }
  // 余下逻辑与初次创建共用
  // 1. 设置workInProgress优先级为NoLanes(最高优先级)
  workInProgress.lanes = NoLanes;
  // 2. 根据workInProgress节点的类型, 用不同的方法派生出子节点
  switch (
    workInProgress.tag // 只列出部分case
  ) {
    case ClassComponent: {
      const Component = workInProgress.type;
      const unresolvedProps = workInProgress.pendingProps;
      const resolvedProps =
        workInProgress.elementType === Component
          ? unresolvedProps
          : resolveDefaultProps(Component, unresolvedProps);
      return updateClassComponent(
        current,
        workInProgress,
        Component,
        resolvedProps,
        renderLanes,
      );
    }
    case HostRoot:
      return updateHostRoot(current, workInProgress, renderLanes);
    case HostComponent:
      return updateHostComponent(current, workInProgress, renderLanes);
    case HostText:
      return updateHostText(current, workInProgress);
    case Fragment:
      return updateFragment(current, workInProgress, renderLanes);
  }
}

// 省略部分无关代码
function bailoutOnAlreadyFinishedWork(
  current: Fiber | null,
  workInProgress: Fiber,
  renderLanes: Lanes,
): Fiber | null {
  if (!includesSomeLane(renderLanes, workInProgress.childLanes)) {
    // 渲染优先级不包括 workInProgress.childLanes, 表明子节点也无需更新. 返回null, 直接进入回溯阶段.
    return null;
  } else {
    // 本fiber虽然不用更新, 但是子节点需要更新. clone并返回子节点
    cloneChildFibers(current, workInProgress);
    return workInProgress.child;
  }
}

// ...省略无关代码
function completeWork(
  current: Fiber | null,
  workInProgress: Fiber,
  renderLanes: Lanes,
): Fiber | null {
  const newProps = workInProgress.pendingProps;
  switch (workInProgress.tag) {
    case HostComponent: {
      // 非文本节点
      popHostContext(workInProgress);
      const rootContainerInstance = getRootHostContainer();
      const type = workInProgress.type;
      if (current !== null && workInProgress.stateNode != null) {
        // 处理改动
        updateHostComponent(
          current,
          workInProgress,
          type,
          newProps,
          rootContainerInstance,
        );
        if (current.ref !== workInProgress.ref) {
          markRef(workInProgress);
        }
      } else {
        // ...省略无关代码
      }
      return null;
    }
    case HostText: {
      // 文本节点
      const newText = newProps;
      if (current && workInProgress.stateNode != null) {
        const oldText = current.memoizedProps;
        // 处理改动
        updateHostText(current, workInProgress, oldText, newText);
      } else {
        // ...省略无关代码
      }
      return null;
    }
  }
}

updateHostComponent = function (
  current: Fiber,
  workInProgress: Fiber,
  type: Type,
  newProps: Props,
  rootContainerInstance: Container,
) {
  const oldProps = current.memoizedProps;
  if (oldProps === newProps) {
    return;
  }
  const instance: Instance = workInProgress.stateNode;
  const currentHostContext = getHostContext();
  const updatePayload = prepareUpdate(
    instance,
    type,
    oldProps,
    newProps,
    rootContainerInstance,
    currentHostContext,
  );
  workInProgress.updateQueue = (updatePayload: any);
  // 如果有属性变动, 设置fiber.flags |= Update, 等待`commit`阶段的处理
  if (updatePayload) {
    markUpdate(workInProgress);
  }
};
updateHostText = function (
  current: Fiber,
  workInProgress: Fiber,
  oldText: string,
  newText: string,
) {
  // 如果有属性变动, 设置fiber.flags |= Update, 等待`commit`阶段的处理
  if (oldText !== newText) {
    markUpdate(workInProgress);
  }
};

function commitRoot(root) {
  const renderPriorityLevel = getCurrentPriorityLevel();
  runWithPriority(
    ImmediateSchedulerPriority,
    commitRootImpl.bind(null, root, renderPriorityLevel),
  );
  return null;
}

// ... 省略部分无关代码
function commitRootImpl(root, renderPriorityLevel) {
  // ============ 渲染前: 准备 ============

  const finishedWork = root.finishedWork;
  const lanes = root.finishedLanes;

  // 清空FiberRoot对象上的属性
  root.finishedWork = null;
  root.finishedLanes = NoLanes;
  root.callbackNode = null;

  if (root === workInProgressRoot) {
    // 重置全局变量
    workInProgressRoot = null;
    workInProgress = null;
    workInProgressRootRenderLanes = NoLanes;
  }

  // 再次更新副作用队列
  let firstEffect;
  if (finishedWork.flags > PerformedWork) {
    // 默认情况下fiber节点的副作用队列是不包括自身的
    // 如果根节点有副作用, 则将根节点添加到副作用队列的末尾
    if (finishedWork.lastEffect !== null) {
      finishedWork.lastEffect.nextEffect = finishedWork;
      firstEffect = finishedWork.firstEffect;
    } else {
      firstEffect = finishedWork;
    }
  } else {
    firstEffect = finishedWork.firstEffect;
  }

  // ============ 渲染 ============
  let firstEffect = finishedWork.firstEffect;
  if (firstEffect !== null) {
    const prevExecutionContext = executionContext;
    executionContext |= CommitContext;
    // 阶段1: dom突变之前
    nextEffect = firstEffect;
    do {
      commitBeforeMutationEffects();
    } while (nextEffect !== null);

    // 阶段2: dom突变, 界面发生改变
    nextEffect = firstEffect;
    do {
      commitMutationEffects(root, renderPriorityLevel);
    } while (nextEffect !== null);
    // 恢复界面状态
    resetAfterCommit(root.containerInfo);
    // 切换current指针
    root.current = finishedWork;

    // 阶段3: layout阶段, 调用生命周期componentDidUpdate和回调函数等
    nextEffect = firstEffect;
    do {
      commitLayoutEffects(root, lanes);
    } while (nextEffect !== null);
    nextEffect = null;
    executionContext = prevExecutionContext;
  }

  // ============ 渲染后: 重置与清理 ============
  if (rootDoesHavePassiveEffects) {
    // 有被动作用(使用useEffect), 保存一些全局变量
  } else {
    // 分解副作用队列链表, 辅助垃圾回收
    // 如果有被动作用(使用useEffect), 会把分解操作放在flushPassiveEffects函数中
    nextEffect = firstEffect;
    while (nextEffect !== null) {
      const nextNextEffect = nextEffect.nextEffect;
      nextEffect.nextEffect = null;
      if (nextEffect.flags & Deletion) {
        detachFiberAfterEffects(nextEffect);
      }
      nextEffect = nextNextEffect;
    }
  }
  // 重置一些全局变量(省略这部分代码)...
  // 下面代码用于检测是否有新的更新任务
  // 比如在componentDidMount函数中, 再次调用setState()

  // 1. 检测常规(异步)任务, 如果有则会发起异步调度(调度中心`scheduler`只能异步调用)
  ensureRootIsScheduled(root, now());
  // 2. 检测同步任务, 如果有则主动调用flushSyncCallbackQueue(无需再次等待scheduler调度), 再次进入fiber树构造循环
  flushSyncCallbackQueue();

  return null;
}

// ... 省略部分无关代码
function commitBeforeMutationEffects() {
  while (nextEffect !== null) {
    const current = nextEffect.alternate;
    const flags = nextEffect.flags;
    // 处理`Snapshot`标记
    if ((flags & Snapshot) !== NoFlags) {
      commitBeforeMutationEffectOnFiber(current, nextEffect);
    }
    // 处理`Passive`标记
    if ((flags & Passive) !== NoFlags) {
      // Passive标记只在使用了hook, useEffect会出现. 所以此处是针对hook对象的处理
      if (!rootDoesHavePassiveEffects) {
        rootDoesHavePassiveEffects = true;
        scheduleCallback(NormalSchedulerPriority, () => {
          flushPassiveEffects();
          return null;
        });
      }
    }
    nextEffect = nextEffect.nextEffect;
  }
}

function commitBeforeMutationLifeCycles(
  current: Fiber | null,
  finishedWork: Fiber,
): void {
  switch (finishedWork.tag) {
    case FunctionComponent:
    case ForwardRef:
    case SimpleMemoComponent:
    case Block: {
      return;
    }
    case ClassComponent: {
      if (finishedWork.flags & Snapshot) {
        if (current !== null) {
          const prevProps = current.memoizedProps;
          const prevState = current.memoizedState;
          const instance = finishedWork.stateNode;

          const snapshot = instance.getSnapshotBeforeUpdate(
            finishedWork.elementType === finishedWork.type
              ? prevProps
              : resolveDefaultProps(finishedWork.type, prevProps),
            prevState,
          );
          instance.__reactInternalSnapshotBeforeUpdate = snapshot;
        }
      }
      return;
    }
    case HostRoot: {
      if (supportsMutation) {
        if (finishedWork.flags & Snapshot) {
          const root = finishedWork.stateNode;
          clearContainer(root.containerInfo);
        }
      }
      return;
    }
    case HostComponent:
    case HostText:
    case HostPortal:
    case IncompleteClassComponent:
      return;
  }
}

// 以下示例代码中的输出顺序为 1, 3, 4, 2
function Test() {
  console.log(1);
  useEffect(() => {
    console.log(2);
  });
  console.log(3);
  Promise.resolve(() => {
    console.log(4);
  });
  return <div>test</div>;
}

// ...省略部分无关代码
function commitMutationEffects(
  root: FiberRoot,
  renderPriorityLevel: ReactPriorityLevel,
) {
  // 处理Ref
  if (flags & Ref) {
    const current = nextEffect.alternate;
    if (current !== null) {
      // 先清空ref, 在commitRoot的第三阶段(dom变更后), 再重新赋值
      commitDetachRef(current);
    }
  }
  // 处理DOM突变
  while (nextEffect !== null) {
    const flags = nextEffect.flags;
    const primaryFlags = flags & (Placement | Update | Deletion | Hydrating);
    switch (primaryFlags) {
      case Placement: {
        // 新增节点
        commitPlacement(nextEffect);
        nextEffect.flags &= ~Placement; // 注意Placement标记会被清除
        break;
      }
      case PlacementAndUpdate: {
        // Placement
        commitPlacement(nextEffect);
        nextEffect.flags &= ~Placement;
        // Update
        const current = nextEffect.alternate;
        commitWork(current, nextEffect);
        break;
      }
      case Update: {
        // 更新节点
        const current = nextEffect.alternate;
        commitWork(current, nextEffect);
        break;
      }
      case Deletion: {
        // 删除节点
        commitDeletion(root, nextEffect, renderPriorityLevel);
        break;
      }
    }
    nextEffect = nextEffect.nextEffect;
  }
}

// ...省略部分无关代码
function commitLayoutEffects(root: FiberRoot, committedLanes: Lanes) {
  while (nextEffect !== null) {
    const flags = nextEffect.flags;
    // 处理 Update和Callback标记
    if (flags & (Update | Callback)) {
      const current = nextEffect.alternate;
      commitLayoutEffectOnFiber(root, current, nextEffect, committedLanes);
    }
    if (flags & Ref) {
      // 重新设置ref
      commitAttachRef(nextEffect);
    }
    nextEffect = nextEffect.nextEffect;
  }
}

// ...省略部分无关代码
function commitLifeCycles(
  finishedRoot: FiberRoot,
  current: Fiber | null,
  finishedWork: Fiber,
  committedLanes: Lanes,
): void {
  switch (finishedWork.tag) {
    case ClassComponent: {
      const instance = finishedWork.stateNode;
      if (finishedWork.flags & Update) {
        if (current === null) {
          // 初次渲染: 调用 componentDidMount
          instance.componentDidMount();
        } else {
          const prevProps =
            finishedWork.elementType === finishedWork.type
              ? current.memoizedProps
              : resolveDefaultProps(finishedWork.type, current.memoizedProps);
          const prevState = current.memoizedState;
          // 更新阶段: 调用 componentDidUpdate
          instance.componentDidUpdate(
            prevProps,
            prevState,
            instance.__reactInternalSnapshotBeforeUpdate,
          );
        }
      }
      const updateQueue: UpdateQueue<*> | null =
        (finishedWork.updateQueue: any);
      if (updateQueue !== null) {
        // 处理update回调函数 如: this.setState({}, callback)
        commitUpdateQueue(finishedWork, updateQueue, instance);
      }
      return;
    }
    case HostComponent: {
      const instance: Instance = finishedWork.stateNode;
      if (current === null && finishedWork.flags & Update) {
        const type = finishedWork.type;
        const props = finishedWork.memoizedProps;
        // 设置focus等原生状态
        commitMount(instance, type, props, finishedWork);
      }
      return;
    }
  }
}

// 清除副作用队列
if (rootDoesHavePassiveEffects) {
  // 有被动作用(使用useEffect), 保存一些全局变量
} else {
  // 分解副作用队列链表, 辅助垃圾回收.
  // 如果有被动作用(使用useEffect), 会把分解操作放在flushPassiveEffects函数中
  nextEffect = firstEffect;
  while (nextEffect !== null) {
    const nextNextEffect = nextEffect.nextEffect;
    nextEffect.nextEffect = null;
    if (nextEffect.flags & Deletion) {
      detachFiberAfterEffects(nextEffect);
    }
    nextEffect = nextNextEffect;
  }
}
// 重置一些全局变量(省略这部分代码)...
// 下面代码用于检测是否有新的更新任务
// 比如在componentDidMount函数中, 再次调用setState()

// 1. 检测常规(异步)任务, 如果有则会发起异步调度(调度中心`scheduler`只能异步调用)
ensureRootIsScheduled(root, now());
// 2. 检测同步任务, 如果有则主动调用flushSyncCallbackQueue(无需再次等待scheduler调度), 再次进入fiber树构造循环
flushSyncCallbackQueue();

export type Fiber = {|
  // 1. fiber节点自身状态相关
  pendingProps: any,
  memoizedProps: any,
  updateQueue: mixed,
  memoizedState: any,

  // 2. fiber节点副作用(Effect)相关
  flags: Flags,
  subtreeFlags: Flags, // v17.0.2未启用
  deletions: Array<Fiber> | null, // v17.0.2未启用
  nextEffect: Fiber | null,
  firstEffect: Fiber | null,
  lastEffect: Fiber | null,
|};

class App extends React.Component {
  constructor() {
    this.state = {
      // 初始状态
      a: 1,
    };
  }
  changeState = () => {
    this.setState({ a: ++this.state.a }); // 进入reconciler流程
  };

  // 生命周期函数: 状态相关
  static getDerivedStateFromProps(nextProps, prevState) {
    console.log('getDerivedStateFromProps');
    return prevState;
  }

  // 生命周期函数: 状态相关
  shouldComponentUpdate(newProps, newState, nextContext) {
    console.log('shouldComponentUpdate');
    return true;
  }

  // 生命周期函数: 副作用相关 fiber.flags |= Update
  componentDidMount() {
    console.log('componentDidMount');
  }

  // 生命周期函数: 副作用相关 fiber.flags |= Snapshot
  getSnapshotBeforeUpdate(prevProps, prevState) {
    console.log('getSnapshotBeforeUpdate');
  }

  // 生命周期函数: 副作用相关 fiber.flags |= Update
  componentDidUpdate() {
    console.log('componentDidUpdate');
  }

  render() {
    // 返回下级ReactElement对象
    return <button onClick={this.changeState}>{this.state.a}</button>;
  }
}

function App() {
  // 状态相关: 初始状态
  const [a, setA] = useState(1);
  const changeState = () => {
    setA(++a); // 进入reconciler流程
  };

  // 副作用相关: fiber.flags |= Update | Passive;
  useEffect(() => {
    console.log(`useEffect`);
  }, []);

  // 副作用相关: fiber.flags |= Update;
  useLayoutEffect(() => {
    console.log(`useLayoutEffect`);
  }, []);

  // 返回下级ReactElement对象
  return <button onClick={changeState}>{a}</button>;
}

export type Fiber = {|
  // 1. fiber节点自身状态相关
  pendingProps: any,
  memoizedProps: any,
  updateQueue: mixed,
  memoizedState: any,

  // 2. fiber节点副作用(Effect)相关
  flags: Flags,
  nextEffect: Fiber | null,
  firstEffect: Fiber | null,
  lastEffect: Fiber | null,
|};

type Update<S, A> = {|
  lane: Lane,
  action: A,
  eagerReducer: ((S, A) => S) | null,
  eagerState: S | null,
  next: Update<S, A>,
  priority?: ReactPriorityLevel,
|};

type UpdateQueue<S, A> = {|
  pending: Update<S, A> | null,
  dispatch: ((A) => mixed) | null,
  lastRenderedReducer: ((S, A) => S) | null,
  lastRenderedState: S | null,
|};

export type Hook = {|
  memoizedState: any, // 当前状态
  baseState: any, // 基状态
  baseQueue: Update<any, any> | null, // 基队列
  queue: UpdateQueue<any, any> | null, // 更新队列
  next: Hook | null, // next指针
|};

export type HookType =
  | 'useState'
  | 'useReducer'
  | 'useContext'
  | 'useRef'
  | 'useEffect'
  | 'useLayoutEffect'
  | 'useCallback'
  | 'useMemo'
  | 'useImperativeHandle'
  | 'useDebugValue'
  | 'useDeferredValue'
  | 'useTransition'
  | 'useMutableSource'
  | 'useOpaqueIdentifier';

// 使用useEffect时, 需要传入一个副作用回调函数.
// 在fiber树构造完成之后, commitRoot阶段会处理这些副作用回调
useEffect(() => {
  console.log('这是一个副作用回调函数');
}, []);

import { useState, useEffect } from 'react';

function useFriendStatus(friendID) {
  // 1. 调用useState, 创建一个状态Hook
  const [isOnline, setIsOnline] = useState(null);

  // 2. 调用useEffect, 创建一个副作用Hook
  useEffect(() => {
    function handleStatusChange(status) {
      setIsOnline(status.isOnline);
    }
    ChatAPI.subscribeToFriendStatus(friendID, handleStatusChange);
    return () => {
      ChatAPI.unsubscribeFromFriendStatus(friendID, handleStatusChange);
    };
  });
  return isOnline;
}

// 只保留FunctionComponent相关:
function beginWork(
  current: Fiber | null,
  workInProgress: Fiber,
  renderLanes: Lanes,
): Fiber | null {
  const updateLanes = workInProgress.lanes;
  switch (workInProgress.tag) {
    case FunctionComponent: {
      const Component = workInProgress.type;
      const unresolvedProps = workInProgress.pendingProps;
      const resolvedProps =
        workInProgress.elementType === Component
          ? unresolvedProps
          : resolveDefaultProps(Component, unresolvedProps);
      return updateFunctionComponent(
        current,
        workInProgress,
        Component,
        resolvedProps,
        renderLanes,
      );
    }
  }
}

function updateFunctionComponent(
  current,
  workInProgress,
  Component,
  nextProps: any,
  renderLanes,
) {
  // ...省略无关代码
  let context;
  let nextChildren;
  prepareToReadContext(workInProgress, renderLanes);

  // 进入Hooks相关逻辑, 最后返回下级ReactElement对象
  nextChildren = renderWithHooks(
    current,
    workInProgress,
    Component,
    nextProps,
    context,
    renderLanes,
  );
  // 进入reconcile函数, 生成下级fiber节点
  reconcileChildren(current, workInProgress, nextChildren, renderLanes);
  // 返回下级fiber节点
  return workInProgress.child;
}

// 渲染优先级
let renderLanes: Lanes = NoLanes;

// 当前正在构造的fiber, 等同于 workInProgress, 为了和当前hook区分, 所以将其改名
let currentlyRenderingFiber: Fiber = (null: any);

// Hooks被存储在fiber.memoizedState 链表上
let currentHook: Hook | null = null; // currentHook = fiber(current).memoizedState

let workInProgressHook: Hook | null = null; // workInProgressHook = fiber(workInProgress).memoizedState

// 在function的执行过程中, 是否再次发起了更新. 只有function被完全执行之后才会重置.
// 当render异常时, 通过该变量可以决定是否清除render过程中的更新.
let didScheduleRenderPhaseUpdate: boolean = false;

// 在本次function的执行过程中, 是否再次发起了更新. 每一次调用function都会被重置
let didScheduleRenderPhaseUpdateDuringThisPass: boolean = false;

// 在本次function的执行过程中, 重新发起更新的最大次数
const RE_RENDER_LIMIT = 25;

// ...省略无关代码
export function renderWithHooks<Props, SecondArg>(
  current: Fiber | null,
  workInProgress: Fiber,
  Component: (p: Props, arg: SecondArg) => any,
  props: Props,
  secondArg: SecondArg,
  nextRenderLanes: Lanes,
): any {
  // --------------- 1. 设置全局变量 -------------------
  renderLanes = nextRenderLanes; // 当前渲染优先级
  currentlyRenderingFiber = workInProgress; // 当前fiber节点, 也就是function组件对应的fiber节点

  // 清除当前fiber的遗留状态
  workInProgress.memoizedState = null;
  workInProgress.updateQueue = null;
  workInProgress.lanes = NoLanes;

  // --------------- 2. 调用function,生成子级ReactElement对象 -------------------
  // 指定dispatcher, 区分mount和update
  ReactCurrentDispatcher.current =
    current === null || current.memoizedState === null
      ? HooksDispatcherOnMount
      : HooksDispatcherOnUpdate;
  // 执行function函数, 其中进行分析Hooks的使用
  let children = Component(props, secondArg);

  // --------------- 3. 重置全局变量,并返回 -------------------
  // 执行function之后, 还原被修改的全局变量, 不影响下一次调用
  renderLanes = NoLanes;
  currentlyRenderingFiber = (null: any);

  currentHook = null;
  workInProgressHook = null;
  didScheduleRenderPhaseUpdate = false;

  return children;
}

export default function App() {
  // 1. useState
  const [a, setA] = useState(1);
  // 2. useEffect
  useEffect(() => {
    console.log(`effect 1 created`);
  });
  // 3. useState
  const [b] = useState(2);
  // 4. useEffect
  useEffect(() => {
    console.log(`effect 2 created`);
  });
  return (
    <>
      <button onClick={() => setA(a + 1)}>{a}</button>
      <button>{b}</button>
    </>
  );
}

function mountState<S>(
  initialState: (() => S) | S,
): [S, Dispatch<BasicStateAction<S>>] {
  const hook = mountWorkInProgressHook();
  // ...省略部分本节不讨论
  return [hook.memoizedState, dispatch];
}

function mountEffectImpl(fiberFlags, hookFlags, create, deps): void {
  const hook = mountWorkInProgressHook();
  // ...省略部分本节不讨论
}

function mountWorkInProgressHook(): Hook {
  const hook: Hook = {
    memoizedState: null,

    baseState: null,
    baseQueue: null,
    queue: null,

    next: null,
  };

  if (workInProgressHook === null) {
    // 链表中首个hook
    currentlyRenderingFiber.memoizedState = workInProgressHook = hook;
  } else {
    // 将hook添加到链表末尾
    workInProgressHook = workInProgressHook.next = hook;
  }
  return workInProgressHook;
}

// ----- 状态Hook --------
function updateReducer<S, I, A>(
  reducer: (S, A) => S,
  initialArg: I,
  init?: (I) => S,
): [S, Dispatch<A>] {
  const hook = updateWorkInProgressHook();
  // ...省略部分本节不讨论
}

// ----- 副作用Hook --------
function updateEffectImpl(fiberFlags, hookFlags, create, deps): void {
  const hook = updateWorkInProgressHook();
  // ...省略部分本节不讨论
}

function updateWorkInProgressHook(): Hook {
  // 1. 移动currentHook指针
  let nextCurrentHook: null | Hook;
  if (currentHook === null) {
    const current = currentlyRenderingFiber.alternate;
    if (current !== null) {
      nextCurrentHook = current.memoizedState;
    } else {
      nextCurrentHook = null;
    }
  } else {
    nextCurrentHook = currentHook.next;
  }

  // 2. 移动workInProgressHook指针
  let nextWorkInProgressHook: null | Hook;
  if (workInProgressHook === null) {
    nextWorkInProgressHook = currentlyRenderingFiber.memoizedState;
  } else {
    nextWorkInProgressHook = workInProgressHook.next;
  }

  if (nextWorkInProgressHook !== null) {
    // 渲染时更新: 本节不讨论
  } else {
    currentHook = nextCurrentHook;
    // 3. 克隆currentHook作为新的workInProgressHook.
    // 随后逻辑与mountWorkInProgressHook一致
    const newHook: Hook = {
      memoizedState: currentHook.memoizedState,

      baseState: currentHook.baseState,
      baseQueue: currentHook.baseQueue,
      queue: currentHook.queue,

      next: null, // 注意next指针是null
    };
    if (workInProgressHook === null) {
      currentlyRenderingFiber.memoizedState = workInProgressHook = newHook;
    } else {
      workInProgressHook = workInProgressHook.next = newHook;
    }
  }
  return workInProgressHook;
}

function mountState<S>(
  initialState: (() => S) | S,
): [S, Dispatch<BasicStateAction<S>>] {
  // 1. 创建hook
  const hook = mountWorkInProgressHook();
  if (typeof initialState === 'function') {
    initialState = initialState();
  }
  // 2. 初始化hook的属性
  // 2.1 设置 hook.memoizedState/hook.baseState
  // 2.2 设置 hook.queue
  hook.memoizedState = hook.baseState = initialState;
  const queue = (hook.queue = {
    pending: null,
    dispatch: null,
    // queue.lastRenderedReducer是内置函数
    lastRenderedReducer: basicStateReducer,
    lastRenderedState: (initialState: any),
  });
  // 2.3 设置 hook.dispatch
  const dispatch: Dispatch<BasicStateAction<S>> = (queue.dispatch =
    (dispatchAction.bind(null, currentlyRenderingFiber, queue): any));

  // 3. 返回[当前状态, dispatch函数]
  return [hook.memoizedState, dispatch];
}

function mountReducer<S, I, A>(
  reducer: (S, A) => S,
  initialArg: I,
  init?: (I) => S,
): [S, Dispatch<A>] {
  // 1. 创建hook
  const hook = mountWorkInProgressHook();
  let initialState;
  if (init !== undefined) {
    initialState = init(initialArg);
  } else {
    initialState = ((initialArg: any): S);
  }
  // 2. 初始化hook的属性
  // 2.1 设置 hook.memoizedState/hook.baseState
  hook.memoizedState = hook.baseState = initialState;
  // 2.2 设置 hook.queue
  const queue = (hook.queue = {
    pending: null,
    dispatch: null,
    // queue.lastRenderedReducer是由外传入
    lastRenderedReducer: reducer,
    lastRenderedState: (initialState: any),
  });
  // 2.3 设置 hook.dispatch
  const dispatch: Dispatch<A> = (queue.dispatch = (dispatchAction.bind(
    null,
    currentlyRenderingFiber,
    queue,
  ): any));

  // 3. 返回[当前状态, dispatch函数]
  return [hook.memoizedState, dispatch];
}

function basicStateReducer<S>(state: S, action: BasicStateAction<S>): S {
  return typeof action === 'function' ? action(state) : action;
}

const [state, dispatch] = useState({ count: 0 });

// 等价于
const [state, dispatch] = useReducer(
  function basicStateReducer(state, action) {
    return typeof action === 'function' ? action(state) : action;
  },
  { count: 0 },
);

// 当需要更新state时, 有2种方式
dispatch({ count: 1 }); // 1.直接设置
dispatch((state) => ({ count: state.count + 1 })); // 2.通过回调函数设置

const [state, dispatch] = useReducer(
  function reducer(state, action) {
    switch (action.type) {
      case 'increment':
        return { count: state.count + 1 };
      case 'decrement':
        return { count: state.count - 1 };
      default:
        throw new Error();
    }
  },
  { count: 0 },
);

// 当需要更新state时, 只有1种方式
dispatch({ type: 'decrement' });

export default function App() {
  const [count, dispatch] = useState(0);
  return (
    <button
      onClick={() => {
        dispatch(1);
        dispatch(3);
        dispatch(2);
      }}
    >
      {count}
    </button>
  );
}

function dispatchAction<S, A>(
  fiber: Fiber,
  queue: UpdateQueue<S, A>,
  action: A,
) {
  // 1. 创建update对象
  const eventTime = requestEventTime();
  const lane = requestUpdateLane(fiber); // Legacy模式返回SyncLane
  const update: Update<S, A> = {
    lane,
    action,
    eagerReducer: null,
    eagerState: null,
    next: (null: any),
  };

  // 2. 将update对象添加到hook.queue.pending队列
  const pending = queue.pending;
  if (pending === null) {
    // 首个update, 创建一个环形链表
    update.next = update;
  } else {
    update.next = pending.next;
    pending.next = update;
  }
  queue.pending = update;

  const alternate = fiber.alternate;
  if (
    fiber === currentlyRenderingFiber ||
    (alternate !== null && alternate === currentlyRenderingFiber)
  ) {
    // 渲染时更新, 做好全局标记
    didScheduleRenderPhaseUpdateDuringThisPass =
      didScheduleRenderPhaseUpdate = true;
  } else {
    // ...省略性能优化部分, 下文介绍

    // 3. 发起调度更新, 进入`reconciler 运作流程`中的输入阶段.
    scheduleUpdateOnFiber(fiber, lane, eventTime);
  }
}

function updateState<S>(
  initialState: (() => S) | S,
): [S, Dispatch<BasicStateAction<S>>] {
  return updateReducer(basicStateReducer, (initialState: any));
}

function updateReducer<S, I, A>(
  reducer: (S, A) => S,
  initialArg: I,
  init?: (I) => S,
): [S, Dispatch<A>] {
  // 1. 获取workInProgressHook对象
  const hook = updateWorkInProgressHook();
  const queue = hook.queue;
  queue.lastRenderedReducer = reducer;
  const current: Hook = (currentHook: any);
  let baseQueue = current.baseQueue;

  // 2. 链表拼接: 将 hook.queue.pending 拼接到 current.baseQueue
  const pendingQueue = queue.pending;
  if (pendingQueue !== null) {
    if (baseQueue !== null) {
      const baseFirst = baseQueue.next;
      const pendingFirst = pendingQueue.next;
      baseQueue.next = pendingFirst;
      pendingQueue.next = baseFirst;
    }
    current.baseQueue = baseQueue = pendingQueue;
    queue.pending = null;
  }
  // 3. 状态计算
  if (baseQueue !== null) {
    const first = baseQueue.next;
    let newState = current.baseState;

    let newBaseState = null;
    let newBaseQueueFirst = null;
    let newBaseQueueLast = null;
    let update = first;

    do {
      const updateLane = update.lane;
      // 3.1 优先级提取update
      if (!isSubsetOfLanes(renderLanes, updateLane)) {
        // 优先级不够: 加入到baseQueue中, 等待下一次render
        const clone: Update<S, A> = {
          lane: updateLane,
          action: update.action,
          eagerReducer: update.eagerReducer,
          eagerState: update.eagerState,
          next: (null: any),
        };
        if (newBaseQueueLast === null) {
          newBaseQueueFirst = newBaseQueueLast = clone;
          newBaseState = newState;
        } else {
          newBaseQueueLast = newBaseQueueLast.next = clone;
        }
        currentlyRenderingFiber.lanes = mergeLanes(
          currentlyRenderingFiber.lanes,
          updateLane,
        );
        markSkippedUpdateLanes(updateLane);
      } else {
        // 优先级足够: 状态合并
        if (newBaseQueueLast !== null) {
          // 更新baseQueue
          const clone: Update<S, A> = {
            lane: NoLane,
            action: update.action,
            eagerReducer: update.eagerReducer,
            eagerState: update.eagerState,
            next: (null: any),
          };
          newBaseQueueLast = newBaseQueueLast.next = clone;
        }
        if (update.eagerReducer === reducer) {
          // 性能优化: 如果存在 update.eagerReducer, 直接使用update.eagerState.避免重复调用reducer
          newState = ((update.eagerState: any): S);
        } else {
          const action = update.action;
          // 调用reducer获取最新状态
          newState = reducer(newState, action);
        }
      }
      update = update.next;
    } while (update !== null && update !== first);

    // 3.2. 更新属性
    if (newBaseQueueLast === null) {
      newBaseState = newState;
    } else {
      newBaseQueueLast.next = (newBaseQueueFirst: any);
    }
    if (!is(newState, hook.memoizedState)) {
      markWorkInProgressReceivedUpdate();
    }
    // 把计算之后的结果更新到workInProgressHook上
    hook.memoizedState = newState;
    hook.baseState = newBaseState;
    hook.baseQueue = newBaseQueueLast;
    queue.lastRenderedState = newState;
  }

  const dispatch: Dispatch<A> = (queue.dispatch: any);
  return [hook.memoizedState, dispatch];
}

function dispatchAction<S, A>(
  fiber: Fiber,
  queue: UpdateQueue<S, A>,
  action: A,
) {
  // ...省略无关代码 ...只保留性能优化部分代码:

  // 下面这个if判断, 能保证当前创建的update, 是`queue.pending`中第一个`update`. 为什么? 发起更新之后fiber.lanes会被改动(可以回顾`fiber 树构造(对比更新)`章节), 如果`fiber.lanes && alternate.lanes`没有被改动, 自然就是首个update
  if (
    fiber.lanes === NoLanes &&
    (alternate === null || alternate.lanes === NoLanes)
  ) {
    const lastRenderedReducer = queue.lastRenderedReducer;
    if (lastRenderedReducer !== null) {
      let prevDispatcher;
      const currentState: S = (queue.lastRenderedState: any);
      const eagerState = lastRenderedReducer(currentState, action);
      // 暂存`eagerReducer`和`eagerState`, 如果在render阶段reducer==update.eagerReducer, 则可以直接使用无需再次计算
      update.eagerReducer = lastRenderedReducer;
      update.eagerState = eagerState;
      if (is(eagerState, currentState)) {
        // 快速通道, eagerState与currentState相同, 无需调度更新
        // 注: update已经被添加到了queue.pending, 并没有丢弃. 之后需要更新的时候, 此update还是会起作用
        return;
      }
    }
  }
  // 发起调度更新, 进入`reconciler 运作流程`中的输入阶段.
  scheduleUpdateOnFiber(fiber, lane, eventTime);
}

function mountEffect(
  create: () => (() => void) | void,
  deps: Array<mixed> | void | null,
): void {
  return mountEffectImpl(
    UpdateEffect | PassiveEffect, // fiberFlags
    HookPassive, // hookFlags
    create,
    deps,
  );
}

function mountLayoutEffect(
  create: () => (() => void) | void,
  deps: Array<mixed> | void | null,
): void {
  return mountEffectImpl(
    UpdateEffect, // fiberFlags
    HookLayout, // hookFlags
    create,
    deps,
  );
}

function mountEffectImpl(fiberFlags, hookFlags, create, deps): void {
  // 1. 创建hook
  const hook = mountWorkInProgressHook();
  const nextDeps = deps === undefined ? null : deps;
  // 2. 设置workInProgress的副作用标记
  currentlyRenderingFiber.flags |= fiberFlags; // fiberFlags 被标记到workInProgress
  // 2. 创建Effect, 挂载到hook.memoizedState上
  hook.memoizedState = pushEffect(
    HookHasEffect | hookFlags, // hookFlags用于创建effect
    create,
    undefined,
    nextDeps,
  );
}

function pushEffect(tag, create, destroy, deps) {
  // 1. 创建effect对象
  const effect: Effect = {
    tag,
    create,
    destroy,
    deps,
    next: (null: any),
  };
  // 2. 把effect对象添加到环形链表末尾
  let componentUpdateQueue: null | FunctionComponentUpdateQueue =
    (currentlyRenderingFiber.updateQueue: any);
  if (componentUpdateQueue === null) {
    // 新建 workInProgress.updateQueue 用于挂载effect对象
    componentUpdateQueue = createFunctionComponentUpdateQueue();
    currentlyRenderingFiber.updateQueue = (componentUpdateQueue: any);
    // updateQueue.lastEffect是一个环形链表
    componentUpdateQueue.lastEffect = effect.next = effect;
  } else {
    const lastEffect = componentUpdateQueue.lastEffect;
    if (lastEffect === null) {
      componentUpdateQueue.lastEffect = effect.next = effect;
    } else {
      const firstEffect = lastEffect.next;
      lastEffect.next = effect;
      effect.next = firstEffect;
      componentUpdateQueue.lastEffect = effect;
    }
  }
  // 3. 返回effect
  return effect;
}

export type Effect = {|
  tag: HookFlags,
  create: () => (() => void) | void,
  destroy: (() => void) | void,
  deps: Array<mixed> | null,
  next: Effect,
|};

export const NoFlags = /*  */ 0b000;
export const HasEffect = /* */ 0b001; // 有副作用, 可以被触发
export const Layout = /*    */ 0b010; // Layout, dom突变后同步触发
export const Passive = /*   */ 0b100; // Passive, dom突变前异步触发

function commitBeforeMutationEffects() {
  while (nextEffect !== null) {
    // ...省略无关代码, 只保留Hook相关

    // 处理`Passive`标记
    const flags = nextEffect.flags;
    if ((flags & Passive) !== NoFlags) {
      if (!rootDoesHavePassiveEffects) {
        rootDoesHavePassiveEffects = true;
        scheduleCallback(NormalSchedulerPriority, () => {
          flushPassiveEffects();
          return null;
        });
      }
    }
    nextEffect = nextEffect.nextEffect;
  }
}

function commitMutationEffects(
  root: FiberRoot,
  renderPriorityLevel: ReactPriorityLevel,
) {
  // ...省略无关代码, 只保留Hook相关
  while (nextEffect !== null) {
    const flags = nextEffect.flags;
    const primaryFlags = flags & (Placement | Update | Deletion | Hydrating);
    switch (primaryFlags) {
      case Update: {
        // useEffect,useLayoutEffect都会设置Update标记
        // 更新节点
        const current = nextEffect.alternate;
        commitWork(current, nextEffect);
        break;
      }
    }
    nextEffect = nextEffect.nextEffect;
  }
}

function commitWork(current: Fiber | null, finishedWork: Fiber): void {
  // ...省略无关代码, 只保留Hook相关
  switch (finishedWork.tag) {
    case FunctionComponent:
    case ForwardRef:
    case MemoComponent:
    case SimpleMemoComponent:
    case Block: {
      // 在突变阶段调用销毁函数, 保证所有的effect.destroy函数都会在effect.create之前执行
      commitHookEffectListUnmount(HookLayout | HookHasEffect, finishedWork);
      return;
    }
  }
}

// 依次执行: effect.destroy
function commitHookEffectListUnmount(tag: number, finishedWork: Fiber) {
  const updateQueue: FunctionComponentUpdateQueue | null =
    (finishedWork.updateQueue: any);
  const lastEffect = updateQueue !== null ? updateQueue.lastEffect : null;
  if (lastEffect !== null) {
    const firstEffect = lastEffect.next;
    let effect = firstEffect;
    do {
      if ((effect.tag & tag) === tag) {
        // 根据传入的tag过滤 effect链表.
        const destroy = effect.destroy;
        effect.destroy = undefined;
        if (destroy !== undefined) {
          destroy();
        }
      }
      effect = effect.next;
    } while (effect !== firstEffect);
  }
}

function commitLayoutEffects(root: FiberRoot, committedLanes: Lanes) {
  // ...省略无关代码, 只保留Hook相关
  while (nextEffect !== null) {
    const flags = nextEffect.flags;
    if (flags & (Update | Callback)) {
      // useEffect,useLayoutEffect都会设置Update标记
      const current = nextEffect.alternate;
      commitLayoutEffectOnFiber(root, current, nextEffect, committedLanes);
    }
    nextEffect = nextEffect.nextEffect;
  }
}

function commitLifeCycles(
  finishedRoot: FiberRoot,
  current: Fiber | null,
  finishedWork: Fiber,
  committedLanes: Lanes,
): void {
  // ...省略无关代码, 只保留Hook相关
  switch (finishedWork.tag) {
    case FunctionComponent:
    case ForwardRef:
    case SimpleMemoComponent:
    case Block: {
      // 在此之前commitMutationEffects函数中, effect.destroy已经被调用, 所以effect.destroy永远不会影响到effect.create
      commitHookEffectListMount(HookLayout | HookHasEffect, finishedWork);

      schedulePassiveEffects(finishedWork);
      return;
    }
  }
}

function commitHookEffectListMount(tag: number, finishedWork: Fiber) {
  const updateQueue: FunctionComponentUpdateQueue | null =
    (finishedWork.updateQueue: any);
  const lastEffect = updateQueue !== null ? updateQueue.lastEffect : null;
  if (lastEffect !== null) {
    const firstEffect = lastEffect.next;
    let effect = firstEffect;
    do {
      if ((effect.tag & tag) === tag) {
        const create = effect.create;
        effect.destroy = create();
      }
      effect = effect.next;
    } while (effect !== firstEffect);
  }
}

function schedulePassiveEffects(finishedWork: Fiber) {
  // 1. 获取 fiber.updateQueue
  const updateQueue: FunctionComponentUpdateQueue | null =
    (finishedWork.updateQueue: any);
  // 2. 获取 effect环形队列
  const lastEffect = updateQueue !== null ? updateQueue.lastEffect : null;
  if (lastEffect !== null) {
    const firstEffect = lastEffect.next;
    let effect = firstEffect;
    do {
      const { next, tag } = effect;
      // 3. 筛选出由useEffect()创建的`effect`
      if (
        (tag & HookPassive) !== NoHookEffect &&
        (tag & HookHasEffect) !== NoHookEffect
      ) {
        // 把effect添加到全局数组, 等待`flushPassiveEffects`处理
        enqueuePendingPassiveHookEffectUnmount(finishedWork, effect);
        enqueuePendingPassiveHookEffectMount(finishedWork, effect);
      }
      effect = next;
    } while (effect !== firstEffect);
  }
}

export function enqueuePendingPassiveHookEffectUnmount(
  fiber: Fiber,
  effect: HookEffect,
): void {
  // unmount effects 数组
  pendingPassiveHookEffectsUnmount.push(effect, fiber);
}

export function enqueuePendingPassiveHookEffectMount(
  fiber: Fiber,
  effect: HookEffect,
): void {
  // mount effects 数组
  pendingPassiveHookEffectsMount.push(effect, fiber);
}

export function flushPassiveEffects(): boolean {
  // Returns whether passive effects were flushed.
  if (pendingPassiveEffectsRenderPriority !== NoSchedulerPriority) {
    const priorityLevel =
      pendingPassiveEffectsRenderPriority > NormalSchedulerPriority
        ? NormalSchedulerPriority
        : pendingPassiveEffectsRenderPriority;
    pendingPassiveEffectsRenderPriority = NoSchedulerPriority;
    // `runWithPriority`设置Schedule中的调度优先级, 如果在flushPassiveEffectsImpl中处理effect时又发起了新的更新, 那么新的update.lane将会受到这个priorityLevel影响.
    return runWithPriority(priorityLevel, flushPassiveEffectsImpl);
  }
  return false;
}

// ...省略无关代码, 只保留Hook相关
function flushPassiveEffectsImpl() {
  if (rootWithPendingPassiveEffects === null) {
    return false;
  }
  rootWithPendingPassiveEffects = null;
  pendingPassiveEffectsLanes = NoLanes;

  // 1. 执行 effect.destroy()
  const unmountEffects = pendingPassiveHookEffectsUnmount;
  pendingPassiveHookEffectsUnmount = [];
  for (let i = 0; i < unmountEffects.length; i += 2) {
    const effect = ((unmountEffects[i]: any): HookEffect);
    const fiber = ((unmountEffects[i + 1]: any): Fiber);
    const destroy = effect.destroy;
    effect.destroy = undefined;
    if (typeof destroy === 'function') {
      destroy();
    }
  }

  // 2. 执行新 effect.create(), 重新赋值到 effect.destroy
  const mountEffects = pendingPassiveHookEffectsMount;
  pendingPassiveHookEffectsMount = [];
  for (let i = 0; i < mountEffects.length; i += 2) {
    const effect = ((mountEffects[i]: any): HookEffect);
    const fiber = ((mountEffects[i + 1]: any): Fiber);
    effect.destroy = create();
  }
}

function updateEffectImpl(fiberFlags, hookFlags, create, deps): void {
  // 1. 获取当前hook
  const hook = updateWorkInProgressHook();
  const nextDeps = deps === undefined ? null : deps;
  let destroy = undefined;
  // 2. 分析依赖
  if (currentHook !== null) {
    const prevEffect = currentHook.memoizedState;
    // 继续使用先前effect.destroy
    destroy = prevEffect.destroy;
    if (nextDeps !== null) {
      const prevDeps = prevEffect.deps;
      // 比较依赖是否变化
      if (areHookInputsEqual(nextDeps, prevDeps)) {
        // 2.1 如果依赖不变, 新建effect(tag不含HookHasEffect)
        pushEffect(hookFlags, create, destroy, nextDeps);
        return;
      }
    }
  }
  // 2.2 如果依赖改变, 更改fiber.flag, 新建effect
  currentlyRenderingFiber.flags |= fiberFlags;

  hook.memoizedState = pushEffect(
    HookHasEffect | hookFlags,
    create,
    destroy,
    nextDeps,
  );
}

// ...省略无关代码
function commitMutationEffects(
  root: FiberRoot,
  renderPriorityLevel: ReactPriorityLevel,
) {
  while (nextEffect !== null) {
    const primaryFlags = flags & (Placement | Update | Deletion | Hydrating);
    switch (primaryFlags) {
      case Deletion: {
        commitDeletion(root, nextEffect, renderPriorityLevel);
        break;
      }
    }
  }
}

export function createContext<T>(
  defaultValue: T,
  calculateChangedBits: ?(a: T, b: T) => number,
): ReactContext<T> {
  if (calculateChangedBits === undefined) {
    calculateChangedBits = null;
  }
  const context: ReactContext<T> = {
    $typeof: REACT_CONTEXT_TYPE,
    _calculateChangedBits: calculateChangedBits,
    // As a workaround to support multiple concurrent renderers, we categorize
    // some renderers as primary and others as secondary. We only expect
    // there to be two concurrent renderers at most: React Native (primary) and
    // Fabric (secondary); React DOM (primary) and React ART (secondary).
    // Secondary renderers store their context values on separate fields.
    _currentValue: defaultValue,
    _currentValue2: defaultValue,
    _threadCount: 0,
    Provider: (null: any),
    Consumer: (null: any),
  };

  context.Provider = {
    $typeof: REACT_PROVIDER_TYPE,
    _context: context,
  };
  context.Consumer = context;
  return context;
}

function beginWork(
  current: Fiber | null,
  workInProgress: Fiber,
  renderLanes: Lanes,
): Fiber | null {
  const updateLanes = workInProgress.lanes;
  workInProgress.lanes = NoLanes;
  // ...省略无关代码
  switch (workInProgress.tag) {
    case ContextProvider:
      return updateContextProvider(current, workInProgress, renderLanes);
    case ContextConsumer:
      return updateContextConsumer(current, workInProgress, renderLanes);
  }
}

function updateContextProvider(
  current: Fiber | null,
  workInProgress: Fiber,
  renderLanes: Lanes,
) {
  // ...省略无关代码
  const providerType: ReactProviderType<any> = workInProgress.type;
  const context: ReactContext<any> = providerType._context;

  const newProps = workInProgress.pendingProps;
  const oldProps = workInProgress.memoizedProps;
  // 接收新value
  const newValue = newProps.value;

  // 更新 ContextProvider._currentValue
  pushProvider(workInProgress, newValue);

  if (oldProps !== null) {
    // ... 省略更新context的逻辑, 下文讨论
  }

  const newChildren = newProps.children;
  reconcileChildren(current, workInProgress, newChildren, renderLanes);
  return workInProgress.child;
}

// ...省略无关代码
export function pushProvider<T>(providerFiber: Fiber, nextValue: T): void {
  const context: ReactContext<T> = providerFiber.type._context;
  push(valueCursor, context._currentValue, providerFiber);
  context._currentValue = nextValue;
}

function updateContextConsumer(
  current: Fiber | null,
  workInProgress: Fiber,
  renderLanes: Lanes,
) {
  let context: ReactContext<any> = workInProgress.type;
  const newProps = workInProgress.pendingProps;
  const render = newProps.children;

  // 读取context
  prepareToReadContext(workInProgress, renderLanes);
  const newValue = readContext(context, newProps.unstable_observedBits);
  let newChildren;

  // ...省略无关代码
}

// ... 省略无关代码
export function prepareToReadContext(
  workInProgress: Fiber,
  renderLanes: Lanes,
): void {
  // 1. 设置全局变量, 为readContext做准备
  currentlyRenderingFiber = workInProgress;
  lastContextDependency = null;
  lastContextWithAllBitsObserved = null;

  const dependencies = workInProgress.dependencies;
  if (dependencies !== null) {
    const firstContext = dependencies.firstContext;
    if (firstContext !== null) {
      if (includesSomeLane(dependencies.lanes, renderLanes)) {
        // Context list has a pending update. Mark that this fiber performed work.
        markWorkInProgressReceivedUpdate();
      }
      // Reset the work-in-progress list
      dependencies.firstContext = null;
    }
  }
}
// ... 省略无关代码
export function readContext<T>(
  context: ReactContext<T>,
  observedBits: void | number | boolean,
): T {
  const contextItem = {
    context: ((context: any): ReactContext<mixed>),
    observedBits: resolvedObservedBits,
    next: null,
  };
  // 1. 构造一个contextItem, 加入到 workInProgress.dependencies链表之后
  if (lastContextDependency === null) {
    lastContextDependency = contextItem;
    currentlyRenderingFiber.dependencies = {
      lanes: NoLanes,
      firstContext: contextItem,
      responders: null,
    };
  } else {
    lastContextDependency = lastContextDependency.next = contextItem;
  }
  // 2. 返回 currentValue
  return isPrimaryRenderer ? context._currentValue : context._currentValue2;
}

function updateContextProvider(
  current: Fiber | null,
  workInProgress: Fiber,
  renderLanes: Lanes,
) {
  const providerType: ReactProviderType<any> = workInProgress.type;
  const context: ReactContext<any> = providerType._context;

  const newProps = workInProgress.pendingProps;
  const oldProps = workInProgress.memoizedProps;

  const newValue = newProps.value;

  pushProvider(workInProgress, newValue);

  if (oldProps !== null) {
    // 更新阶段进入
    const oldValue = oldProps.value;
    // 对比 newValue 和 oldValue
    const changedBits = calculateChangedBits(context, newValue, oldValue);
    if (changedBits === 0) {
      // value没有变动, 进入 Bailout 逻辑
      if (
        oldProps.children === newProps.children &&
        !hasLegacyContextChanged()
      ) {
        return bailoutOnAlreadyFinishedWork(
          current,
          workInProgress,
          renderLanes,
        );
      }
    } else {
      // value变动, 查找对应的consumers, 并使其能够被更新
      propagateContextChange(workInProgress, context, changedBits, renderLanes);
    }
  }
  // ... 省略无关代码
}

export function propagateContextChange(
  workInProgress: Fiber,
  context: ReactContext<mixed>,
  changedBits: number,
  renderLanes: Lanes,
): void {
  let fiber = workInProgress.child;
  if (fiber !== null) {
    // Set the return pointer of the child to the work-in-progress fiber.
    fiber.return = workInProgress;
  }
  while (fiber !== null) {
    let nextFiber;
    const list = fiber.dependencies;
    if (list !== null) {
      nextFiber = fiber.child;
      let dependency = list.firstContext;
      while (dependency !== null) {
        // 检查 dependency中依赖的context
        if (
          dependency.context === context &&
          (dependency.observedBits & changedBits) !== 0
        ) {
          // 符合条件, 安排调度
          if (fiber.tag === ClassComponent) {
            // class 组件需要创建一个update对象, 添加到updateQueue队列
            const update = createUpdate(
              NoTimestamp,
              pickArbitraryLane(renderLanes),
            );
            update.tag = ForceUpdate; // 注意ForceUpdate, 保证class组件一定执行render
            enqueueUpdate(fiber, update);
          }
          fiber.lanes = mergeLanes(fiber.lanes, renderLanes);
          const alternate = fiber.alternate;
          if (alternate !== null) {
            alternate.lanes = mergeLanes(alternate.lanes, renderLanes);
          }
          // 向上
          scheduleWorkOnParentPath(fiber.return, renderLanes);

          // 标记优先级
          list.lanes = mergeLanes(list.lanes, renderLanes);

          // 退出查找
          break;
        }
        dependency = dependency.next;
      }
    }

    // ...省略无关代码
    // ...省略无关代码

    fiber = nextFiber;
  }
}

export function scheduleWorkOnParentPath(
  parent: Fiber | null,
  renderLanes: Lanes,
) {
  // Update the child lanes of all the ancestors, including the alternates.
  let node = parent;
  while (node !== null) {
    const alternate = node.alternate;
    if (!isSubsetOfLanes(node.childLanes, renderLanes)) {
      node.childLanes = mergeLanes(node.childLanes, renderLanes);
      if (alternate !== null) {
        alternate.childLanes = mergeLanes(
          alternate.childLanes,
          renderLanes,
        );
      }
    } else if (
      alternate !== null &&
      !isSubsetOfLanes(alternate.childLanes, renderLanes)
    ) {
      alternate.childLanes = mergeLanes(alternate.childLanes, renderLanes);
    } else {
      // Neither alternate was updated, which means the rest of the
      // ancestor path already has sufficient priority.
      break;
    }
    node = node.return;
  }
}

function createRootImpl(
  container: Container,
  tag: RootTag,
  options: void | RootOptions,
) {
  // ... 省略无关代码
  if (enableEagerRootListeners) {
    const rootContainerElement =
      container.nodeType === COMMENT_NODE ? container.parentNode : container;
    listenToAllSupportedEvents(rootContainerElement);
  }
  // ... 省略无关代码
}

// ... 省略无关代码
export function listenToAllSupportedEvents(rootContainerElement: EventTarget) {
  if (enableEagerRootListeners) {
    // 1. 节流优化, 保证全局注册只被调用一次
    if ((rootContainerElement: any)[listeningMarker]) {
      return;
    }
    (rootContainerElement: any)[listeningMarker] = true;
    // 2. 遍历allNativeEvents 监听冒泡和捕获阶段的事件
    allNativeEvents.forEach((domEventName) => {
      if (!nonDelegatedEvents.has(domEventName)) {
        listenToNativeEvent(
          domEventName,
          false, // 冒泡阶段监听
          ((rootContainerElement: any): Element),
          null,
        );
      }
      listenToNativeEvent(
        domEventName,
        true, // 捕获阶段监听
        ((rootContainerElement: any): Element),
        null,
      );
    });
  }
}

// ... 省略无关代码
export function listenToNativeEvent(
  domEventName: DOMEventName,
  isCapturePhaseListener: boolean,
  rootContainerElement: EventTarget,
  targetElement: Element | null,
  eventSystemFlags?: EventSystemFlags = 0,
): void {
  let target = rootContainerElement;

  const listenerSet = getEventListenerSet(target);
  const listenerSetKey = getListenerSetKey(
    domEventName,
    isCapturePhaseListener,
  );
  // 利用set数据结构, 保证相同的事件类型只会被注册一次.
  if (!listenerSet.has(listenerSetKey)) {
    if (isCapturePhaseListener) {
      eventSystemFlags |= IS_CAPTURE_PHASE;
    }
    // 注册事件监听
    addTrappedEventListener(
      target,
      domEventName,
      eventSystemFlags,
      isCapturePhaseListener,
    );
    listenerSet.add(listenerSetKey);
  }
}

// ... 省略无关代码
function addTrappedEventListener(
  targetContainer: EventTarget,
  domEventName: DOMEventName,
  eventSystemFlags: EventSystemFlags,
  isCapturePhaseListener: boolean,
  isDeferredListenerForLegacyFBSupport?: boolean,
) {
  // 1. 构造listener
  let listener = createEventListenerWrapperWithPriority(
    targetContainer,
    domEventName,
    eventSystemFlags,
  );
  let unsubscribeListener;
  // 2. 注册事件监听
  if (isCapturePhaseListener) {
    unsubscribeListener = addEventCaptureListener(
      targetContainer,
      domEventName,
      listener,
    );
  } else {
    unsubscribeListener = addEventBubbleListener(
      targetContainer,
      domEventName,
      listener,
    );
  }
}

// 注册原生事件 冒泡
export function addEventBubbleListener(
  target: EventTarget,
  eventType: string,
  listener: Function,
): Function {
  target.addEventListener(eventType, listener, false);
  return listener;
}

// 注册原生事件 捕获
export function addEventCaptureListener(
  target: EventTarget,
  eventType: string,
  listener: Function,
): Function {
  target.addEventListener(eventType, listener, true);
  return listener;
}

export function createEventListenerWrapperWithPriority(
  targetContainer: EventTarget,
  domEventName: DOMEventName,
  eventSystemFlags: EventSystemFlags,
): Function {
  // 1. 根据优先级设置 listenerWrapper
  const eventPriority = getEventPriorityForPluginSystem(domEventName);
  let listenerWrapper;
  switch (eventPriority) {
    case DiscreteEvent:
      listenerWrapper = dispatchDiscreteEvent;
      break;
    case UserBlockingEvent:
      listenerWrapper = dispatchUserBlockingUpdate;
      break;
    case ContinuousEvent:
    default:
      listenerWrapper = dispatchEvent;
      break;
  }
  // 2. 返回 listenerWrapper
  return listenerWrapper.bind(
    null,
    domEventName,
    eventSystemFlags,
    targetContainer,
  );
}

// ...省略无关代码
export function dispatchEvent(
  domEventName: DOMEventName,
  eventSystemFlags: EventSystemFlags,
  targetContainer: EventTarget,
  nativeEvent: AnyNativeEvent,
): void {
  if (!_enabled) {
    return;
  }
  const blockedOn = attemptToDispatchEvent(
    domEventName,
    eventSystemFlags,
    targetContainer,
    nativeEvent,
  );
}

export function attemptToDispatchEvent(
  domEventName: DOMEventName,
  eventSystemFlags: EventSystemFlags,
  targetContainer: EventTarget,
  nativeEvent: AnyNativeEvent,
): null | Container | SuspenseInstance {
  // ...省略无关代码

  // 1. 定位原生DOM节点
  const nativeEventTarget = getEventTarget(nativeEvent);
  // 2. 获取与DOM节点对应的fiber节点
  let targetInst = getClosestInstanceFromNode(nativeEventTarget);
  // 3. 通过插件系统, 派发事件
  dispatchEventForPluginEventSystem(
    domEventName,
    eventSystemFlags,
    nativeEvent,
    targetInst,
    targetContainer,
  );
  return null;
}

function extractEvents(
  dispatchQueue: DispatchQueue,
  domEventName: DOMEventName,
  targetInst: null | Fiber,
  nativeEvent: AnyNativeEvent,
  nativeEventTarget: null | EventTarget,
  eventSystemFlags: EventSystemFlags,
  targetContainer: EventTarget,
): void {
  const reactName = topLevelEventsToReactNames.get(domEventName);
  if (reactName === undefined) {
    return;
  }
  let SyntheticEventCtor = SyntheticEvent;
  let reactEventType: string = domEventName;

  const inCapturePhase = (eventSystemFlags & IS_CAPTURE_PHASE) !== 0;
  const accumulateTargetOnly = !inCapturePhase && domEventName === 'scroll';
  // 1. 收集所有监听该事件的函数.
  const listeners = accumulateSinglePhaseListeners(
    targetInst,
    reactName,
    nativeEvent.type,
    inCapturePhase,
    accumulateTargetOnly,
  );
  if (listeners.length > 0) {
    // 2. 构造合成事件, 添加到派发队列
    const event = new SyntheticEventCtor(
      reactName,
      reactEventType,
      null,
      nativeEvent,
      nativeEventTarget,
    );
    dispatchQueue.push({ event, listeners });
  }
}

export function accumulateSinglePhaseListeners(
  targetFiber: Fiber | null,
  reactName: string | null,
  nativeEventType: string,
  inCapturePhase: boolean,
  accumulateTargetOnly: boolean,
): Array<DispatchListener> {
  const captureName = reactName !== null ? reactName + 'Capture' : null;
  const reactEventName = inCapturePhase ? captureName : reactName;
  const listeners: Array<DispatchListener> = [];

  let instance = targetFiber;
  let lastHostComponent = null;

  // 从targetFiber开始, 向上遍历, 直到 root 为止
  while (instance !== null) {
    const { stateNode, tag } = instance;
    // 当节点类型是HostComponent时(如: div, span, button等类型)
    if (tag === HostComponent && stateNode !== null) {
      lastHostComponent = stateNode;
      if (reactEventName !== null) {
        // 获取标准的监听函数 (如onClick , onClickCapture等)
        const listener = getListener(instance, reactEventName);
        if (listener != null) {
          listeners.push(
            createDispatchListener(instance, listener, lastHostComponent),
          );
        }
      }
    }
    // 如果只收集目标节点, 则不用向上遍历, 直接退出
    if (accumulateTargetOnly) {
      break;
    }
    instance = instance.return;
  }
  return listeners;
}

export function processDispatchQueue(
  dispatchQueue: DispatchQueue,
  eventSystemFlags: EventSystemFlags,
): void {
  const inCapturePhase = (eventSystemFlags & IS_CAPTURE_PHASE) !== 0;
  for (let i = 0; i < dispatchQueue.length; i++) {
    const { event, listeners } = dispatchQueue[i];
    processDispatchQueueItemsInOrder(event, listeners, inCapturePhase);
  }
  // ...省略无关代码
}

function processDispatchQueueItemsInOrder(
  event: ReactSyntheticEvent,
  dispatchListeners: Array<DispatchListener>,
  inCapturePhase: boolean,
): void {
  let previousInstance;
  if (inCapturePhase) {
    // 1. capture事件: 倒序遍历listeners
    for (let i = dispatchListeners.length - 1; i >= 0; i--) {
      const { instance, currentTarget, listener } = dispatchListeners[i];
      if (instance !== previousInstance && event.isPropagationStopped()) {
        return;
      }
      executeDispatch(event, listener, currentTarget);
      previousInstance = instance;
    }
  } else {
    // 2. bubble事件: 顺序遍历listeners
    for (let i = 0; i < dispatchListeners.length; i++) {
      const { instance, currentTarget, listener } = dispatchListeners[i];
      if (instance !== previousInstance && event.isPropagationStopped()) {
        return;
      }
      executeDispatch(event, listener, currentTarget);
      previousInstance = instance;
    }
  }
}

// 只保留主干逻辑
function reconcileSingleElement(
  returnFiber: Fiber,
  currentFirstChild: Fiber | null,
  element: ReactElement,
  lanes: Lanes,
): Fiber {
  const key = element.key;
  let child = currentFirstChild;

  while (child !== null) {
    // currentFirstChild !== null, 表明是对比更新阶段
    if (child.key === key) {
      // 1. key相同, 进一步判断 child.elementType === element.type
      switch (child.tag) {
        // 只看核心逻辑
        default: {
          if (child.elementType === element.type) {
            // 1.1 已经匹配上了, 如果有兄弟节点, 需要给兄弟节点打上Deletion标记
            deleteRemainingChildren(returnFiber, child.sibling);
            // 1.2 构造fiber节点, 新的fiber对象会复用current.stateNode, 即可复用DOM对象
            const existing = useFiber(child, element.props);
            existing.ref = coerceRef(returnFiber, child, element);
            existing.return = returnFiber;
            return existing;
          }
          break;
        }
      }
      // Didn't match. 给当前节点点打上Deletion标记
      deleteRemainingChildren(returnFiber, child);
      break;
    } else {
      // 2. key不相同, 匹配失败, 给当前节点打上Deletion标记
      deleteChild(returnFiber, child);
    }
    child = child.sibling;
  }

  {
    // ...省略部分代码, 只看核心逻辑
  }

  // 新建节点
  const created = createFiberFromElement(element, returnFiber.mode, lanes);
  created.ref = coerceRef(returnFiber, currentFirstChild, element);
  created.return = returnFiber;
  return created;
}

function reconcileChildFibers(
  returnFiber: Fiber,
  currentFirstChild: Fiber | null,
  newChild: any,
  lanes: Lanes,
): Fiber | null {
  if (isArray(newChild)) {
    return reconcileChildrenArray(
      returnFiber,
      currentFirstChild,
      newChild,
      lanes,
    );
  }
  if (getIteratorFn(newChild)) {
    return reconcileChildrenIterator(
      returnFiber,
      currentFirstChild,
      newChild,
      lanes,
    );
  }
}

function reconcileChildrenArray(
  returnFiber: Fiber,
  currentFirstChild: Fiber | null,
  newChildren: Array<*>,
  lanes: Lanes,
): Fiber | null {
  let resultingFirstChild: Fiber | null = null;
  let previousNewFiber: Fiber | null = null;

  let oldFiber = currentFirstChild;
  let lastPlacedIndex = 0;
  let newIdx = 0;
  let nextOldFiber = null;
  // 1. 第一次循环: 遍历最长公共序列(key相同), 公共序列的节点都视为可复用
  for (; oldFiber !== null && newIdx < newChildren.length; newIdx++) {
    // 后文分析
  }

  if (newIdx === newChildren.length) {
    // 如果newChildren序列被遍历完, 那么oldFiber序列中剩余节点都视为删除(打上Deletion标记)
    deleteRemainingChildren(returnFiber, oldFiber);
    return resultingFirstChild;
  }

  if (oldFiber === null) {
    // 如果oldFiber序列被遍历完, 那么newChildren序列中剩余节点都视为新增(打上Placement标记)
    for (; newIdx < newChildren.length; newIdx++) {
      // 后文分析
    }
    return resultingFirstChild;
  }

  // ==================分割线==================
  const existingChildren = mapRemainingChildren(returnFiber, oldFiber);

  // 2. 第二次循环: 遍历剩余非公共序列, 优先复用oldFiber序列中的节点
  for (; newIdx < newChildren.length; newIdx++) {}

  if (shouldTrackSideEffects) {
    // newChildren已经遍历完, 那么oldFiber序列中剩余节点都视为删除(打上Deletion标记)
    existingChildren.forEach((child) => deleteChild(returnFiber, child));
  }

  return resultingFirstChild;
}

// 1. 第一次循环: 遍历最长公共序列(key相同), 公共序列的节点都视为可复用
for (; oldFiber !== null && newIdx < newChildren.length; newIdx++) {
  if (oldFiber.index > newIdx) {
    nextOldFiber = oldFiber;
    oldFiber = null;
  } else {
    nextOldFiber = oldFiber.sibling;
  }
  // new槽位和old槽位进行比较, 如果key不同, 返回null
  // key相同, 比较type是否一致. type一致则执行useFiber(update逻辑), type不一致则运行createXXX(insert逻辑)
  const newFiber = updateSlot(
    returnFiber,
    oldFiber,
    newChildren[newIdx],
    lanes,
  );

  if (newFiber === null) {
    // 如果返回null, 表明key不同. 无法满足公共序列条件, 退出循环
    if (oldFiber === null) {
      oldFiber = nextOldFiber;
    }
    break;
  }
  if (shouldTrackSideEffects) {
    // 若是新增节点, 则给老节点打上Deletion标记
    if (oldFiber && newFiber.alternate === null) {
      deleteChild(returnFiber, oldFiber);
    }
  }

  // lastPlacedIndex 记录被移动的节点索引
  // 如果当前节点可复用, 则要判断位置是否移动.
  lastPlacedIndex = placeChild(newFiber, lastPlacedIndex, newIdx);

  // 更新resultingFirstChild结果序列
  if (previousNewFiber === null) {
    resultingFirstChild = newFiber;
  } else {
    previousNewFiber.sibling = newFiber;
  }
  previousNewFiber = newFiber;
  oldFiber = nextOldFiber;
}

// 1. 将第一次循环后, oldFiber剩余序列加入到一个map中. 目的是为了第二次循环能顺利的找到可复用节点
const existingChildren = mapRemainingChildren(returnFiber, oldFiber);

// 2. 第二次循环: 遍历剩余非公共序列, 优先复用oldFiber序列中的节点
for (; newIdx < newChildren.length; newIdx++) {
  const newFiber = updateFromMap(
    existingChildren,
    returnFiber,
    newIdx,
    newChildren[newIdx],
    lanes,
  );
  if (newFiber !== null) {
    if (shouldTrackSideEffects) {
      if (newFiber.alternate !== null) {
        // 如果newFiber是通过复用创建的, 则清理map中对应的老节点
        existingChildren.delete(newFiber.key === null ? newIdx : newFiber.key);
      }
    }
    lastPlacedIndex = placeChild(newFiber, lastPlacedIndex, newIdx);
    // 更新resultingFirstChild结果序列
    if (previousNewFiber === null) {
      resultingFirstChild = newFiber;
    } else {
      previousNewFiber.sibling = newFiber;
    }
    previousNewFiber = newFiber;
  }
}
// 3. 善后工作, 第二次循环完成之后, existingChildren中剩余的fiber节点就是将要被删除的节点, 打上Deletion标记
if (shouldTrackSideEffects) {
  existingChildren.forEach((child) => deleteChild(returnFiber, child));
}

The production A : A @ B, where @ is one of the bitwise operators in the productions above, is evaluated as follows:
1. Let lref be the result of evaluating A.
2. Let lval be GetValue(lref).
3. Let rref be the result of evaluating B.
4. Let rval be GetValue(rref).
5. Let lnum be ToInt32(lval).
6. Let rnum be ToInt32(rval).
7. Return the result of applying the bitwise operator @ to lnum and rnum. The result is a signed 32 bit integer.

Before: 11100110111110100000000000000110000000000001
After:              10100000000000000110000000000001

'str' >>> 0; //  ===> Number('str') >>> 0  ===> NaN >>> 0 = 0

const A = 1 << 0; // 0b00000001
const B = 1 << 1; // 0b00000010
const C = 1 << 2; // 0b00000100

const A = 1 << 0; // 0b00000001
const B = 1 << 1; // 0b00000010
const C = 1 << 2; // 0b00000100

// 增加属性
const ABC = A | B | C; // 0b00000111
// 删除属性
const AB = ABC & ~C; // 0b00000011

// 属性比较
// 1. AB当中包含B
console.log((AB & B) === B); // true
// 2. AB当中不包含C
console.log((AB & C) === 0); // true
// 3. A和B相等
console.log(A === B); // false

//类型定义
export opaque type Lanes = number;
export opaque type Lane = number;

// 变量定义
export const NoLanes: Lanes = /*                        */ 0b0000000000000000000000000000000;
export const NoLane: Lane = /*                          */ 0b0000000000000000000000000000000;

export const SyncLane: Lane = /*                        */ 0b0000000000000000000000000000001;
export const SyncBatchedLane: Lane = /*                 */ 0b0000000000000000000000000000010;

export const InputDiscreteHydrationLane: Lane = /*      */ 0b0000000000000000000000000000100;
const InputDiscreteLanes: Lanes = /*                    */ 0b0000000000000000000000000011000;

const InputContinuousHydrationLane: Lane = /*           */ 0b0000000000000000000000000100000;
const InputContinuousLanes: Lanes = /*                  */ 0b0000000000000000000000011000000;
// ...
// ...

const NonIdleLanes = /*                                 */ 0b0000111111111111111111111111111;

export const IdleHydrationLane: Lane = /*               */ 0b0001000000000000000000000000000;
const IdleLanes: Lanes = /*                             */ 0b0110000000000000000000000000000;

export const OffscreenLane: Lane = /*                   */ 0b1000000000000000000000000000000;

function getHighestPriorityLanes(lanes: Lanes | Lane): Lanes {
  // 判断 lanes中是否包含 SyncLane
  if ((SyncLane & lanes) !== NoLanes) {
    return_highestLanePriority = SyncLanePriority;
    return SyncLane;
  }
  // 判断 lanes中是否包含 SyncBatchedLane
  if ((SyncBatchedLane & lanes) !== NoLanes) {
    return_highestLanePriority = SyncBatchedLanePriority;
    return SyncBatchedLane;
  }
  // ...
  // ... 省略其他代码
  return lanes;
}

function getHighestPriorityLane(lanes: Lanes) {
  return lanes & -lanes;
}

function getLowestPriorityLane(lanes: Lanes): Lane {
  // This finds the most significant non-zero bit.
  const index = 31 - clz32(lanes);
  return index < 0 ? NoLanes : 1 << index;
}

export const NoContext = /*             */ 0b0000000;
const BatchedContext = /*               */ 0b0000001;
const EventContext = /*                 */ 0b0000010;
const DiscreteEventContext = /*         */ 0b0000100;
const LegacyUnbatchedContext = /*       */ 0b0001000;
const RenderContext = /*                */ 0b0010000;
const CommitContext = /*                */ 0b0100000;
export const RetryAfterError = /*       */ 0b1000000;

// ...

// Describes where we are in the React execution stack
let executionContext: ExecutionContext = NoContext;

// scheduleUpdateOnFiber函数中包含了好多关于executionContext的判断(都是使用位运算)
export function scheduleUpdateOnFiber(
  fiber: Fiber,
  lane: Lane,
  eventTime: number,
) {
  if (root === workInProgressRoot) {
    // 判断: executionContext 不包含 RenderContext
    if (
      deferRenderPhaseUpdateToNextBatch ||
      (executionContext & RenderContext) === NoContext
    ) {
      // ...
    }
  }
  if (lane === SyncLane) {
    if (
      // 判断: executionContext 包含 LegacyUnbatchedContext
      (executionContext & LegacyUnbatchedContext) !== NoContext &&
      // 判断: executionContext 不包含 RenderContext或CommitContext
      (executionContext & (RenderContext | CommitContext)) === NoContext
    ) {
      // ...
    }
  }
  // ...
}

function Node() {
  this.name = '';
  this.children = [];
}

function dfs(node) {
  console.log('探寻阶段: ', node.name);
  node.children.forEach((child) => {
    dfs(child);
  });
  console.log('回溯阶段: ', node.name);
}

function Node() {
  this.name = '';
  this.children = [];

  // 因为要分辨探寻阶段和回溯阶段, 所以必须要一个属性来记录是否已经访问过该节点
  // 如果不打印探寻和回溯, 就不需要此属性
  this.visited = false;
}

function dfs(node) {
  const stack = [];
  stack.push(node);
  // 栈顶元素还存在, 就继续循环
  while ((node = stack[stack.length - 1])) {
    if (node.visited) {
      console.log('回溯阶段: ', node.name);
      // 回溯完成, 弹出该元素
      stack.pop();
    } else {
      console.log('探寻阶段: ', node.name);
      node.visited = true;
      // 利用栈的先进后出的特性, 倒序将节点送入栈中
      for (let i = node.children.length - 1; i >= 0; i--) {
        stack.push(node.children[i]);
      }
    }
  }
}

function workLoopSync() {
  // 1. 最外层循环, 保证每一个节点都能遍历, 不会遗漏
  while (workInProgress !== null) {
    performUnitOfWork(workInProgress);
  }
}

function performUnitOfWork(unitOfWork: Fiber): void {
  const current = unitOfWork.alternate;
  let next;
  // 2. beginWork是向下探寻阶段
  next = beginWork(current, unitOfWork, subtreeRenderLanes);
  if (next === null) {
    // 3. completeUnitOfWork 是回溯阶段
    completeUnitOfWork(unitOfWork);
  } else {
    workInProgress = next;
  }
}

function completeUnitOfWork(unitOfWork: Fiber): void {
  let completedWork = unitOfWork;
  do {
    const current = completedWork.alternate;
    const returnFiber = completedWork.return;
    let next;
    // 3.1 回溯并处理节点
    next = completeWork(current, completedWork, subtreeRenderLanes);
    if (next !== null) {
      // 判断在处理节点的过程中, 是否派生出新的节点
      workInProgress = next;
      return;
    }
    const siblingFiber = completedWork.sibling;
    // 3.2 判断是否有旁支
    if (siblingFiber !== null) {
      workInProgress = siblingFiber;
      return;
    }
    // 3.3 没有旁支 继续回溯
    completedWork = returnFiber;
    workInProgress = completedWork;
  } while (completedWork !== null);
}

class App extends React.Component {
  render() {
    return (
      <div className="app">
        <header>header</header>
        <Content />
        <footer>footer</footer>
      </div>
    );
  }
}

class Content extends React.Component {
  render() {
    return (
      <React.Fragment>
        <p>1</p>
        <p>2</p>
        <p>3</p>
      </React.Fragment>
    );
  }
}

export default App;

export function propagateContextChange(
  workInProgress: Fiber,
  context: ReactContext<mixed>,
  changedBits: number,
  renderLanes: Lanes,
): void {
  let fiber = workInProgress.child;
  while (fiber !== null) {
    let nextFiber;
    // Visit this fiber.
    const list = fiber.dependencies;
    if (list !== null) {
      // 匹配context等逻辑, 和dfs无关, 此处可以暂时忽略
      // ...
    } else {
      // 向下探寻
      nextFiber = fiber.child;
    }
    fiber = nextFiber;
  }
}

const minHeapSort = (arr) => {
  // 1. 构造最小堆
  buildMinHeap(arr);
  // 2. 循环提取根节点arr[0], 直到全部提取完
  for (let i = arr.length - 1; i > 0; i--) {
    let tmp = arr[0];
    arr[0] = arr[i];
    arr[i] = tmp;
    siftDown(arr, 0, i - 1);
  }
};

// 把整个数组构造成最小堆
const buildMinHeap = (arr) => {
  if (arr.length < 2) {
    return arr;
  }
  const startIndex = Math.floor(arr.length / 2 - 1);
  for (let i = startIndex; i >= 0; i--) {
    siftDown(arr, i, arr.length - 1);
  }
};

// 从startIndex索引开始, 向下调整最小堆
const siftDown = (arr, startIndex, endIndex) => {
  const leftChildIndx = 2 * startIndex + 1;
  const rightChildIndx = 2 * startIndex + 2;
  let swapIndex = startIndex;
  let tmpNode = arr[startIndex];
  if (leftChildIndx <= endIndex) {
    if (arr[leftChildIndx] < tmpNode) {
      // 待定是否交换, 因为right子节点有可能更小
      tmpNode = arr[leftChildIndx];
      swapIndex = leftChildIndx;
    }
  }
  if (rightChildIndx <= endIndex) {
    if (arr[rightChildIndx] < tmpNode) {
      // 比left节点更小, 替换swapIndex
      tmpNode = arr[rightChildIndx];
      swapIndex = rightChildIndx;
    }
  }
  if (swapIndex !== startIndex) {
    // 1.交换节点
    arr[swapIndex] = arr[startIndex];
    arr[startIndex] = tmpNode;

    // 2. 递归调用, 继续向下调整
    siftDown(arr, swapIndex, endIndex);
  }
};

var arr1 = [5, 8, 0, 10, 4, 6, 1];
minHeapSort(arr1);
console.log(arr1); // [10, 8, 6, 5,4, 1, 0]

var arr2 = [5];
minHeapSort(arr2);
console.log(arr2); // [ 5 ]

var arr3 = [5, 1];
minHeapSort(arr3);
console.log(arr3); //[ 5, 1 ]

type Heap = Array<Node>;
type Node = {|
  id: number,
  sortIndex: number,
|};

// 添加新节点, 添加之后, 需要调用`siftUp`函数向上调整堆.
export function push(heap: Heap, node: Node): void {
  const index = heap.length;
  heap.push(node);
  siftUp(heap, node, index);
}

// 查看堆的顶点, 也就是优先级最高的`task`或`timer`
export function peek(heap: Heap): Node | null {
  const first = heap[0];
  return first === undefined ? null : first;
}

// 将堆的顶点提取出来, 并删除顶点之后, 需要调用`siftDown`函数向下调整堆.
export function pop(heap: Heap): Node | null {
  const first = heap[0];
  if (first !== undefined) {
    const last = heap.pop();
    if (last !== first) {
      heap[0] = last;
      siftDown(heap, last, 0);
    }
    return first;
  } else {
    return null;
  }
}

// 当插入节点之后, 需要向上调整堆结构, 保证数组是一个最小堆.
function siftUp(heap, node, i) {
  let index = i;
  while (true) {
    const parentIndex = (index - 1) >>> 1;
    const parent = heap[parentIndex];
    if (parent !== undefined && compare(parent, node) > 0) {
      // The parent is larger. Swap positions.
      heap[parentIndex] = node;
      heap[index] = parent;
      index = parentIndex;
    } else {
      // The parent is smaller. Exit.
      return;
    }
  }
}

// 向下调整堆结构, 保证数组是一个最小堆.
function siftDown(heap, node, i) {
  let index = i;
  const length = heap.length;
  while (index < length) {
    const leftIndex = (index + 1) * 2 - 1;
    const left = heap[leftIndex];
    const rightIndex = leftIndex + 1;
    const right = heap[rightIndex];

    // If the left or right node is smaller, swap with the smaller of those.
    if (left !== undefined && compare(left, node) < 0) {
      if (right !== undefined && compare(right, left) < 0) {
        heap[index] = right;
        heap[rightIndex] = node;
        index = rightIndex;
      } else {
        heap[index] = left;
        heap[leftIndex] = node;
        index = leftIndex;
      }
    } else if (right !== undefined && compare(right, node) < 0) {
      heap[index] = right;
      heap[rightIndex] = node;
      index = rightIndex;
    } else {
      // Neither child is smaller. Exit.
      return;
    }
  }
}

// 定义Node节点类型
function Node(name) {
  this.name = name;
  this.next = null;
}

// 链表
function LinkedList() {
  this.head = new Node('head');

  // 查找node节点的前一个节点
  this.findPrevious = function (node) {
    let currentNode = this.head;
    while (currentNode && currentNode.next !== node) {
      currentNode = currentNode.next;
    }
    return currentNode;
  };

  // 在node后插入新节点newElement
  this.insert = function (name, node) {
    const newNode = new Node(name);
    newNode.next = node.next;
    node.next = newNode;
  };

  // 删除节点
  this.remove = function (node) {
    const previousNode = this.findPrevious(node);
    if (previousNode) {
      previousNode.next = node.next;
    }
  };

  // 反转链表
  this.reverse = function () {
    let prev = null;
    let current = this.head;
    while (current) {
      const tempNode = current.next;
      // 重新设置next指针, 使其指向前一个节点
      current.next = prev;
      // 游标后移
      prev = current;
      current = tempNode;
    }
    // 重新设置head节点
    this.head = prev;
  };
}

export function enqueueUpdate<State>(fiber: Fiber, update: Update<State>) {
  const updateQueue = fiber.updateQueue;
  // ...
  const sharedQueue: SharedQueue<State> = (updateQueue: any).shared;
  // 将新的update对象添加到fiber.updateQueue.shared.pending链表上
  const pending = sharedQueue.pending;
  if (pending === null) {
    update.next = update;
  } else {
    update.next = pending.next;
    pending.next = update;
  }
  sharedQueue.pending = update;
}

export function processUpdateQueue<State>(
  workInProgress: Fiber,
  props: any,
  instance: any,
  renderLanes: Lanes,
): void {
  // This is always non-null on a ClassComponent or HostRoot
  const queue: UpdateQueue<State> = (workInProgress.updateQueue: any);
  let firstBaseUpdate = queue.firstBaseUpdate;
  let lastBaseUpdate = queue.lastBaseUpdate;
  // Check if there are pending updates. If so, transfer them to the base queue.
  let pendingQueue = queue.shared.pending;
  if (pendingQueue !== null) {
    queue.shared.pending = null;
    // The pending queue is circular. Disconnect the pointer between first
    // and last so that it's non-circular.
    const lastPendingUpdate = pendingQueue;
    const firstPendingUpdate = lastPendingUpdate.next;
    lastPendingUpdate.next = null;
    // Append pending updates to base queue
    if (lastBaseUpdate === null) {
      firstBaseUpdate = firstPendingUpdate;
    } else {
      lastBaseUpdate.next = firstPendingUpdate;
    }
    lastBaseUpdate = lastPendingUpdate;
  }
}

function dispatchAction<S, A>(
  fiber: Fiber,
  queue: UpdateQueue<S, A>,
  action: A,
) {
  // ... 省略部分代码
  const pending = queue.pending;
  if (pending === null) {
    // This is the first update. Create a circular list.
    update.next = update;
  } else {
    update.next = pending.next;
    pending.next = update;
  }
  queue.pending = update;
}

function updateReducer<S, I, A>(
  reducer: (S, A) => S,
  initialArg: I,
  init?: (I) => S,
): [S, Dispatch<A>] {
  // ... 省略部分代码
  if (pendingQueue !== null) {
    if (baseQueue !== null) {
      // 在这里进行队列的合并
      const baseFirst = baseQueue.next;
      const pendingFirst = pendingQueue.next;
      baseQueue.next = pendingFirst;
      pendingQueue.next = baseFirst;
    }
    current.baseQueue = baseQueue = pendingQueue;
    queue.pending = null;
  }
}

class Stack {
  constructor() {
    this.dataStore = [];
    this.top = 0;
  }

  // 压栈
  push(element) {
    this.dataStore[this.top++] = element;
  }

  // 弹栈
  pop() {
    return this.dataStore[--this.top];
  }

  // 预览栈顶元素
  peek() {
    return this.dataStore[this.top - 1];
  }

  // 检测栈内存储了多少个元素
  length() {
    return this.top;
  }

  // 清空栈
  clear() {
    this.top = 0;
  }
}

const test = () => {
  const stack = new Stack();
  console.log('压栈a: ');
  stack.push('a');
  console.log('压栈b: ');
  stack.push('b');
  console.log('压栈c: ');
  stack.push('c');
  console.log('栈高度: ', stack.length());
  console.log('栈顶元素: ', stack.peek());
  console.log('弹出: ', stack.pop());
  console.log('栈顶元素: ', stack.peek());
  console.log('压栈d: ');
  stack.push('d');
  console.log('栈顶元素: ', stack.peek());
  console.log('清空栈: ');
  stack.clear();
  console.log('栈高度: ', stack.length());
  console.log('压栈e: ');
  stack.push('e');
  console.log('栈顶元素: ', stack.peek());
};

export type StackCursor<T> = {| current: T |};

// 维护一个全局stack
const valueStack: Array<any> = [];
let index = -1;

// 一个工厂函数, 创建StackCursor对象
function createCursor<T>(defaultValue: T): StackCursor<T> {
  return {
    current: defaultValue,
  };
}
function isEmpty(): boolean {
  return index === -1;
}
// 出栈
function pop<T>(cursor: StackCursor<T>, fiber: Fiber): void {
  if (index < 0) {
    return;
  }
  cursor.current = valueStack[index];
  valueStack[index] = null;
  index--;
}
// 入栈
function push<T>(cursor: StackCursor<T>, value: T, fiber: Fiber): void {
  index++;
  // 注意: 这里存储的是 cursor当前值, 随后更新了cursor.current为
  valueStack[index] = cursor.current;
  cursor.current = value;
}

// 定义全局 valueCursor, 用于管理<Context.Provider/>组件的value
const valueCursor: StackCursor<mixed> = createCursor(null);

// ...省略无关代码

// 将context当前的值保存到valueCursor中, 并设置context._currentValue为最新值
// 运行完成之后context为最新状态
export function pushProvider<T>(providerFiber: Fiber, nextValue: T): void {
  const context: ReactContext<T> = providerFiber.type._context;
  push(valueCursor, context._currentValue, providerFiber);
  context._currentValue = nextValue;
}

// 取出valueCursor中保存的旧值, 设置到context._currentValue上.
// 运行完成之后context恢复到上一个状态
export function popProvider(providerFiber: Fiber): void {
  const currentValue = valueCursor.current;
  pop(valueCursor, providerFiber);
  const context: ReactContext<any> = providerFiber.type._context;
  context._currentValue = currentValue;
}

const MyContext = React.createContext(0);

export default function App() {
  return (
    // 第一级
    <MyContext.Provider value={1}>
      <MyContext.Consumer>
        {(value1) => (
          //第二级嵌套
          <MyContext.Provider value={2}>
            <MyContext.Consumer>
              {(value2) => (
                // 第三级嵌套
                <MyContext.Provider value={3}>
                  <MyContext.Consumer>
                    {(value3) => (
                      <span>
                        {value1}-{value2}-{value3}
                      </span>
                    )}
                  </MyContext.Consumer>
                </MyContext.Provider>
              )}
            </MyContext.Consumer>
          </MyContext.Provider>
        )}
      </MyContext.Consumer>
    </MyContext.Provider>
  );
}

export function batchedUpdates<A, R>(fn: (A) => R, a: A): R {
  // 在执行回调之前, 先改变 executionContext
  const prevExecutionContext = executionContext;
  executionContext |= BatchedContext;
  try {
    return fn(a);
  } finally {
    // 回调执行完毕之后, 再恢复到以前的值 prevExecutionContext
    executionContext = prevExecutionContext;
    // ... 省略无关代码
  }
}

export function unbatchedUpdates<A, R>(fn: (a: A) => R, a: A): R {
  const prevExecutionContext = executionContext;
  executionContext &= ~BatchedContext;
  executionContext |= LegacyUnbatchedContext;
  try {
    return fn(a);
  } finally {
    executionContext = prevExecutionContext;
    // ... 省略无关代码
  }
}

// ... 省略其他函数

export function scheduleUpdateOnFiber(
  fiber: Fiber,
  expirationTime: ExpirationTime,
) {
  const priorityLevel = getCurrentPriorityLevel();

  if (expirationTime === Sync) {
    if (
      // Check if we're inside unbatchedUpdates
      (executionContext & LegacyUnbatchedContext) !== NoContext &&
      // Check if we're not already rendering
      (executionContext & (RenderContext | CommitContext)) === NoContext
    ) {
      performSyncWorkOnRoot(root);
    } else {
      ensureRootIsScheduled(root);
      schedulePendingInteractions(root, expirationTime);
      if (executionContext === NoContext) {
        // Flush the synchronous work now, unless we're already working or inside
        // a batch. This is intentionally inside scheduleUpdateOnFiber instead of
        // scheduleCallbackForFiber to preserve the ability to schedule a callback
        // without immediately flushing it. We only do this for user-initiated
        // updates, to preserve historical behavior of legacy mode.
        flushSyncCallbackQueue();
      }
    }
  } else {
    // Schedule a discrete update but only if it's not Sync.
    if (
      (executionContext & DiscreteEventContext) !== NoContext &&
      // Only updates at user-blocking priority or greater are considered
      // discrete, even inside a discrete event.
      (priorityLevel === UserBlockingPriority ||
        priorityLevel === ImmediatePriority)
    ) {
      // This is the result of a discrete event. Track the lowest priority
      // discrete update per root so we can flush them early, if needed.
      if (rootsWithPendingDiscreteUpdates === null) {
        rootsWithPendingDiscreteUpdates = new Map([[root, expirationTime]]);
      } else {
        const lastDiscreteTime = rootsWithPendingDiscreteUpdates.get(root);
        if (
          lastDiscreteTime === undefined ||
          lastDiscreteTime > expirationTime
        ) {
          rootsWithPendingDiscreteUpdates.set(root, expirationTime);
        }
      }
    }
    // Schedule other updates after in case the callback is sync.
    ensureRootIsScheduled(root);
    schedulePendingInteractions(root, expirationTime);
  }
}

export default class App extends React.Component {
  state = {
    count: 0
  };

  changeState = () => {
    const newCount = this.state.count + 1;
    this.setState({
      count: this.state.count + 1
    });
    if (newCount === this.state.count) {
      console.log("同步执行render");
    } else {
      console.log("异步执行render");
    }
  };

  changeState2 = () => {
    const newCount = this.state.count + 1;
    Promise.resolve().then(() => {
      this.setState({
        count: this.state.count + 1
      });
      if (newCount === this.state.count) {
        console.log("同步执行render");
      } else {
        console.log("异步执行render");
      }
    });
  };

  render() {
    return (
      <div>
        <p>当前count={this.state.count}</p>
        <button onClick={this.changeState}>异步+1</button>
        <button onClick={this.changeState2}>同步+1</button>
      </div>
    );
  }
}

/**
 * https://github.com/reactjs/rfcs/pull/107
 * @param {*} type
 * @param {object} props
 * @param {string} key
 */
export function jsx(type, config, maybeKey) {
  let propName;

  // 1. key的默认值是null
  let key = null;

  // Currently, key can be spread in as a prop. This causes a potential
  // issue if key is also explicitly declared (ie. <div {...props} key="Hi" />
  // or <div key="Hi" {...props} /> ). We want to deprecate key spread,
  // but as an intermediary step, we will use jsxDEV for everything except
  // <div {...props} key="Hi" />, because we aren't currently able to tell if
  // key is explicitly declared to be undefined or not.
  if (maybeKey !== undefined) {
    key = '' + maybeKey;
  }

  if (hasValidKey(config)) {
    // 2. 将key转换成字符串
    key = '' + config.key;
  }
  // 3. 将key传入构造函数
  return ReactElement(
    type,
    key,
    ref,
    undefined,
    undefined,
    ReactCurrentOwner.current,
    props,
  );
}

/**
 * Create and return a new ReactElement of the given type.
 * See https://reactjs.org/docs/react-api.html#createelement
 */
export function createElement(type, config, children) {
  let propName;

  // Reserved names are extracted
  const props = {};

  let key = null;
  let ref = null;
  let self = null;
  let source = null;

  if (config != null) {
    if (hasValidKey(config)) {
      key = '' + config.key; // key转换成字符串
    }
  }

  return ReactElement(
    type,
    key,
    ref,
    self,
    source,
    ReactCurrentOwner.current,
    props,
  );
}

// ReactElement的构造函数: 本节就先只关注其中的key属性
const ReactElement = function (type, key, ref, self, source, owner, props) {
  const element = {
    $typeof: REACT_ELEMENT_TYPE,
    type: type,
    key: key,
    ref: ref,
    props: props,
    _owner: owner,
  };
  return element;
};

function FiberNode(
  tag: WorkTag,
  pendingProps: mixed,
  key: null | string,
  mode: TypeOfMode,
) {
  this.tag = tag;
  this.key = key; // 重点: key也是`fiber`对象的一个属性

  // ...
  this.elementType = null;
  this.type = null;
  this.stateNode = null;
  // ... 省略无关代码
}

function ChildReconciler(shouldTrackSideEffects) {
  function reconcileChildFibers(
    returnFiber: Fiber,
    currentFirstChild: Fiber | null,
    newChild: any,
    lanes: Lanes,
  ): Fiber | null {
    // Handle object types
    const isObject = typeof newChild === 'object' && newChild !== null;

    if (isObject) {
      switch (newChild.$typeof) {
        case REACT_ELEMENT_TYPE:
          // newChild是单节点
          return placeSingleChild(
            reconcileSingleElement(
              returnFiber,
              currentFirstChild,
              newChild,
              lanes,
            ),
          );
      }
    }
    //  newChild是多节点
    if (isArray(newChild)) {
      return reconcileChildrenArray(
        returnFiber,
        currentFirstChild,
        newChild,
        lanes,
      );
    }
    // ...
  }

  return reconcileChildFibers;
}

function reconcileSingleElement(
  returnFiber: Fiber,
  currentFirstChild: Fiber | null,
  element: ReactElement,
  lanes: Lanes,
): Fiber {
  const key = element.key;
  let child = currentFirstChild;
  while (child !== null) {
    //重点1: key是单节点是否复用的第一判断条件
    if (child.key === key) {
      switch (child.tag) {
        default: {
          if (child.elementType === element.type) {
            // 第二判断条件
            deleteRemainingChildren(returnFiber, child.sibling);
            // 节点复用: 调用useFiber
            const existing = useFiber(child, element.props);
            existing.ref = coerceRef(returnFiber, child, element);
            existing.return = returnFiber;
            return existing;
          }
          break;
        }
      }
      // Didn't match.
      deleteRemainingChildren(returnFiber, child);
      break;
    }
    child = child.sibling;
  }
  // 重点2: fiber节点创建, `key`是随着`element`对象被传入`fiber`的构造函数
  const created = createFiberFromElement(element, returnFiber.mode, lanes);
  created.ref = coerceRef(returnFiber, currentFirstChild, element);
  created.return = returnFiber;
  return created;
}

function reconcileChildrenArray(
  returnFiber: Fiber,
  currentFirstChild: Fiber | null,
  newChildren: Array<*>,
  lanes: Lanes,
): Fiber | null {
  if (__DEV__) {
    // First, validate keys.
    let knownKeys = null;
    for (let i = 0; i < newChildren.length; i++) {
      const child = newChildren[i];
      // 1. 在dev环境下, 执行warnOnInvalidKey.
      //  - 如果没有设置key, 会警告提示, 希望能显式设置key
      //  - 如果key重复, 会错误提示.
      knownKeys = warnOnInvalidKey(child, knownKeys, returnFiber);
    }
  }

  let resultingFirstChild: Fiber | null = null;
  let previousNewFiber: Fiber | null = null;

  let oldFiber = currentFirstChild;
  let lastPlacedIndex = 0;
  let newIdx = 0;
  let nextOldFiber = null;
  // 第一次循环: 只会在更新阶段发生
  for (; oldFiber !== null && newIdx < newChildren.length; newIdx++) {
    if (oldFiber.index > newIdx) {
      nextOldFiber = oldFiber;
      oldFiber = null;
    } else {
      nextOldFiber = oldFiber.sibling;
    }
    // 1. 调用updateSlot, 处理公共序列中的fiber
    const newFiber = updateSlot(
      returnFiber,
      oldFiber,
      newChildren[newIdx],
      lanes,
    );
    if (newFiber === null) {
      // 如果无法复用, 则退出公共序列的遍历
      if (oldFiber === null) {
        oldFiber = nextOldFiber;
      }
      break;
    }
  }

  // 第二次循环
  if (oldFiber === null) {
    for (; newIdx < newChildren.length; newIdx++) {
      // 2. 调用createChild直接创建新fiber
      const newFiber = createChild(returnFiber, newChildren[newIdx], lanes);
    }
    return resultingFirstChild;
  }

  for (; newIdx < newChildren.length; newIdx++) {
    // 3. 调用updateFromMap处理非公共序列中的fiber
    const newFiber = updateFromMap(
      existingChildren,
      returnFiber,
      newIdx,
      newChildren[newIdx],
      lanes,
    );
  }

  return resultingFirstChild;
}

function updateSlot(
  returnFiber: Fiber,
  oldFiber: Fiber | null,
  newChild: any,
  lanes: Lanes,
): Fiber | null {
  const key = oldFiber !== null ? oldFiber.key : null;

  if (typeof newChild === 'object' && newChild !== null) {
    switch (newChild.$typeof) {
      case REACT_ELEMENT_TYPE: {
        //重点: key用于是否复用的第一判断条件
        if (newChild.key === key) {
          return updateElement(returnFiber, oldFiber, newChild, lanes);
        } else {
          return null;
        }
      }
    }
  }

  return null;
}

function createChild(
  returnFiber: Fiber,
  newChild: any,
  lanes: Lanes,
): Fiber | null {
  if (typeof newChild === 'object' && newChild !== null) {
    switch (newChild.$typeof) {
      case REACT_ELEMENT_TYPE: {
        // 重点: 调用构造函数进行创建
        const created = createFiberFromElement(
          newChild,
          returnFiber.mode,
          lanes,
        );
        return created;
      }
    }
  }

  return null;
}

function updateFromMap(
  existingChildren: Map<string | number, Fiber>,
  returnFiber: Fiber,
  newIdx: number,
  newChild: any,
  lanes: Lanes,
): Fiber | null {
  if (typeof newChild === 'object' && newChild !== null) {
    switch (newChild.$typeof) {
      case REACT_ELEMENT_TYPE: {
        //重点: key用于是否复用的第一判断条件
        const matchedFiber =
          existingChildren.get(
            newChild.key === null ? newIdx : newChild.key,
          ) || null;
        return updateElement(returnFiber, matchedFiber, newChild, lanes);
      }
    }
    return null;
  }
}