6 posts tagged with "react"

{
  create: Function, // 传入use（Layout）Effect函数的第一个参数，即回调函数
  destroy: Function, // 回调函数return的函数，在组件销毁的时候执行
  deps: any[], // 依赖项
  next: Effect, // 指向下一个effect
  tag: string, // effect的类型，区分是useEffect还是useLayoutEffect
}

function mountEffectImpl(fiberFlags, hookFlags, create, deps) {
  const hook = mountWorkInProgressHook(); // 创建hook对象
  const nextDeps = deps === undefined ? null : deps; // 依赖

  // NOTE 给 fiber 节点打上副作用的 effectTag
  currentlyRenderingFiber.flags = fiberFlags;

  // 创建 effect 对象，挂载到 hook 的 memoizedState 上
  hook.memoizedState = pushEffect(
    HookHasEffect | hookFlags,
    create,
    undefined,
    nextDeps
  );
}

function updateEffectImpl(fiberFlags, hookFlags, create, deps): void {
  const hook = updateWorkInProgressHook();
  const nextDeps = deps === undefined ? null : deps;
  let destroy = undefined;

  if (currentHook !== null) {
    const prevEffect = currentHook.memoizedState;
    // 获取上一次 effect 的 destroy 函数
    destroy = prevEffect.destroy;
    if (nextDeps !== null) {
      const prevDeps = prevEffect.deps;
      // NOTE 如果前后依赖相同
      if (areHookInputsEqual(nextDeps, prevDeps)) {
        pushEffect(hookFlags, create, destroy, nextDeps);
        return;
      }
    }
  }

  // NOTE flags的作用
  currentlyRenderingFiber.flags |= fiberFlags;
  // 如果前后依赖有变，在 effect 的 tag 中加入 HookHasEffect
  // 并将新的 effect 挂载到 hook.memoizedState 上
  hook.memoizedState = pushEffect(
    HookHasEffect | hookFlags,
    create,
    destroy,
    nextDeps
  );
}

function pushEffect(tag, create, destroy, deps) {
  const effect: Effect = {
    tag,
    create,
    destroy,
    deps,
    next,
  };

  // NOTE 从 workInProgress 上取到 updateQueue（和 effectList 的区别）
  let componentUpdateQueue = currentlyRenderingFiber.updateQueue;
  if (componentUpdateQueue === null) {
    // 如果 updateQueue 为空，把 effect 放到链表中，和它自己形成闭环
    componentUpdateQueue = createFunctionComponentUpdateQueue();
    currentlyRenderingFiber.updateQueue = componentUpdateQueue;
    componentUpdateQueue.lastEffect = effect.next = effect;
  } else {
    // updateQueue 不为空，将 effect 追加到链表上
    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;
    }
  }
  return effect;
}

function completeUnitOfWork(unitOfWork: Fiber): void {
  let completedWork = unitOfWork;
  do {
    const current = completedWork.alternate;
    const returnFiber = completedWork.return;

    let next = completeWork(current, completedWork, subtreeRenderLanes);

    // effect list构建
    if (returnFiber !== null && (returnFiber.flags & Incomplete) === NoFlags) {
      // 层层拷贝
      if (returnFiber.firstEffect === null) {
        returnFiber.firstEffect = completedWork.firstEffect;
      }
      if (completedWork.lastEffect !== null) {
        // 说明当前节点是兄弟节点，子节点有effect，已经给returnFiber.lastEffect赋值过了
        if (returnFiber.lastEffect !== null) {
          // 连接兄弟节点的effect
          returnFiber.lastEffect.nextEffect = completedWork.firstEffect;
        }
        returnFiber.lastEffect = completedWork.lastEffect;
      }

      const flags = completedWork.flags;

      // NOTE: 该 fiber 节点有 effect
      if (flags > PerformedWork) {
        // 当前节点有effect连接上effect list
        if (returnFiber.lastEffect !== null) {
          returnFiber.lastEffect.nextEffect = completedWork;
        } else {
          // returnFiber没有firstEffect的情况是第一次遇见有effect的节点
          returnFiber.firstEffect = completedWork;
        }
        returnFiber.lastEffect = completedWork;
      }
    }

    // 兄弟元素遍历再到返返回父级
    const siblingFiber = completedWork.sibling;
    if (siblingFiber !== null) {
      workInProgress = siblingFiber;
      return;
    }
    completedWork = returnFiber;
    workInProgress = completedWork;
  } while (completedWork !== null);
}

function commitRootImpl(root, renderPriorityLevel) {
  // 进入commit阶段，先执行一次之前未执行的 useEffect
  do {
    flushPassiveEffects();
  } while (rootWithPendingPassiveEffects !== null);
  // ...
  do {
    try {
      // beforeMutation阶段的处理函数：commitBeforeMutationEffects内部，
      // 异步调度 useEffect
      commitBeforeMutationEffects();
    } catch (error) {
      ...
    }
  } while (nextEffect !== null);
  // ...
  const rootDidHavePassiveEffects = rootDoesHavePassiveEffects;

  if (rootDoesHavePassiveEffects) {
    // NOTE 记录有副作用的 effect
    rootWithPendingPassiveEffects = root;
  }
}

function commitBeforeMutationEffects() {
  while (nextEffect !== null) {
    // ...
    if ((flags & Passive) !== NoFlags) {
      // 如果fiber节点上的flags存在Passive调度useEffect
      if (!rootDoesHavePassiveEffects) {
        // 锁住调度状态，防止多次调度
        rootDoesHavePassiveEffects = true;
        // NOTE NormalSchedulerPriority 一般优先级
        scheduleCallback(NormalSchedulerPriority, () => {
          flushPassiveEffects();
          return null;
        });
      }
    }
    nextEffect = nextEffect.nextEffect;
  }
}

function commitLifeCycles(
  finishedRoot: FiberRoot,
  current: Fiber | null,
  finishedWork: Fiber,
  committedLanes: Lanes,
): void {
  switch (finishedWork.tag) {
    case FunctionComponent:
    case ForwardRef:
    case SimpleMemoComponent:
    case Block: {
      // ...
      // layout阶段填充effect执行数组
      schedulePassiveEffects(finishedWork);
      return;
    }
}

function schedulePassiveEffects(finishedWork: Fiber) {
  // 获取到函数组件的 updateQueue
  const updateQueue = finishedWork.updateQueue;
  // 获取 effect 链表
  const lastEffect = updateQueue !== null ? updateQueue.lastEffect : null;
  if (lastEffect !== null) {
    const firstEffect = lastEffect.next;
    let effect = firstEffect;
    // 循环 effect 链表
    do {
      const { next, tag } = effect;
      if (
        (tag & HookPassive) !== NoHookEffect &&
        (tag & HookHasEffect) !== NoHookEffect
      ) {
        // 当 effect 的 tag 含有 HookPassive 和 HookHasEffect 时
        // 分别插入销毁函数数组和创建函数数组
        enqueuePendingPassiveHookEffectUnmount(finishedWork, effect);
        enqueuePendingPassiveHookEffectMount(finishedWork, effect);
      }
      effect = next;
    } while (effect !== firstEffect);
  }
}

function flushPassiveEffectsImpl() {
  // 先校验，如果root上没有 Passive effectTag 的节点，则直接return
  if (rootWithPendingPassiveEffects === null) {
    return false;
  }
  // ...
  // 执行effect的销毁
  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") {
      try {
        destroy();
      } catch (error) {
        captureCommitPhaseError(fiber, error);
      }
    }
  }

  // 再执行effect的创建
  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);
    try {
      const create = effect.create;
      effect.destroy = create();
    } catch (error) {
      captureCommitPhaseError(fiber, error);
    }
  }
  // ...
  return true;
}

// 调用卸载layout effect的函数，传入layout有关的effectTag和说明effect有变化的effectTag：HookLayout | HookHasEffect
commitHookEffectListUnmount(HookLayout | HookHasEffect, finishedWork);

function commitHookEffectListUnmount(tag: number, finishedWork: Fiber) {
  const updateQueue = finishedWork.updateQueue;
  const lastEffect = updateQueue !== null ? updateQueue.lastEffect : null;

  // 循环updateQueue上的effect链表
  if (lastEffect !== null) {
    const firstEffect = lastEffect.next;
    let effect = firstEffect;
    do {
      if ((effect.tag & tag) === tag) {
        const destroy = effect.destroy;
        effect.destroy = undefined;
        if (destroy !== undefined) {
          destroy();
        }
      }
      effect = effect.next;
    } while (effect !== firstEffect);
  }
}

// 调用创建layout effect的函数
commitHookEffectListMount(HookLayout | HookHasEffect, finishedWork);

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

// scheduleUpdateOnFiber
// ...
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();
}
// ...

// beforeMutation
// 将当前上下文标记为CommitContext，作为commit阶段的标志
// ...
const prevExecutionContext = executionContext;
executionContext |= CommitContext;
// ...

/*
Hooks are stored as a linked list on the fiber's memoizedState field.  
hooks 以链表的形式存储在fiber节点的memoizedState属性上
The current hook list is the list that belongs to the current fiber.
当前的hook链表就是当前正在遍历的fiber节点上的
The work-in-progress hook list is a new list that will be added to the work-in-progress fiber.
work-in-progress hook 就是即将被添加到正在遍历fiber节点的hooks新链表
*/
let currentHook: Hook | null = null;
let nextCurrentHook: Hook | null = null;

{
    baseQueue: null,     // 当前 update
    baseState: 'hook1',  // 初始值，即 useState 入参
    memoizedState: null, // 当前状态（更新时表示上一次的状态）
    queue: null,         // 待执行的更新队列(queue.pending)
    next: {              // 下一个 hook
        baseQueue: null,
        baseState: null,
        memoizedState: 'hook2',
        next: null
        queue: null
    }
}

// renderWithHooks
function renderWithHooks(current, workInProgress, Component, props, secondArg, nextRenderLanes) {
  ...
  // 当前fiber
  currentlyRenderingFiber$1 = workInProgress;
  // hooks队列
  workInProgress.memoizedState = null;
  // 副作用队列
  workInProgress.updateQueue = null;
  {
    if (current !== null && current.memoizedState !== null) {
      ReactCurrentDispatcher$1.current = HooksDispatcherOnUpdateInDEV;
    } else if (hookTypesDev !== null) {
      ReactCurrentDispatcher$1.current = HooksDispatcherOnMountWithHookTypesInDEV;
    } else {
      ReactCurrentDispatcher$1.current = HooksDispatcherOnMountInDEV;
    }
  }
   ...
  return children;
}

/**
 * ReactCurrentDispatcher 是一个内部储存状态的状态机.
 * 主要作用还是用于切换不同执行时机的 dispatcher 对象
 * */
const ReactCurrentDispatcher = { current: null };
function resolveDispatcher() {
  const dispatcher = ReactCurrentDispatcher.current;
  return dispatcher;
}
function useState(initialState) {
  var dispatcher = resolveDispatcher();
  return dispatcher.useState(initialState);
}

// resolveDispatcher
...
useState: function (initialState) {
      ...
      try {
        return mountState(initialState);
      } finally {
        ReactCurrentDispatcher$1.current = prevDispatcher;
      }
},
...

// mountWorkInProgressHook
function mountWorkInProgressHook() {
  var hook = {
    memoizedState: null,
    baseState: null,
    baseQueue: null,
    queue: null,
    next: null,
  };

  if (workInProgressHook === null) {
    currentlyRenderingFiber$1.memoizedState = workInProgressHook = hook;
  } else {
    workInProgressHook = workInProgressHook.next = hook;
  }

  return workInProgressHook;
}

// mountState
function mountState(initialState) {
  // 创建 hook 对象
  var hook = mountWorkInProgressHook();

  if (typeof initialState === "function") {
    initialState = initialState();
  }

  hook.memoizedState = hook.baseState = initialState;

  var queue = (hook.queue = {
    pending: null, // update 队列
    dispatch: null, // 触发函数，会触发更新，useState返回值的第二个参数
    lastRenderedReducer: basicStateReducer, // 最后一次使用的 reducer，初始化时使用最基础的 reducer
    lastRenderedState: initialState, // 上一次的值，挂载阶段该值为初始值
  });
  var dispatch = (queue.dispatch = dispatchAction.bind(
    null,
    currentlyRenderingFiber$1,
    queue
  ));
  // dispatch是一个闭包函数
  return [hook.memoizedState, dispatch];
}

function basicStateReducer(state, action) {
  return typeof action === "function" ? action(state) : action;
}

// mountState
...
var dispatch = queue.dispatch = dispatchAction.bind(null, currentlyRenderingFiber$1, queue);
return [hook.memoizedState, dispatch];

function dispatchAction(fiber, queue, action) {
  ...
  // 更新对象，是一个链表节点
  var update = {
    lane: lane,         // 优先级
    action: action,     // 新值，也就是 setState 传入的参数
    next: null,         // 指向下一个更新对象
    eagerReducer: null, // 上一次使用的 reducer
    eagerState: null,   // 旧值
  };

  var pending = queue.pending;
  if (pending === null) {
    // 在队列上添加当前hook的首个更新对象，并保持循环
    update.next = update;
  } else {
    // 非首个更新对象，会追加到更新队列之后
    update.next = pending.next;
    pending.next = update;
  }
  queue.pending = update;

  // fiber.alternate 指向旧 fiber 链表
  var alternate = fiber.alternate;

  if (fiber === currentlyRenderingFiber$1 || alternate !== null && alternate === currentlyRenderingFiber$1) {
    didScheduleRenderPhaseUpdateDuringThisPass = didScheduleRenderPhaseUpdate = true;
  } else {
    if (fiber.lanes === NoLanes && (alternate === null || alternate.lanes === NoLanes)) {
      var lastRenderedReducer = queue.lastRenderedReducer;

      if (lastRenderedReducer !== null) {
        var prevDispatcher;

        {
          // NOTE 暂不清楚这一步的作用
          prevDispatcher = ReactCurrentDispatcher$1.current;
          ReactCurrentDispatcher$1.current = InvalidNestedHooksDispatcherOnUpdateInDEV;
        }

        try {
          var currentState = queue.lastRenderedState;
          // 计算新的 state
          var eagerState = lastRenderedReducer(currentState, action);

          update.eagerReducer = lastRenderedReducer;
          update.eagerState = eagerState;

          if (objectIs(eagerState, currentState)) {
            // 前后值没变，没必要更新
            return;
          }
        } catch (error) {
            // Suppress the error. It will throw again in the render phase.
        } finally {
          {
            ReactCurrentDispatcher$1.current = prevDispatcher;
          }
        }
      }
    }
    ...
    // 发起一次调度更新
    scheduleUpdateOnFiber(fiber, lane, eventTime);
  }
}

function updateWorkInProgressHook() {
  var nextCurrentHook;

  if (currentHook === null) {
    var current = currentlyRenderingFiber$1.alternate;

    if (current !== null) {
      nextCurrentHook = current.memoizedState;
    } else {
      nextCurrentHook = null;
    }
  } else {
    nextCurrentHook = currentHook.next;
  }

  var nextWorkInProgressHook;

  if (workInProgressHook === null) {
    nextWorkInProgressHook = currentlyRenderingFiber$1.memoizedState;
  } else {
    nextWorkInProgressHook = workInProgressHook.next;
  }

  if (nextWorkInProgressHook !== null) {
    workInProgressHook = nextWorkInProgressHook;
    nextWorkInProgressHook = workInProgressHook.next;
    currentHook = nextCurrentHook;
  } else {
    ...
    currentHook = nextCurrentHook;
    var newHook = {
      memoizedState: currentHook.memoizedState,
      baseState: currentHook.baseState,
      baseQueue: currentHook.baseQueue,
      queue: currentHook.queue,
      next: null
    };

    if (workInProgressHook === null) {
      currentlyRenderingFiber$1.memoizedState = workInProgressHook = newHook;
    } else {
      workInProgressHook = workInProgressHook.next = newHook;
    }
  }

  return workInProgressHook;
}

const HooksDispatcherOnUpdate: Dispatcher = {
  ...
  useState: updateState,
}
function updateState(initialState) {
  return updateReducer(basicStateReducer, initialState);
}
...
// updateReducer
function updateReducer(reducer, initialArg, init) {
  var hook = updateWorkInProgressHook();
  var queue = hook.queue;
  ...
    do {
      ...
        if (update.eagerReducer === reducer) {
          newState = update.eagerState;
        } else {
          // 遍历更新队列，得到最新值
          var action = update.action;
          newState = reducer(newState, action);
        }
      ...
      update = update.next;
    } while (update !== null && update !== first);
  ...
  var dispatch = queue.dispatch;
  return [hook.memoizedState, dispatch];
}

onClick = () => {
  setState(0);
  setState(1);
  setState(2);
};

// ...
  constructor() {
    super();
    this.state = {
      val: 0
    };
  }

  componentDidMount() {
    this.setState({val: this.state.val + 1});
    console.log(this.state.val);
    this.setState({val: this.state.val + 1});
    console.log(this.state.val);
    setTimeout(() => {
      this.setState({val: this.state.val + 1});
      console.log(this.state.val);
      this.setState({val: this.state.val + 1});
      console.log(this.state.val);
    }, 0);
  }
// ...
};

var Transaction = {
  // 重置 Transaction，刷新之前的数据
  reinitializeTransaction: function () {
    // getTransactionWrappers 其实是一个获取wrapper的抽象方法，需要具体实现
    // 用于获取当前事务需要的所有 wrappers
    this.transactionWrappers = this.getTransactionWrappers();
    if (this.wrapperInitData) {
      this.wrapperInitData.length = 0;
    } else {
      this.wrapperInitData = [];
    }
    this._isInTransaction = false;
  },

  _isInTransaction: false,

  getTransactionWrappers: null,

  // 判断当前事务是否正在执行
  // 可用于防止当前事务被打断
  isInTransaction: function () {
    return !!this._isInTransaction;
  },

  perform: () => {
    // this._isInTransaction = true 表明正处于事务中
    // this.initializeAll
    // 执行method
    // this.closeAll
    // this._isInTransaction = false
  },
  initializeAll: () => {
    // 遍历所有wrappers，执行initialize方法
  }
  closeAll: () => {
    // 遍历所有wrappers，执行所有close方法
  }

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

// ...
  enqueueSetState: function(publicInstance, partialState) {
    var internalInstance = getInternalInstanceReadyForUpdate(
      publicInstance,
      'setState',
    );

    var queue =
      internalInstance._pendingStateQueue ||
      (internalInstance._pendingStateQueue = []);
    queue.push(partialState);

    enqueueUpdate(internalInstance);
  },
// ...

function enqueueUpdate(internalInstance) {
  ReactUpdates.enqueueUpdate(internalInstance);
}
// ...
function enqueueUpdate(component) {
  // ...
  if (!batchingStrategy.isBatchingUpdates) {
    batchingStrategy.batchedUpdates(enqueueUpdate, component);
    return;
  }
  dirtyComponents.push(component);
}

// 更新事务的 wrappers
var TRANSACTION_WRAPPERS = [FLUSH_BATCHED_UPDATES, RESET_BATCHED_UPDATES];
// 修改批量更新状态
var RESET_BATCHED_UPDATES = {
  initialize: emptyFunction,
  close: function () {
    ReactDefaultBatchingStrategy.isBatchingUpdates = false;
  }
};
// 遍历所有的 dirtyComponents 更新组件
var FLUSH_BATCHED_UPDATES = {
  initialize: emptyFunction,
  close: ReactUpdates.flushBatchedUpdates.bind(ReactUpdates)
};
// ...
// 更新策略对象
var ReactDefaultBatchingStrategy = {
  // 是否批量更新
  isBatchingUpdates: false,

  batchedUpdates: function (callback, a, b, c, d, e) {
    var alreadyBatchingUpdates = ReactDefaultBatchingStrategy.isBatchingUpdates;
    ReactDefaultBatchingStrategy.isBatchingUpdates = true;
    if (alreadyBatchingUpdates) {
      return callback(a, b, c, d, e);
    } else {
      return transaction.perform(callback, null, a, b, c, d, e);
    }
  }
};
// ...
// 更新事务对象，是 Transition 对象的子类
function ReactDefaultBatchingStrategyTransaction() {
  // 初始化事务
  this.reinitializeTransaction();
}
// ...
// 实现了 getTransactionWrappers 这个抽象方法，即注入更新事务的 wrappers
Object.assign(ReactDefaultBatchingStrategyTransaction.prototype, Transaction, {
  getTransactionWrappers: function() {
    return TRANSACTION_WRAPPERS;
  },
});
...
var transaction = new ReactDefaultBatchingStrategyTransaction();

// react/packages/react/src/ReactBaseClasses.js
Component.prototype.setState = function (partialState, callback) {
  // ...
  this.updater.enqueueSetState(this, partialState, callback, "setState");
};

// react/packages/react-reconciler/src/ReactFiberClassComponent.js
const classComponentUpdater = {
  // ...
  enqueueSetState(inst, payload, callback) {
    const fiber = getInstance(inst);
    const currentTime = requestCurrentTime();
    const expirationTime = computeExpirationForFiber(currentTime, fiber);

    // 创建一个更新队列
    const update = createUpdate(expirationTime);
    update.payload = payload;
    if (callback !== undefined && callback !== null) {
      // ...
      update.callback = callback;
    }
    // TODO 暂不清楚这个的具体作用
    flushPassiveEffects();
    // 将更新任务挂载到 fiber 上
    enqueueUpdate(fiber, update);
    // 调度更新
    scheduleWork(fiber, expirationTime);
  },
  // ...
};

// react/packages/react-reconciler/src/ReactUpdateQueue.js
export function enqueueUpdate<State>(fiber: Fiber, update: Update<State>) {
  // Update queues are created lazily.
  const alternate = fiber.alternate;
  let queue1;
  let queue2;
  // ...
  {
    appendUpdateToQueue(queue1, update);
  }
  // ...
}
// ...
function appendUpdateToQueue<State>(
  queue: UpdateQueue<State>,
  update: Update<State>
) {
  // 将 update 对象挂载到更新队列 updateQueue 上，形成一个环状单向链表
  if (queue.lastUpdate === null) {
    queue.firstUpdate = queue.lastUpdate = update;
  } else {
    queue.lastUpdate.next = update;
    queue.lastUpdate = update;
  }
}

// react/packages/react-reconciler/src/ReactUpdateQueue.js
function updateClassInstance(
  current: Fiber,
  workInProgress: Fiber,
  ctor: any,
  newProps: any,
  renderExpirationTime: ExpirationTime,
): boolean {
  // ...
  const oldState = workInProgress.memoizedState;
  let newState = (instance.state = oldState);
  let updateQueue = workInProgress.updateQueue;
  if (updateQueue !== null) {
    // 遍历更新队列计算得到新值
    processUpdateQueue(
      workInProgress,
      updateQueue,
      newProps,
      instance,
      renderExpirationTime,
    );
    newState = workInProgress.memoizedState;
  }
  // ...
}
// ...
// 处理更新队列，返回新的 state
export function processUpdateQueue<State>(
  workInProgress: Fiber,
  queue: UpdateQueue<State>,
  props: any,
  instance: any,
  renderExpirationTime: ExpirationTime,
): void {
  // ...
  while (update !== null) {
    // ...
    {
      resultState = getStateFromUpdate(
        workInProgress,
        queue,
        update,
        resultState,
        props,
        instance,
      );
      // 收集 callback，在 commit 阶段执行
      const callback = update.callback;
      if (callback !== null) {
        workInProgress.effectTag |= Callback;
        update.nextEffect = null;
        if (queue.lastEffect === null) {
          queue.firstEffect = queue.lastEffect = update;
        } else {
          queue.lastEffect.nextEffect = update;
          queue.lastEffect = update;
        }
      }
    }
    // Continue to the next update.
    update = update.next;
  }
  // ...
  workInProgress.memoizedState = resultState;
  // ...
}
// ...
function getStateFromUpdate (..., partialState, ...) {
  // ...
  if (typeof _payload2 === 'function') {
    // ...
    // Merge the partial state and the previous state.
    return _assign({}, prevState, partialState);
  }
}