requestEventTime

其实在React执行过程中，会有数不清的任务要去执行，但是他们会有一个优先级的判定，假如两个事件的优先级一样，那么React是怎么去判定他们两谁先执行呢？

// packages/react-reconciler/src/ReactFiberWorkLoop.old.js
export function requestEventTime() {if ((executionContext & (RenderContext | CommitContext)) !== NoContext) {// We're inside React, so it's fine to read the actual time.// react事件正在执行// executionContext// RenderContext 正在计算// CommitContext 正在提交// 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;return now();}// 没有在react事件执行 NoTimestamp === -1if (currentEventTime !== NoTimestamp) { // 浏览器事件正在执行，返回上次的 currentEventTimereturn currentEventTime;}// 重新计算currentEventTime，当执行被中断后currentEventTime = now();return currentEventTime;
}

• RenderContext与CommitContext表示正在计算更新和正在提交更新，返回now()。
• 如果是浏览器事件正在执行中，返回上一次的currentEventTime。
• 如果终止或者中断react任务执行的时候，则重新获取执行时间now()。
• 获取的时间越小，则执行的优先级越高。

now()并不是单纯的new Date()，而是判定两次更新任务的时间是否小于10ms，来决定是否复用上一次的更新时间Scheduler_now的。

export const now = initialTimeMs < 10000 ? Scheduler_now : () => Scheduler_now() - initialTimeMs;

其实各位猜想一下，对于10ms级别的任务间隙时间，几乎是可以忽略不计的，那么这里就可以视为同样的任务，不需要有很大的性能开销，有利于批量更新。

requestUpdateLane

requestEventTime位每一个需要执行的任务打上了触发更新时间标签，那么任务的优先级还需要进一步的确立，requestUpdateLane就是用来获取每一个任务执行的优先级的。

// packages/react-reconciler/src/ReactFiberWorkLoop.old.js
export function requestUpdateLane(fiber: Fiber): Lane {// Special casesconst mode = fiber.mode;if ((mode & BlockingMode) === NoMode) {return (SyncLane: Lane);} else if ((mode & ConcurrentMode) === NoMode) {return getCurrentPriorityLevel() === ImmediateSchedulerPriority? (SyncLane: Lane): (SyncBatchedLane: Lane);} else if (!deferRenderPhaseUpdateToNextBatch &&(executionContext & RenderContext) !== NoContext &&workInProgressRootRenderLanes !== NoLanes) {// This is a render phase update. These are not officially supported. The// old behavior is to give this the same "thread" (expiration time) as// whatever is currently rendering. So if you call `setState` on a component// that happens later in the same render, it will flush. Ideally, we want to// remove the special case and treat them as if they came from an// interleaved event. Regardless, this pattern is not officially supported.// This behavior is only a fallback. The flag only exists until we can roll// out the setState warning, since existing code might accidentally rely on// the current behavior.return pickArbitraryLane(workInProgressRootRenderLanes);}// The algorithm for assigning an update to a lane should be stable for all// updates at the same priority within the same event. To do this, the inputs// to the algorithm must be the same. For example, we use the `renderLanes`// to avoid choosing a lane that is already in the middle of rendering.//// However, the "included" lanes could be mutated in between updates in the// same event, like if you perform an update inside `flushSync`. Or any other// code path that might call `prepareFreshStack`.//// The trick we use is to cache the first of each of these inputs within an// event. Then reset the cached values once we can be sure the event is over.// Our heuristic for that is whenever we enter a concurrent work loop.//// We'll do the same for `currentEventPendingLanes` below.if (currentEventWipLanes === NoLanes) {currentEventWipLanes = workInProgressRootIncludedLanes;}const isTransition = requestCurrentTransition() !== NoTransition;if (isTransition) {if (currentEventPendingLanes !== NoLanes) {currentEventPendingLanes =mostRecentlyUpdatedRoot !== null? mostRecentlyUpdatedRoot.pendingLanes: NoLanes;}return findTransitionLane(currentEventWipLanes, currentEventPendingLanes);}// TODO: Remove this dependency on the Scheduler priority.// To do that, we're replacing it with an update lane priority.// 获取执行任务的优先级，便于调度const schedulerPriority = getCurrentPriorityLevel();// The old behavior was using the priority level of the Scheduler.// This couples React to the Scheduler internals, so we're replacing it// with the currentUpdateLanePriority above. As an example of how this// could be problematic, if we're not inside `Scheduler.runWithPriority`,// then we'll get the priority of the current running Scheduler task,// which is probably not what we want.let lane;if (// TODO: Temporary. We're removing the concept of discrete updates.(executionContext & DiscreteEventContext) !== NoContext &&// 用户block的类型事件schedulerPriority === UserBlockingSchedulerPriority) {// 通过findUpdateLane函数重新计算lanelane = findUpdateLane(InputDiscreteLanePriority, currentEventWipLanes);} else {// 根据优先级计算法则计算laneconst schedulerLanePriority = schedulerPriorityToLanePriority(schedulerPriority,);if (decoupleUpdatePriorityFromScheduler) {// In the new strategy, we will track the current update lane priority// inside React and use that priority to select a lane for this update.// For now, we're just logging when they're different so we can assess.const currentUpdateLanePriority = getCurrentUpdateLanePriority();if (schedulerLanePriority !== currentUpdateLanePriority &&currentUpdateLanePriority !== NoLanePriority) {if (__DEV__) {console.error('Expected current scheduler lane priority %s to match current update lane priority %s',schedulerLanePriority,currentUpdateLanePriority,);}}}// 根据计算得到的 schedulerLanePriority，计算更新的优先级 lanelane = findUpdateLane(schedulerLanePriority, currentEventWipLanes);}return lane;
}

• 通过getCurrentPriorityLevel获得所有执行任务的调度优先级schedulerPriority。
• 通过findUpdateLane计算lane，作为更新中的优先级。

findUpdateLane

export function findUpdateLane(lanePriority: LanePriority,  wipLanes: Lanes,
): Lane {switch (lanePriority) {case NoLanePriority:break;case SyncLanePriority:return SyncLane;case SyncBatchedLanePriority:return SyncBatchedLane;case InputDiscreteLanePriority: {const lane = pickArbitraryLane(InputDiscreteLanes & ~wipLanes);if (lane === NoLane) {// Shift to the next priority levelreturn findUpdateLane(InputContinuousLanePriority, wipLanes);}return lane;}case InputContinuousLanePriority: {const lane = pickArbitraryLane(InputContinuousLanes & ~wipLanes);if (lane === NoLane) {// Shift to the next priority levelreturn findUpdateLane(DefaultLanePriority, wipLanes);}return lane;}case DefaultLanePriority: {let lane = pickArbitraryLane(DefaultLanes & ~wipLanes);if (lane === NoLane) {// If all the default lanes are already being worked on, look for a// lane in the transition range.lane = pickArbitraryLane(TransitionLanes & ~wipLanes);if (lane === NoLane) {// All the transition lanes are taken, too. This should be very// rare, but as a last resort, pick a default lane. This will have// the effect of interrupting the current work-in-progress render.lane = pickArbitraryLane(DefaultLanes);}}return lane;}case TransitionPriority: // Should be handled by findTransitionLane insteadcase RetryLanePriority: // Should be handled by findRetryLane insteadbreak;case IdleLanePriority:let lane = pickArbitraryLane(IdleLanes & ~wipLanes);if (lane === NoLane) {lane = pickArbitraryLane(IdleLanes);}return lane;default:// The remaining priorities are not valid for updatesbreak;}invariant(false,'Invalid update priority: %s. This is a bug in React.',lanePriority,);
}

相关参考视频讲解：进入学习

lanePriority: LanePriority

export opaque type LanePriority =| 0| 1| 2| 3| 4| 5| 6| 7| 8| 9| 10| 11| 12| 13| 14| 15| 16| 17;
export opaque type Lanes = number;
export opaque type Lane = number;
export opaque type LaneMap<T> = Array<T>;import {ImmediatePriority as ImmediateSchedulerPriority,UserBlockingPriority as UserBlockingSchedulerPriority,NormalPriority as NormalSchedulerPriority,LowPriority as LowSchedulerPriority,IdlePriority as IdleSchedulerPriority,NoPriority as NoSchedulerPriority,
} from './SchedulerWithReactIntegration.new';// 同步任务
export const SyncLanePriority: LanePriority = 15;
export const SyncBatchedLanePriority: LanePriority = 14;// 用户事件
const InputDiscreteHydrationLanePriority: LanePriority = 13;
export const InputDiscreteLanePriority: LanePriority = 12;const InputContinuousHydrationLanePriority: LanePriority = 11;
export const InputContinuousLanePriority: LanePriority = 10;const DefaultHydrationLanePriority: LanePriority = 9;
export const DefaultLanePriority: LanePriority = 8;const TransitionHydrationPriority: LanePriority = 7;
export const TransitionPriority: LanePriority = 6;const RetryLanePriority: LanePriority = 5;const SelectiveHydrationLanePriority: LanePriority = 4;const IdleHydrationLanePriority: LanePriority = 3;
const IdleLanePriority: LanePriority = 2;const OffscreenLanePriority: LanePriority = 1;export const NoLanePriority: LanePriority = 0;

createUpdate

export function createUpdate(eventTime: number, lane: Lane): Update<*> {const update: Update<*> = {eventTime, // 更新时间lane, // 优先级tag: UpdateState,//更新payload: null,// 需要更新的内容callback: null, // 更新完后的回调next: null, // 指向下一个更新};return update;
}

createUpdate函数入参为eventTime和lane，输出一个update对象，而对象中的tag表示此对象要进行什么样的操作。

export const UpdateState = 0;// 更新
export const ReplaceState = 1;//替换
export const ForceUpdate = 2;//强制更新
export const CaptureUpdate = 3;//xx更新

• createUpdate就是单纯的给每一个任务进行包装，作为一个个体推入到更新队列中。

enqueueUpdate

export function enqueueUpdate<State>(fiber: Fiber, update: Update<State>) {// 获取当前更新队列？为啥呢？因为无法保证react是不是还有正在更新或者没有更新完毕的任务const updateQueue = fiber.updateQueue;//  如果更新队列为空，则表示fiber还未渲染，直接退出if (updateQueue === null) {// Only occurs if the fiber has been unmounted.return;}const sharedQueue: SharedQueue<State> = (updateQueue: any).shared;const pending = sharedQueue.pending;if (pending === null) {// This is the first update. Create a circular list.// 还记得那个更新对象吗？update.next =>// 如果pedding位null，表示第一次渲染，那么他的指针为update本身update.next = update;} else {// 将update插入到更新队列循环当中update.next = pending.next;pending.next = update;}sharedQueue.pending = update;if (__DEV__) {if (currentlyProcessingQueue === sharedQueue &&!didWarnUpdateInsideUpdate) {console.error('An update (setState, replaceState, or forceUpdate) was scheduled ' +'from inside an update function. Update functions should be pure, ' +'with zero side-effects. Consider using componentDidUpdate or a ' +'callback.',);didWarnUpdateInsideUpdate = true;}}
}

• 这一步就是把需要更新的对象，与fiber更新队列关联起来。

总结

React通过获取事件的优先级，处理具有同样优先级的事件，创建更新对象并与fiber的更新队列关联起来。到这一步updateContainer这个流程就走完了，也下面就是开始他的协调阶段了。

协调与调度

协调与调度的流程大致如图所示：

reconciler流程

React的reconciler流程以scheduleUpdateOnFiber为入口，并在checkForNestedUpdates里面处理任务更新的嵌套层数，如果嵌套层数过大（ >50 ），就会认为是无效更新，则会抛出异常。之后便根据markUpdateLaneFromFiberToRoot对当前的fiber树，自底向上的递归fiber的lane，根据lane做二进制比较或者位运算处理。详情如下：

• 如果当前执行任务的优先级为同步，则去判断有无正在执行的React任务。如果没有则执行ensureRootIsScheduled，进行调度处理。
• 如果当前任务优先级是异步执行，则执行ensureRootIsScheduled进行调度处理。

export function scheduleUpdateOnFiber(fiber: Fiber,  lane: Lane,  eventTime: number,
) {// 检查嵌套层数，避免是循环做无效操作checkForNestedUpdates();warnAboutRenderPhaseUpdatesInDEV(fiber);// 更新当前更新队列里面的任务优先级,自底而上更新child.fiberLanesconst root = markUpdateLaneFromFiberToRoot(fiber, lane);if (root === null) {warnAboutUpdateOnUnmountedFiberInDEV(fiber);return null;}// Mark that the root has a pending update.// 标记root有更新的，执行它markRootUpdated(root, lane, eventTime);if (root === workInProgressRoot) {// Received an update to a tree that's in the middle of rendering. Mark// that there was an interleaved update work on this root. Unless the// `deferRenderPhaseUpdateToNextBatch` flag is off and this is a render// phase update. In that case, we don't treat render phase updates as if// they were interleaved, for backwards compat reasons.if (deferRenderPhaseUpdateToNextBatch ||(executionContext & RenderContext) === NoContext) {workInProgressRootUpdatedLanes = mergeLanes(workInProgressRootUpdatedLanes,lane,);}if (workInProgressRootExitStatus === RootSuspendedWithDelay) {// The root already suspended with a delay, which means this render// definitely won't finish. Since we have a new update, let's mark it as// suspended now, right before marking the incoming update. This has the// effect of interrupting the current render and switching to the update.// TODO: Make sure this doesn't override pings that happen while we've// already started rendering.markRootSuspended(root, workInProgressRootRenderLanes);}}// TODO: requestUpdateLanePriority also reads the priority. Pass the// priority as an argument to that function and this one.// 获取当前优先级层次const priorityLevel = getCurrentPriorityLevel();// 同步任务，采用同步更新的方式if (lane === SyncLane) {if (// Check if we're inside unbatchedUpdates(executionContext & LegacyUnbatchedContext) !== NoContext &&// Check if we're not already rendering(executionContext & (RenderContext | CommitContext)) === NoContext) {// Register pending interactions on the root to avoid losing traced interaction data.// 同步而且没有react任务在执行，调用performSyncWorkOnRootschedulePendingInteractions(root, lane);// This is a legacy edge case. The initial mount of a ReactDOM.render-ed// root inside of batchedUpdates should be synchronous, but layout updates// should be deferred until the end of the batch.performSyncWorkOnRoot(root);} else {// 如果有正在执行的react任务，那么执行它ensureRootIsScheduled去复用当前正在执行的任务// 跟本次更新一起进行ensureRootIsScheduled(root, eventTime);schedulePendingInteractions(root, lane);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.resetRenderTimer();flushSyncCallbackQueue();}}} else {// Schedule a discrete update but only if it's not Sync.// 如果此次是异步任务if ((executionContext & DiscreteEventContext) !== NoContext &&// Only updates at user-blocking priority or greater are considered// discrete, even inside a discrete event.(priorityLevel === UserBlockingSchedulerPriority ||priorityLevel === ImmediateSchedulerPriority)) {// 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 Set([root]);} else {rootsWithPendingDiscreteUpdates.add(root);}}// Schedule other updates after in case the callback is sync.// 可以中断更新，只要调用ensureRootIsScheduled => performConcurrentWorkOnRootensureRootIsScheduled(root, eventTime);schedulePendingInteractions(root, lane);}// We use this when assigning a lane for a transition inside// `requestUpdateLane`. We assume it's the same as the root being updated,// since in the common case of a single root app it probably is. If it's not// the same root, then it's not a huge deal, we just might batch more stuff// together more than necessary.mostRecentlyUpdatedRoot = root;
}

同步任务类型执行机制

当任务的类型为同步任务，并且当前的js主线程空闲，会通过 performSyncWorkOnRoot(root) 方法开始执行同步任务。

performSyncWorkOnRoot 里面主要做了两件事：

• renderRootSync 从根节点开始进行同步渲染任务
• commitRoot 执行 commit 流程

当前js线程中有正在执行的任务时候，就会触发ensureRootIsScheduled函数。 ensureRootIsScheduled里面主要是处理当前加入的更新任务的lane是否有变化：

• 如果没有变化则表示跟当前的schedule一起执行。
• 如果有则创建新的schedule。
• 调用performSyncWorkOnRoot执行同步任务。

function ensureRootIsScheduled(root: FiberRoot, currentTime: number) {const existingCallbackNode = root.callbackNode;// Check if any lanes are being starved by other work. If so, mark them as// expired so we know to work on those next.markStarvedLanesAsExpired(root, currentTime);// Determine the next lanes to work on, and their priority.const nextLanes = getNextLanes(root,root === workInProgressRoot ? workInProgressRootRenderLanes : NoLanes,);// This returns the priority level computed during the `getNextLanes` call.const newCallbackPriority = returnNextLanesPriority();if (nextLanes === NoLanes) {// Special case: There's nothing to work on.if (existingCallbackNode !== null) {cancelCallback(existingCallbackNode);root.callbackNode = null;root.callbackPriority = NoLanePriority;}return;}// Check if there's an existing task. We may be able to reuse it.if (existingCallbackNode !== null) {const existingCallbackPriority = root.callbackPriority;if (existingCallbackPriority === newCallbackPriority) {// The priority hasn't changed. We can reuse the existing task. Exit.return;}// The priority changed. Cancel the existing callback. We'll schedule a new// one below.cancelCallback(existingCallbackNode);}// Schedule a new callback.let newCallbackNode;if (newCallbackPriority === SyncLanePriority) {// Special case: Sync React callbacks are scheduled on a special// internal queue// 同步任务调用performSyncWorkOnRootnewCallbackNode = scheduleSyncCallback(performSyncWorkOnRoot.bind(null, root),);} else if (newCallbackPriority === SyncBatchedLanePriority) {newCallbackNode = scheduleCallback(ImmediateSchedulerPriority,performSyncWorkOnRoot.bind(null, root),);} else {// 异步任务调用 performConcurrentWorkOnRootconst schedulerPriorityLevel = lanePriorityToSchedulerPriority(newCallbackPriority,);newCallbackNode = scheduleCallback(schedulerPriorityLevel,performConcurrentWorkOnRoot.bind(null, root),);}root.callbackPriority = newCallbackPriority;root.callbackNode = newCallbackNode;
}

所以任务类型为同步的时候，不管js线程空闲与否，都会走到performSyncWorkOnRoot，进而走renderRootSync、workLoopSync流程，而在workLoopSync中，只要workInProgress fiber不为null，则会一直循环执行performUnitOfWork，而performUnitOfWork中会去执行beginWork和completeWork，也就是上一章里面说的beginWork流程去创建每一个fiber节点

// packages/react-reconciler/src/ReactFiberWorkLoop.old.jsfunction workLoopSync() {while (workInProgress !== null) {performUnitOfWork(workInProgress);}
}

异步任务类型执行机制

异步任务则会去执行performConcurrentWorkOnRoot，进而去执行renderRootConcurrent、workLoopConcurrent，但是与同步任务不同的是异步任务是可以中断的，这个可中断的关键字就在于shouldYield，它本身返回值是一个false，为true则可以中断。

// packages/react-reconciler/src/ReactFiberWorkLoop.old.jsfunction workLoopConcurrent() {while (workInProgress !== null && !shouldYield()) {performUnitOfWork(workInProgress);}
}

每一次在执行performUnitOfWork之前都会关注一下shouldYield()返回值，也就是说的reconciler过程可中断的意思。

shouldYield

// packages\scheduler\src\SchedulerPostTask.js
export function unstable_shouldYield() {return getCurrentTime() >= deadline;
}

getCurrentTime为new Date()，deadline为浏览器处理每一帧结束时间戳，所以这里表示的是，在浏览器每一帧空闲的时候，才会去处理此任务，如果当前任务在浏览器执行的某一帧里面，则会中断当前任务，等待浏览器当前帧执行完毕，等到下一帧空闲的时候，才会去执行当前任务。

所以不管在workLoopConcurrent还是workLoopSync中，都会根据当前的workInProgress fiber是否为null来进行循环调用performUnitOfWork。根据流程图以及上面说的这一些，可以看得出来从beginWork到completeUnitOfWork这个过程究竟干了什么。

这三章将会讲解fiber树的reconcileChildren过程、completeWork过程、commitMutationEffects…insertOrAppendPlacementNodeIntoContainer(DOM)过程。这里将详细解读v17版本的React的diff算法、虚拟dom到真实dom的创建，函数生命钩子的执行流程等。

performUnitOfWork

function performUnitOfWork(unitOfWork: Fiber): void {// The current, flushed, state of this fiber is the alternate. Ideally// nothing should rely on this, but relying on it here means that we don't// need an additional field on the work in progress.const current = unitOfWork.alternate;setCurrentDebugFiberInDEV(unitOfWork);let next;if (enableProfilerTimer && (unitOfWork.mode & ProfileMode) !== NoMode) {startProfilerTimer(unitOfWork);next = beginWork(current, unitOfWork, subtreeRenderLanes);stopProfilerTimerIfRunningAndRecordDelta(unitOfWork, true);} else {// beginWorknext = beginWork(current, unitOfWork, subtreeRenderLanes);}resetCurrentDebugFiberInDEV();unitOfWork.memoizedProps = unitOfWork.pendingProps;if (next === null) {// If this doesn't spawn new work, complete the current work.// completeUnitOfWorkcompleteUnitOfWork(unitOfWork);} else {workInProgress = next;}ReactCurrentOwner.current = null;
}

所以在performUnitOfWork里面，每一次执行beginWork，进行workIngProgress更新，当遍历完毕整棵fiber树之后便会执行completeUnitOfWork。

beginWork

我们可以看到beginWork就是originBeginWork得实际执行。我们翻开beginWork的源码可以看到，它便是根据不同的workInProgress.tag执行不同组件类型的处理函数，这里就不去拆分的太细，只有有想法便会单独出一篇文章讲述这个的细节，但是最后都会去调用reconcileChildren。

completeUnitOfWork

当遍历完毕执行beginWork，遍历完毕之后就会走completeUnitOfWork。

function completeUnitOfWork(unitOfWork: Fiber): void {// Attempt to complete the current unit of work, then move to the next// sibling. If there are no more siblings, return to the parent fiber.let completedWork = unitOfWork;do {// The current, flushed, state of this fiber is the alternate. Ideally// nothing should rely on this, but relying on it here means that we don't// need an additional field on the work in progress.const current = completedWork.alternate;const returnFiber = completedWork.return;// Check if the work completed or if something threw.if ((completedWork.flags & Incomplete) === NoFlags) {setCurrentDebugFiberInDEV(completedWork);let next;if (!enableProfilerTimer ||(completedWork.mode & ProfileMode) === NoMode) {// 绑定事件，更新props，更新domnext = completeWork(current, completedWork, subtreeRenderLanes);} else {startProfilerTimer(completedWork);next = completeWork(current, completedWork, subtreeRenderLanes);// Update render duration assuming we didn't error.stopProfilerTimerIfRunningAndRecordDelta(completedWork, false);}resetCurrentDebugFiberInDEV();if (next !== null) {// Completing this fiber spawned new work. Work on that next.workInProgress = next;return;}resetChildLanes(completedWork);if (returnFiber !== null &&// Do not append effects to parents if a sibling failed to complete(returnFiber.flags & Incomplete) === NoFlags) {// Append all the effects of the subtree and this fiber onto the effect// list of the parent. The completion order of the children affects the// side-effect order.// 把已收集到的副作用，合并到父级effect lists中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;}// If this fiber had side-effects, we append it AFTER the children's// side-effects. We can perform certain side-effects earlier if needed,// by doing multiple passes over the effect list. We don't want to// schedule our own side-effect on our own list because if end up// reusing children we'll schedule this effect onto itself since we're// at the end.const flags = completedWork.flags;// Skip both NoWork and PerformedWork tags when creating the effect// list. PerformedWork effect is read by React DevTools but shouldn't be// committed.// 跳过NoWork，PerformedWork在commit阶段用不到if (flags > PerformedWork) {if (returnFiber.lastEffect !== null) {returnFiber.lastEffect.nextEffect = completedWork;} else {returnFiber.firstEffect = completedWork;}returnFiber.lastEffect = completedWork;}}} else {// This fiber did not complete because something threw. Pop values off// the stack without entering the complete phase. If this is a boundary,// capture values if possible.const next = unwindWork(completedWork, subtreeRenderLanes);// Because this fiber did not complete, don't reset its expiration time.if (next !== null) {// If completing this work spawned new work, do that next. We'll come// back here again.// Since we're restarting, remove anything that is not a host effect// from the effect tag.next.flags &= HostEffectMask;workInProgress = next;return;}if (enableProfilerTimer &&(completedWork.mode & ProfileMode) !== NoMode) {// Record the render duration for the fiber that errored.stopProfilerTimerIfRunningAndRecordDelta(completedWork, false);// Include the time spent working on failed children before continuing.let actualDuration = completedWork.actualDuration;let child = completedWork.child;while (child !== null) {actualDuration += child.actualDuration;child = child.sibling;}completedWork.actualDuration = actualDuration;}if (returnFiber !== null) {// Mark the parent fiber as incomplete and clear its effect list.returnFiber.firstEffect = returnFiber.lastEffect = null;returnFiber.flags |= Incomplete;}}// 兄弟层指针const siblingFiber = completedWork.sibling;if (siblingFiber !== null) {// If there is more work to do in this returnFiber, do that next.workInProgress = siblingFiber;return;}// Otherwise, return to the parentcompletedWork = returnFiber;// Update the next thing we're working on in case something throws.workInProgress = completedWork;} while (completedWork !== null);// We've reached the root.if (workInProgressRootExitStatus === RootIncomplete) {workInProgressRootExitStatus = RootCompleted;}
}

他的作用便是逐层收集fiber树上已经被打上的副作用标签flags，一直收集到root上面以便于在commit阶段进行dom的增删改。

scheduler流程

在这里应该有很多人不明白，协调和调度是什么意思，通俗来讲：

• 协调就是协同合作
• 调度就是执行命令

所以在React中协调就是一个js线程中，需要安排很多模块去完成整个流程，例如：同步异步lane的处理，reconcileChildren处理fiber节点等，保证整个流程有条不紊的执行。调度表现为让空闲的js线程（帧层面）去执行其他任务，这个过程称之为调度，那么它到底是怎么去做的呢？
我们回到处理异步任务那里，我们会发现performConcurrentWorkOnRoot这个函数外面包裹了一层scheduleCallback：

newCallbackNode = scheduleCallback(schedulerPriorityLevel,performConcurrentWorkOnRoot.bind(null, root),
)

export function scheduleCallback(reactPriorityLevel: ReactPriorityLevel,  callback: SchedulerCallback,  options: SchedulerCallbackOptions | void | null,
) {const priorityLevel = reactPriorityToSchedulerPriority(reactPriorityLevel);return Scheduler_scheduleCallback(priorityLevel, callback, options);
}

我们几经周折找到了声明函数的地方

// packages/scheduler/src/Scheduler.js
function unstable_scheduleCallback(priorityLevel, callback, options) {var currentTime = getCurrentTime();var startTime;if (typeof options === 'object' && options !== null) {var delay = options.delay;if (typeof delay === 'number' && delay > 0) {startTime = currentTime + delay;} else {startTime = currentTime;}} else {startTime = currentTime;}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;var newTask = {id: taskIdCounter++,callback,priorityLevel,startTime,expirationTime,sortIndex: -1,};if (enableProfiling) {newTask.isQueued = false;}if (startTime > currentTime) {// This is a delayed task.newTask.sortIndex = startTime;push(timerQueue, newTask);if (peek(taskQueue) === null && newTask === peek(timerQueue)) {// All tasks are delayed, and this is the task with the earliest delay.if (isHostTimeoutScheduled) {// Cancel an existing timeout.cancelHostTimeout();} else {isHostTimeoutScheduled = true;}// Schedule a timeout.requestHostTimeout(handleTimeout, startTime - currentTime);}} else {newTask.sortIndex = expirationTime;push(taskQueue, newTask);if (enableProfiling) {markTaskStart(newTask, currentTime);newTask.isQueued = true;}// Schedule a host callback, if needed. If we're already performing work,// wait until the next time we yield.if (!isHostCallbackScheduled && !isPerformingWork) {isHostCallbackScheduled = true;requestHostCallback(flushWork);}}return newTask;
}

• 当starttime > currentTime的时候，表示任务超时，插入超时队列。
• 任务没有超时，插入调度队列
• 执行requestHostCallback调度任务。

  // 创建消息通道const channel = new MessageChannel();const port = channel.port2;channel.port1.onmessage = performWorkUntilDeadline;// 告知scheduler开始调度requestHostCallback = function(callback) {scheduledHostCallback = callback;if (!isMessageLoopRunning) {isMessageLoopRunning = true;port.postMessage(null);}};

react通过 new MessageChannel() 创建了消息通道，当发现js线程空闲时，通过postMessage通知 scheduler开始调度。performWorkUntilDeadline函数功能为处理react调度开始时间更新到结束时间。
这里我们要关注一下设备帧速率。

  forceFrameRate = function(fps) {if (fps < 0 || fps > 125) {// Using console['error'] to evade Babel and ESLintconsole['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 framerateyieldInterval = 5;}};

performWorkUntilDeadline

  const performWorkUntilDeadline = () => {if (scheduledHostCallback !== null) {const currentTime = getCurrentTime();// Yield after `yieldInterval` ms, regardless of where we are in the vsync// cycle. This means there's always time remaining at the beginning of// the message event.// 更新当前帧结束时间deadline = currentTime + yieldInterval;const hasTimeRemaining = true;try {const hasMoreWork = scheduledHostCallback(hasTimeRemaining,currentTime,);// 还有任务就继续执行if (!hasMoreWork) {isMessageLoopRunning = false;scheduledHostCallback = null;} else {// If there's more work, schedule the next message event at the end// of the preceding one.// 没有就postMessageport.postMessage(null);}} catch (error) {// If a scheduler task throws, exit the current browser task so the// error can be observed.port.postMessage(null);throw error;}} else {isMessageLoopRunning = false;}// Yielding to the browser will give it a chance to paint, so we can// reset this.needsPaint = false;};

总结

本文讲了React在状态改变的时候，会根据当前任务优先级，等一些列操作去创建workInProgress fiber链表树，在协调阶段，会根据浏览器每一帧去做比较，假如浏览器每一帧执行时间戳高于当前时间，则表示当前帧没有空闲时间，当前任务则必须要等到下一个空闲帧才能去执行的可中断的策略。还有关于beginWork的遍历执行更新fiber的节点。那么到这里这一章就讲述完毕了，下一章讲一讲React的diff算法