scheduler工作阶段在React内部被称为schedule阶段。

在《React源码3》，我们已经将update加入队列并返回到了根容器节点root。

function updateContainer(element, container, parentComponent, callback) {//前面略过var root = enqueueUpdate(current$1, update, lane);if (root !== null) {scheduleUpdateOnFiber(root, current$1, lane, eventTime);entangleTransitions(root, current$1, lane);}return lane;
}

scheduleUpdateOnFiber函数是React中Schedule模块主要函数，用于调度更新，下面几种情况都会调用scheduleUpdateOnFiber函数：

• 页面初次渲染
• 类组件setState/forceUpdate
• 函数组件setState

Schedule模块流程图

一、schdeuleUpdateOnFiber函数

function scheduleUpdateOnFiber(root, fiber, lane, eventTime) {checkForNestedUpdates();{if (isRunningInsertionEffect) {error('useInsertionEffect must not schedule updates.');}}{if (isFlushingPassiveEffects) {didScheduleUpdateDuringPassiveEffects = true;}} // Mark that the root has a pending update.markRootUpdated(root, lane, eventTime);if ((executionContext & RenderContext) !== NoLanes && root === workInProgressRoot) {// This update was dispatched during the render phase. This is a mistake// if the update originates from user space (with the exception of local// hook updates, which are handled differently and don't reach this// function), but there are some internal React features that use this as// an implementation detail, like selective hydration.warnAboutRenderPhaseUpdatesInDEV(fiber); // Track lanes that were updated during the render phase} else {// This is a normal update, scheduled from outside the render phase. For// example, during an input event.{if (isDevToolsPresent) {addFiberToLanesMap(root, fiber, lane);}}warnIfUpdatesNotWrappedWithActDEV(fiber);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 ( (executionContext & RenderContext) === NoContext) {workInProgressRootInterleavedUpdatedLanes = mergeLanes(workInProgressRootInterleavedUpdatedLanes, 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$1(root, workInProgressRootRenderLanes);}}ensureRootIsScheduled(root, eventTime);if (lane === SyncLane && executionContext === NoContext && (fiber.mode & ConcurrentMode) === NoMode && // Treat `act` as if it's inside `batchedUpdates`, even in legacy mode.!( ReactCurrentActQueue$1.isBatchingLegacy)) {// 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();flushSyncCallbacksOnlyInLegacyMode();}}
}

二、markRootUpdated函数      

负责标记根节点有新任务，并为调度系统提供最新的优先级和时间信息。

function markRootUpdated(root, updateLane, eventTime) {root.pendingLanes |= updateLane; // If there are any suspended transitions, it's possible this new update// could unblock them. Clear the suspended lanes so that we can try rendering// them again.//// TODO: We really only need to unsuspend only lanes that are in the// `subtreeLanes` of the updated fiber, or the update lanes of the return// path. This would exclude suspended updates in an unrelated sibling tree,// since there's no way for this update to unblock it.//// We don't do this if the incoming update is idle, because we never process// idle updates until after all the regular updates have finished; there's no// way it could unblock a transition.if (updateLane !== IdleLane) {root.suspendedLanes = NoLanes;root.pingedLanes = NoLanes;}var eventTimes = root.eventTimes;var index = laneToIndex(updateLane); // We can always overwrite an existing timestamp because we prefer the most// recent event, and we assume time is monotonically increasing.eventTimes[index] = eventTime;
}

三、ensureRootIsScheduled函数

确保当前 FiberRoot 上有一个合适优先级的调度任务被安排，即根据最新的更新优先级，决定是否需要新建、复用或取消调度任务，并最终调度同步或异步的渲染入口。

1. 获取下一个需要处理的 lanes（优先级），如果没有任务，取消已存在的调度任务。
2. 判断当前任务优先级是否和已存在的调度任务一致，如果一致则复用，不一致则取消旧任务。
3. 根据优先级安排调度：
   • 如果是同步优先级（SyncLane），调度 performSyncWorkOnRoot。
   • 否则，调度performConcurrentWorkOnRoot ，并传递合适的 scheduler 优先级。
4. 将调度任务的引用和优先级记录到 root 上。

function ensureRootIsScheduled(root, currentTime) {var 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.var nextLanes = getNextLanes(root, root === workInProgressRoot ? workInProgressRootRenderLanes : NoLanes);if (nextLanes === NoLanes) {// Special case: There's nothing to work on.if (existingCallbackNode !== null) {cancelCallback$1(existingCallbackNode);}root.callbackNode = null;root.callbackPriority = NoLane;return;} // We use the highest priority lane to represent the priority of the callback.var newCallbackPriority = getHighestPriorityLane(nextLanes); // Check if there's an existing task. We may be able to reuse it.var existingCallbackPriority = root.callbackPriority;if (existingCallbackPriority === newCallbackPriority && // Special case related to `act`. If the currently scheduled task is a// Scheduler task, rather than an `act` task, cancel it and re-scheduled// on the `act` queue.!( ReactCurrentActQueue$1.current !== null && existingCallbackNode !== fakeActCallbackNode)) {{// If we're going to re-use an existing task, it needs to exist.// Assume that discrete update microtasks are non-cancellable and null.// TODO: Temporary until we confirm this warning is not fired.if (existingCallbackNode == null && existingCallbackPriority !== SyncLane) {error('Expected scheduled callback to exist. This error is likely caused by a bug in React. Please file an issue.');}} // The priority hasn't changed. We can reuse the existing task. Exit.return;}if (existingCallbackNode != null) {// Cancel the existing callback. We'll schedule a new one below.cancelCallback$1(existingCallbackNode);} // Schedule a new callback.var newCallbackNode;if (newCallbackPriority === SyncLane) {// Special case: Sync React callbacks are scheduled on a special// internal queueif (root.tag === LegacyRoot) {if ( ReactCurrentActQueue$1.isBatchingLegacy !== null) {ReactCurrentActQueue$1.didScheduleLegacyUpdate = true;}scheduleLegacySyncCallback(performSyncWorkOnRoot.bind(null, root));} else {scheduleSyncCallback(performSyncWorkOnRoot.bind(null, root));}{// Flush the queue in a microtask.if ( ReactCurrentActQueue$1.current !== null) {// Inside `act`, use our internal `act` queue so that these get flushed// at the end of the current scope even when using the sync version// of `act`.ReactCurrentActQueue$1.current.push(flushSyncCallbacks);} else {scheduleMicrotask(function () {// In Safari, appending an iframe forces microtasks to run.// https://github.com/facebook/react/issues/22459// We don't support running callbacks in the middle of render// or commit so we need to check against that.if ((executionContext & (RenderContext | CommitContext)) === NoContext) {// Note that this would still prematurely flush the callbacks// if this happens outside render or commit phase (e.g. in an event).flushSyncCallbacks();}});}}newCallbackNode = null;} else {var schedulerPriorityLevel;switch (lanesToEventPriority(nextLanes)) {case DiscreteEventPriority:schedulerPriorityLevel = ImmediatePriority;break;case ContinuousEventPriority:schedulerPriorityLevel = UserBlockingPriority;break;case DefaultEventPriority:schedulerPriorityLevel = NormalPriority;break;case IdleEventPriority:schedulerPriorityLevel = IdlePriority;break;default:schedulerPriorityLevel = NormalPriority;break;}newCallbackNode = scheduleCallback$1(schedulerPriorityLevel, performConcurrentWorkOnRoot.bind(null, root));}root.callbackPriority = newCallbackPriority;root.callbackNode = newCallbackNode;
} // This is the entry point for every concurrent task, i.e. anything that
// goes through Scheduler.

四、performSyncWorkOnRoot函数

以同步方式执行根节点的渲染和提交流程，用于处理最高优先级（SyncLane）的更新。

function performSyncWorkOnRoot(root) {{syncNestedUpdateFlag();}if ((executionContext & (RenderContext | CommitContext)) !== NoContext) {throw new Error('Should not already be working.');}flushPassiveEffects();var lanes = getNextLanes(root, NoLanes);if (!includesSomeLane(lanes, SyncLane)) {// There's no remaining sync work left.ensureRootIsScheduled(root, now());return null;}var exitStatus = renderRootSync(root, lanes);if (root.tag !== LegacyRoot && exitStatus === RootErrored) {// If something threw an error, try rendering one more time. We'll render// synchronously to block concurrent data mutations, and we'll includes// all pending updates are included. If it still fails after the second// attempt, we'll give up and commit the resulting tree.var errorRetryLanes = getLanesToRetrySynchronouslyOnError(root);if (errorRetryLanes !== NoLanes) {lanes = errorRetryLanes;exitStatus = recoverFromConcurrentError(root, errorRetryLanes);}}if (exitStatus === RootFatalErrored) {var fatalError = workInProgressRootFatalError;prepareFreshStack(root, NoLanes);markRootSuspended$1(root, lanes);ensureRootIsScheduled(root, now());throw fatalError;}if (exitStatus === RootDidNotComplete) {throw new Error('Root did not complete. This is a bug in React.');} // We now have a consistent tree. Because this is a sync render, we// will commit it even if something suspended.var finishedWork = root.current.alternate;root.finishedWork = finishedWork;root.finishedLanes = lanes;commitRoot(root, workInProgressRootRecoverableErrors, workInProgressTransitions); // Before exiting, make sure there's a callback scheduled for the next// pending level.ensureRootIsScheduled(root, now());return null;
}function flushRoot(root, lanes) {if (lanes !== NoLanes) {markRootEntangled(root, mergeLanes(lanes, SyncLane));ensureRootIsScheduled(root, now());if ((executionContext & (RenderContext | CommitContext)) === NoContext) {resetRenderTimer();flushSyncCallbacks();}}
}

五、performConcurrentWorkOnRoot函数

并发渲染的核心入口，负责调度和执行并发模式下的 Fiber 树渲染。

function performConcurrentWorkOnRoot(root, didTimeout) {{resetNestedUpdateFlag();} // Since we know we're in a React event, we can clear the current// event time. The next update will compute a new event time.currentEventTime = NoTimestamp;currentEventTransitionLane = NoLanes;if ((executionContext & (RenderContext | CommitContext)) !== NoContext) {throw new Error('Should not already be working.');} // Flush any pending passive effects before deciding which lanes to work on,// in case they schedule additional work.var originalCallbackNode = root.callbackNode;var didFlushPassiveEffects = flushPassiveEffects();if (didFlushPassiveEffects) {// Something in the passive effect phase may have canceled the current task.// Check if the task node for this root was changed.if (root.callbackNode !== originalCallbackNode) {// The current task was canceled. Exit. We don't need to call// `ensureRootIsScheduled` because the check above implies either that// there's a new task, or that there's no remaining work on this root.return null;}} // Determine the next lanes to work on, using the fields stored// on the root.var lanes = getNextLanes(root, root === workInProgressRoot ? workInProgressRootRenderLanes : NoLanes);if (lanes === NoLanes) {// Defensive coding. This is never expected to happen.return null;} // We disable time-slicing in some cases: if the work has been CPU-bound// for too long ("expired" work, to prevent starvation), or we're in// sync-updates-by-default mode.// TODO: We only check `didTimeout` defensively, to account for a Scheduler// bug we're still investigating. Once the bug in Scheduler is fixed,// we can remove this, since we track expiration ourselves.var shouldTimeSlice = !includesBlockingLane(root, lanes) && !includesExpiredLane(root, lanes) && ( !didTimeout);var exitStatus = shouldTimeSlice ? renderRootConcurrent(root, lanes) : renderRootSync(root, lanes);if (exitStatus !== RootInProgress) {if (exitStatus === RootErrored) {// If something threw an error, try rendering one more time. We'll// render synchronously to block concurrent data mutations, and we'll// includes all pending updates are included. If it still fails after// the second attempt, we'll give up and commit the resulting tree.var errorRetryLanes = getLanesToRetrySynchronouslyOnError(root);if (errorRetryLanes !== NoLanes) {lanes = errorRetryLanes;exitStatus = recoverFromConcurrentError(root, errorRetryLanes);}}if (exitStatus === RootFatalErrored) {var fatalError = workInProgressRootFatalError;prepareFreshStack(root, NoLanes);markRootSuspended$1(root, lanes);ensureRootIsScheduled(root, now());throw fatalError;}if (exitStatus === RootDidNotComplete) {// The render unwound without completing the tree. This happens in special// cases where need to exit the current render without producing a// consistent tree or committing.//// This should only happen during a concurrent render, not a discrete or// synchronous update. We should have already checked for this when we// unwound the stack.markRootSuspended$1(root, lanes);} else {// The render completed.// Check if this render may have yielded to a concurrent event, and if so,// confirm that any newly rendered stores are consistent.// TODO: It's possible that even a concurrent render may never have yielded// to the main thread, if it was fast enough, or if it expired. We could// skip the consistency check in that case, too.var renderWasConcurrent = !includesBlockingLane(root, lanes);var finishedWork = root.current.alternate;if (renderWasConcurrent && !isRenderConsistentWithExternalStores(finishedWork)) {// A store was mutated in an interleaved event. Render again,// synchronously, to block further mutations.exitStatus = renderRootSync(root, lanes); // We need to check again if something threwif (exitStatus === RootErrored) {var _errorRetryLanes = getLanesToRetrySynchronouslyOnError(root);if (_errorRetryLanes !== NoLanes) {lanes = _errorRetryLanes;exitStatus = recoverFromConcurrentError(root, _errorRetryLanes); // We assume the tree is now consistent because we didn't yield to any// concurrent events.}}if (exitStatus === RootFatalErrored) {var _fatalError = workInProgressRootFatalError;prepareFreshStack(root, NoLanes);markRootSuspended$1(root, lanes);ensureRootIsScheduled(root, now());throw _fatalError;}} // We now have a consistent tree. The next step is either to commit it,// or, if something suspended, wait to commit it after a timeout.root.finishedWork = finishedWork;root.finishedLanes = lanes;finishConcurrentRender(root, exitStatus, lanes);}}ensureRootIsScheduled(root, now());if (root.callbackNode === originalCallbackNode) {// The task node scheduled for this root is the same one that's// currently executed. Need to return a continuation.return performConcurrentWorkOnRoot.bind(null, root);}return null;
}

六、prepareFreshStack函数

prepareFreshStack的作用是为一次新的 Fiber 树渲染初始化全局状态，主要包括：

• 重置当前 root 的渲染相关状态（如 finishedWork、finishedLanes 等）。
• 取消上一次渲染遗留的超时（timeout）。
• 如果上一次渲染被中断，清理未完成的 workInProgress。
• 创建新的 workInProgress Fiber 树（即 root.current 的 alternate），并将全局变量 workInProgress指向它。
• 设置本次渲染的 lanes（优先级）。
• 重置本次渲染相关的全局变量（如 workInProgressRoot、workInProgressRootRenderLanes 等）。
• 清空上次渲染遗留的错误、跳过的 lanes、pinged lanes 等。

function prepareFreshStack(root, lanes) {root.finishedWork = null;root.finishedLanes = NoLanes;var timeoutHandle = root.timeoutHandle;if (timeoutHandle !== noTimeout) {// The root previous suspended and scheduled a timeout to commit a fallback// state. Now that we have additional work, cancel the timeout.root.timeoutHandle = noTimeout; // $FlowFixMe Complains noTimeout is not a TimeoutID, despite the check abovecancelTimeout(timeoutHandle);}if (workInProgress !== null) {var interruptedWork = workInProgress.return;while (interruptedWork !== null) {var current = interruptedWork.alternate;unwindInterruptedWork(current, interruptedWork);interruptedWork = interruptedWork.return;}}workInProgressRoot = root;var rootWorkInProgress = createWorkInProgress(root.current, null);workInProgress = rootWorkInProgress;workInProgressRootRenderLanes = subtreeRenderLanes = workInProgressRootIncludedLanes = lanes;workInProgressRootExitStatus = RootInProgress;workInProgressRootFatalError = null;workInProgressRootSkippedLanes = NoLanes;workInProgressRootInterleavedUpdatedLanes = NoLanes;workInProgressRootPingedLanes = NoLanes;workInProgressRootConcurrentErrors = null;workInProgressRootRecoverableErrors = null;finishQueueingConcurrentUpdates();{ReactStrictModeWarnings.discardPendingWarnings();}return rootWorkInProgress;
}