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react 源码中的协调与调度

作者:flyzz177
  • 2022-11-08
    浙江
  • 本文字数:17690 字

    阅读完需:约 58 分钟

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.jsexport 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 === -1  if (currentEventTime !== NoTimestamp) {     // 浏览器事件正在执行,返回上次的 currentEventTime    return currentEventTime;  }  // 重新计算currentEventTime,当执行被中断后  currentEventTime = now();  return currentEventTime;}
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  • RenderContextCommitContext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;
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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.jsexport function requestUpdateLane(fiber: Fiber): Lane {  // Special cases  const 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lane lane = findUpdateLane(InputDiscreteLanePriority, currentEventWipLanes); } else { // 根据优先级计算法则计算lane const 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 lane lane = findUpdateLane(schedulerLanePriority, currentEventWipLanes); }
return lane;}
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  • 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 level        return findUpdateLane(InputContinuousLanePriority, wipLanes);      }      return lane;    }    case InputContinuousLanePriority: {      const lane = pickArbitraryLane(InputContinuousLanes & ~wipLanes);      if (lane === NoLane) {        // Shift to the next priority level        return 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 instead    case RetryLanePriority: // Should be handled by findRetryLane instead      break;    case IdleLanePriority:      let lane = pickArbitraryLane(IdleLanes & ~wipLanes);      if (lane === NoLane) {        lane = pickArbitraryLane(IdleLanes);      }      return lane;    default:      // The remaining priorities are not valid for updates      break;  }  invariant(    false,    'Invalid update priority: %s. This is a bug in React.',    lanePriority,  );}
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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;
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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;}
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createUpdate函数入参为eventTimelane,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
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  • 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; } }}
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  • 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

协调调度的流程大致如图所示:


reconciler 流程

Reactreconciler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fiberlane,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.fiberLanes const 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performSyncWorkOnRoot schedulePendingInteractions(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 => performConcurrentWorkOnRoot ensureRootIsScheduled(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;}
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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performSyncWorkOnRoot newCallbackNode = scheduleSyncCallback( performSyncWorkOnRoot.bind(null, root), ); } else if (newCallbackPriority === SyncBatchedLanePriority) { newCallbackNode = scheduleCallback( ImmediateSchedulerPriority, performSyncWorkOnRoot.bind(null, root), ); } else { // 异步任务调用 performConcurrentWorkOnRoot const schedulerPriorityLevel = lanePriorityToSchedulerPriority( newCallbackPriority, ); newCallbackNode = scheduleCallback( schedulerPriorityLevel, performConcurrentWorkOnRoot.bind(null, root), ); }
root.callbackPriority = newCallbackPriority; root.callbackNode = newCallbackNode;}
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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renderRootSyncworkLoopSync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beginWorkcompleteWork,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.js
function workLoopSync() { while (workInProgress !== null) { performUnitOfWork(workInProgress); }}
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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renderRootConcurrentworkLoopConcurrent,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.js
function workLoopConcurrent() { while (workInProgress !== null && !shouldYield()) { performUnitOfWork(workInProgress); }}
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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.jsexport function unstable_shouldYield() {  return getCurrentTime() >= deadline;}
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getCurrentTimenew 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beginWorkcompleteUnitOfWork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Reactdiff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 { // beginWork next = beginWork(current, unitOfWork, subtreeRenderLanes); }
resetCurrentDebugFiberInDEV(); unitOfWork.memoizedProps = unitOfWork.pendingProps; if (next === null) { // If this doesn't spawn new work, complete the current work. // completeUnitOfWork completeUnitOfWork(unitOfWork); } else { workInProgress = next; }
ReactCurrentOwner.current = null;}
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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dom next = 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 parent completedWork = 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; }}
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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增删改


scheduler 流程

在这里应该有很多人不明白,协调调度是什么意思,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),)
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export function scheduleCallback(  reactPriorityLevel: ReactPriorityLevel,  callback: SchedulerCallback,  options: SchedulerCallbackOptions | void | null,) {  const priorityLevel = reactPriorityToSchedulerPriority(reactPriorityLevel);  return Scheduler_scheduleCallback(priorityLevel, callback, options);}
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我们几经周折找到了声明函数的地方


// packages/scheduler/src/Scheduler.jsfunction 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;}
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  • 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); } };
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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 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;    }  };
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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postMessage          port.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;  };
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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


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