react源码层分析协调与调度

目录

• requestEventTime
• requestUpdateLane
• findUpdateLane
• lanePriority LanePriority
• createUpdate
• enqueueUpdate
• 总结
• 协调与调度
• reconciler流程
• 同步任务类型执行机制
• 异步任务类型执行机制
• shouldYield
• performUnitOfWork
• beginWork
• completeUnitOfWork
• scheduler流程
• performWorkUntilDeadline
• 总结

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 === -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;
}

• 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 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;
}

• 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,
  );
}

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.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;
}

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;
}

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.js
function 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.js
function 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 {
    // 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;
}

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;
  }
}

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 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;
    }
  };

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;
  };

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

到此这篇关于react源码层分析协调与调度的文章就介绍到这了,更多相关react协调与调度内容请搜索我们以前的文章或继续浏览下面的相关文章希望大家以后多多支持我们！