1、setState,useEffect等hook执行时会触发调度器调度。

在useState函数调用栈一章中的第6段讲到会将更新任务使用scheduleCallback方法加入调度器的调度队 列。而useEffect函数调用栈一章也讲到副作用的处理也是通过scheduleCallback方法加入调度队列进行调度。这两点也导致了setState时，state值发生改变的异步现象，和useEffect副作用的执行发生在渲染完成之后的现象。所以调度器触发的入口函数应当就是在这些地方用到的scheduleCallback方法： 在packages\react-reconciler\src\Scheduler.js源代码中你可以看到这样一段代码： 这表明scheduleCallback方法来自Scheduler的unstable_scheduleCallback。

2、unstable_scheduleCallback

在packages\scheduler\src\forks\Scheduler.js中你可以很容易的找到该方法的源码：

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 85 function unstable_scheduleCallback(
  priorityLevel: PriorityLevel,
  callback: Callback,
  options?: {delay: number},
): Task {
  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: Task = {
    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();
    }
  }

  return newTask;
}

3、调度循环

要不断的处理调度队列中的任务当然要进行循环了，可以参考浏览器的事件循环。沿着 unstable_scheduleCallback->requestHostCallback->schedulePerformWorkUntilDeadline->performWorkUntilDeadline->flushWork->workLoop的调用栈，你会找到该循环：

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 66 function workLoop(initialTime: number) {
  let currentTime = initialTime;
  advanceTimers(currentTime);
  currentTask = peek(taskQueue);
  while (
    currentTask !== null &&
    !(enableSchedulerDebugging && isSchedulerPaused)
  ) {
    if (currentTask.expirationTime > currentTime && shouldYieldToHost()) {
      // This currentTask hasn't expired, and we've reached the deadline.
      break;
    }
    // $FlowFixMe[incompatible-use] found when upgrading Flow
    const callback = currentTask.callback;
    if (typeof callback === 'function') {
      // $FlowFixMe[incompatible-use] found when upgrading Flow
      currentTask.callback = null;
      // $FlowFixMe[incompatible-use] found when upgrading Flow
      currentPriorityLevel = currentTask.priorityLevel;
      // $FlowFixMe[incompatible-use] found when upgrading Flow
      const didUserCallbackTimeout = currentTask.expirationTime <= currentTime;
      if (enableProfiling) {
        // $FlowFixMe[incompatible-call] found when upgrading Flow
        markTaskRun(currentTask, currentTime);
      }
      const continuationCallback = callback(didUserCallbackTimeout);
      currentTime = getCurrentTime();
      if (typeof continuationCallback === 'function') {
        // If a continuation is returned, immediately yield to the main thread
        // regardless of how much time is left in the current time slice.
        // $FlowFixMe[incompatible-use] found when upgrading Flow
        currentTask.callback = continuationCallback;
        if (enableProfiling) {
          // $FlowFixMe[incompatible-call] found when upgrading Flow
          markTaskYield(currentTask, currentTime);
        }
        advanceTimers(currentTime);
        return true;
      } else {
        if (enableProfiling) {
          // $FlowFixMe[incompatible-call] found when upgrading Flow
          markTaskCompleted(currentTask, currentTime);
          // $FlowFixMe[incompatible-use] found when upgrading Flow
          currentTask.isQueued = false;
        }
        if (currentTask === peek(taskQueue)) {
          pop(taskQueue);
        }
        advanceTimers(currentTime);
      }
    } else {
      pop(taskQueue);
    }
    currentTask = peek(taskQueue);
  }
  // Return whether there's additional work
  if (currentTask !== null) {
    return true;
  } else {
    const firstTimer = peek(timerQueue);
    if (firstTimer !== null) {
      requestHostTimeout(handleTimeout, firstTimer.startTime - currentTime);
    }
    return false;
  }
}

4、中断和恢复

为了避免长任务阻塞，react需要在一定时间内中断任务执行，等待浏览器处理其他任务之后再恢复执行。 在workLoop的源码中你能看到诸多循环终止的代码，在某些条件下会触发。进而中断这个过程而至于恢复，沿着workLoop->requestHostTimeout->localSetTimeout以及：

可见：react在中断任务时会使用js的定时器，在一段时间后重新执行调度任务。

总结：调度器其实就是循环处理调度列表，在达到一定时间片长度时还未执行完会发生中断避免长任务阻塞，但是在中断时会使用定时器在一定时间后继续执行。