为什么需要调度

在 React 中，组件最终体现为 Fiber，并形成 FiberTree，Fiber 的目的是提高渲染性能，将原先的 React 渲染任务拆分为多个小的微任务，这样做的目的是可以灵活的让出主线程，可以随时打断渲染，后面再继续执行。由于需要让出主线程，需要将任务保存起来，一个个排队执行，需要的时候进行切换，为了实现排队、切换，就需要实现一个调度引擎，哪个任务先执行，哪个任务后执行，暂停之后怎么继续执行。

优先级队列

React 中，Scheduler.js 定义了定义了调度的核心逻辑，Task 包存在一个 PriorityQueue 中，优先级高的任务会先进行处理。

WorkLoop

React 中，WorkLoop 是指循环Fiber Tree 去找需要处理的任务，WorkLoop 分为同步和并发，这两个逻辑几乎一样，并发处理是，要检查是否要暂停并释放主线程给浏览器执行其他任务。

## 同步，不能中断
function workLoopSync() {
  while (workInProgress !== null) {
    performUnitOfWork(workInProgress);
  }
}

function workLoopConcurrent() {
  while (workInProgress !== null && !shouldYield()) {
    performUnitOfWork(workInProgress);
  }
}

shouldYield 大于 5 毫秒就会释放，打开 React Tools Profile 可以看到这个渲染过程

function shouldYieldToHost() {
  const timeElapsed = getCurrentTime() - startTime;
  if (timeElapsed < frameInterval) {
    // The main thread has only been blocked for a really short amount of time;
    // smaller than a single frame. Don't yield yet.
    return false;
  }

调度

首先，调度创建任务，任务通过 callback function 创建，源码 Sheduler.js -> unstable_scheduleCallback，可以看到，优先级是通过过期时间进行定义的，越早过期的优先级越高，正常的任务 5 秒过期，USER_BLOCKING_PRIORITY_TIMEOUT 阻碍用户操作的任务 250 毫秒。

  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,
  };

任务加入队列并执行

  if (startTime > currentTime) {
    // This is a delayed task.
    newTask.sortIndex = startTime;
    ##########加入队列
    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;
	##########加入队列
    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);
    }

requestHostCallback 最终会调用 schedulePerformWorkUntilDeadline，schedulePerformWorkUntilDeadline会调用 performWorkUntilDeadline。

### 调用 schedulePerformWorkUntilDeadline
function requestHostCallback(callback) {
  scheduledHostCallback = callback;
  if (!isMessageLoopRunning) {
    isMessageLoopRunning = true;
    schedulePerformWorkUntilDeadline();
  }
}

## 最终调用 performWorkUntilDeadline
let schedulePerformWorkUntilDeadline;
if (typeof localSetImmediate === 'function') {
  // Node.js and old IE.
  // There's a few reasons for why we prefer setImmediate.
  //
  // Unlike MessageChannel, it doesn't prevent a Node.js process from exiting.
  // (Even though this is a DOM fork of the Scheduler, you could get here
  // with a mix of Node.js 15+, which has a MessageChannel, and jsdom.)
  // https://github.com/facebook/react/issues/20756
  //
  // But also, it runs earlier which is the semantic we want.
  // If other browsers ever implement it, it's better to use it.
  // Although both of these would be inferior to native scheduling.
  schedulePerformWorkUntilDeadline = () => {
    localSetImmediate(performWorkUntilDeadline);
  };
} else if (typeof MessageChannel !== 'undefined') {
  // DOM and Worker environments.
  // We prefer MessageChannel because of the 4ms setTimeout clamping.
  const channel = new MessageChannel();
  const port = channel.port2;
  channel.port1.onmessage = performWorkUntilDeadline;
  schedulePerformWorkUntilDeadline = () => {
    port.postMessage(null);
  };
} else {
  // We should only fallback here in non-browser environments.
  schedulePerformWorkUntilDeadline = () => {
    localSetTimeout(performWorkUntilDeadline, 0);
  };
}

scheduledHostCallback(hasTimeRemaining, currentTime) ，通过上面的代码的定义可知scheduledHostCallback 就是flushWork，所以这里是调用 flushWork(hasTimeRemaining, currentTime)

const performWorkUntilDeadline = () => {
  if (scheduledHostCallback !== null) {
    const currentTime = getCurrentTime();
    // Keep track of the start time so we can measure how long the main thread
    // has been blocked.
    startTime = currentTime;
    const hasTimeRemaining = true;

    // If a scheduler task throws, exit the current browser task so the
    // error can be observed.
    //
    // Intentionally not using a try-catch, since that makes some debugging
    // techniques harder. Instead, if `scheduledHostCallback` errors, then
    // `hasMoreWork` will remain true, and we'll continue the work loop.
    let hasMoreWork = true;
    try {
      hasMoreWork = scheduledHostCallback(hasTimeRemaining, currentTime);
    } finally {
      if (hasMoreWork) {
        // If there's more work, schedule the next message event at the end
        // of the preceding one.
        schedulePerformWorkUntilDeadline();
      } else {
        isMessageLoopRunning = false;
        scheduledHostCallback = null;
      }
    }
  } else {
    isMessageLoopRunning = false;
  }
  // Yielding to the browser will give it a chance to paint, so we can
  // reset this.
  needsPaint = false;
};

flushWork 在调用 workLoop， workLoop 是 scheduler 核心

function flushWork(hasTimeRemaining, initialTime) {
  // 省略一部分代码
  isPerformingWork = true;
  const previousPriorityLevel = currentPriorityLevel;
  try {
    if (enableProfiling) {
      try {
        return workLoop(hasTimeRemaining, initialTime);

workLoop 是 Scheduler 的核心方法，当 currentTask 的 callback 是一个方法，这部分处理暂停任务，如果callback返回一个 function，那么在一次执行中还是执行当前任务，否则任务完成。

 const callback = currentTask.callback;
    if (typeof callback === 'function') {
      currentTask.callback = null;
      currentPriorityLevel = currentTask.priorityLevel;
      const didUserCallbackTimeout = currentTask.expirationTime <= currentTime;
      if (enableProfiling) {
        markTaskRun(currentTask, currentTime);
      }
      const continuationCallback = callback(didUserCallbackTimeout);
      currentTime = getCurrentTime();
      //**************是否是暂停任务
      if (typeof continuationCallback === 'function') {
        currentTask.callback = continuationCallback;
        if (enableProfiling) {
          markTaskYield(currentTask, currentTime);
        }
      } else {
        if (enableProfiling) {
          markTaskCompleted(currentTask, currentTime);
          currentTask.isQueued = false;
        }
        if (currentTask === peek(taskQueue)) {
          pop(taskQueue);
        }
      }
      advanceTimers(currentTime);
    } else {
      pop(taskQueue);
    }

总结

React 调度是用来调度 fiber 任务的，任务可以暂停并将控制权交给浏览器，调度的核心是通过 Priority Queue 实现，根据重要程度进行排序，过期时间作为排序字段，先到期的先执行。