深入"时间管理大师" —— React Scheduler

let newCallbackNode;
// 同步
if (newCallbackPriority === SyncLane) {
  // 执行 scheduleSyncCallback 方法
  // 只不过要区分下 legacy 模式还是 concurrent 模式
  // scheduleSyncCallback 自己有个 syncQueue, 用来承载同步任务
  // 并交由 flushSyncCallbacks 处理这些同步任务后, 再交由下面 scheduleCallback
  // 以最高优先级让 Scheduler 调度
  if (root.tag === LegacyRoot) {
    scheduleLegacySyncCallback(performSyncWorkOnRoot.bind(null, root));
  } else {
    scheduleSyncCallback(performSyncWorkOnRoot.bind(null, root));
  }

  // 这里我们只谈 scheduleCallback, 即以最高优先级
  // ImmediateSchedulerPriority 来执行同步任务
  if (supportsMicrotasks) {
    scheduleMicrotask(flushSyncCallbacks);
  } else {
    scheduleCallback(ImmediateSchedulerPriority, flushSyncCallbacks);
  }
  newCallbackNode = null;
} else {
  // 异步
  let schedulerPriorityLevel;
  // 需要将 lane 转换为 Scheduler 可识别的优先级
  switch (lanesToEventPriority(nextLanes)) {
    case DiscreteEventPriority:
      schedulerPriorityLevel = ImmediateSchedulerPriority;
      break;
    case ContinuousEventPriority:
      schedulerPriorityLevel = UserBlockingSchedulerPriority;
      break;
    case DefaultEventPriority:
      schedulerPriorityLevel = NormalSchedulerPriority;
      break;
    case IdleEventPriority:
      schedulerPriorityLevel = IdleSchedulerPriority;
      break;
    default:
      schedulerPriorityLevel = NormalSchedulerPriority;
      break;
  }
  // 通过 scheduleCallback 将任务及其优先级传入到 Scheduler 中
  newCallbackNode = scheduleCallback(
    schedulerPriorityLevel,
    performConcurrentWorkOnRoot.bind(null, root)
  );
}

export const NoPriority = 0; // 没有任何优先级
export const ImmediatePriority = 1; // 立即执行的优先级, 级别最高
export const UserBlockingPriority = 2; // 用户阻塞级别的优先级, 比如用户输入, 拖拽这些
export const NormalPriority = 3; // 正常的优先级
export const LowPriority = 4; // 低优先级
export const IdlePriority = 5; // 最低阶的优先级, 可以被闲置的那种

function unstable_scheduleCallback(priorityLevel, callback, options) {
  /*
   * (1
   */
  var currentTime = getCurrentTime();

  // timerQueue 根据 startTime 排序
  // 任务进来的时候, 开始时间默认是当前时间, 如果进入调度的时候传了延迟时间
  // 开始时间则是当前时间与延迟时间的和
  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;
  }

  /*
   * (2
   */
  // taskQueue 根据 expirationTime 排序
  var timeout;
  switch (priorityLevel) {
    case ImmediatePriority:
      timeout = IMMEDIATE_PRIORITY_TIMEOUT; // -1
      break;
    case UserBlockingPriority:
      timeout = USER_BLOCKING_PRIORITY_TIMEOUT; // 250
      break;
    case IdlePriority:
      timeout = IDLE_PRIORITY_TIMEOUT; // 1073741823 (2^30 - 1)
      break;
    case LowPriority:
      timeout = LOW_PRIORITY_TIMEOUT; // 10000
      break;
    case NormalPriority:
    default:
      timeout = NORMAL_PRIORITY_TIMEOUT; // 5000
      break;
  }

  // 计算任务的过期时间, 任务开始时间 + timeout
  // 若是立即执行的优先级(IMMEDIATE_PRIORITY_TIMEOUT(-1))
  // 它的过期时间是 startTime - 1, 意味着立刻就过期
  var expirationTime = startTime + timeout;

  /*
   * (3
   */
  // 创建调度任务
  var newTask = {
    id: taskIdCounter++,
    callback, // 调度的任务
    priorityLevel, // 任务优先级
    startTime, // 任务开始的时间, 表示任务何时才能执行
    expirationTime, // 任务的过期时间
    sortIndex: -1, // 在小顶堆队列中排序的依据
  };

  if (enableProfiling) {
    newTask.isQueued = false;
  }

  /*
   * (4
   */
  // startTime > currentTime 说明任务无需立刻执行
  // 故放到 timerQueue 中
  if (startTime > currentTime) {
    // timerQueue 是通过 startTime 判断优先级的,
    // 故将 startTime 设为 sortIndex 作为优先级依据
    newTask.sortIndex = startTime;
    push(timerQueue, newTask);

    // 如果 taskQueue 是空的, 并且当前任务优先级最高
    // 那么这个任务就应该优先被设为 isHostTimeoutScheduled
    if (peek(taskQueue) === null && newTask === peek(timerQueue)) {
      // 如果超时调度已经在执行了, 就取消掉
      // 因为当前这个任务是最高优的, 需要先处理当前这个任务
      if (isHostTimeoutScheduled) {
        cancelHostTimeout();
      } else {
        isHostTimeoutScheduled = true;
      }
      // Schedule a timeout.
      requestHostTimeout(handleTimeout, startTime - currentTime);
    }
  } else {
    // startTime <= currentTime 说明任务已过期
    // 需将任务放到 taskQueue
    newTask.sortIndex = expirationTime;
    push(taskQueue, newTask);

    if (enableProfiling) {
      markTaskStart(newTask, currentTime);
      newTask.isQueued = true;
    }

    // 如果目前正在对某个过期任务进行调度,
    // 当前任务需要等待下次时间片让出时才能执行
    if (!isHostCallbackScheduled && !isPerformingWork) {
      isHostCallbackScheduled = true;
      requestHostCallback(flushWork);
    }
  }

  return newTask;
}

let getCurrentTime;
const hasPerformanceNow =
  typeof performance === "object" && typeof performance.now === "function";

if (hasPerformanceNow) {
  const localPerformance = performance;
  getCurrentTime = () => localPerformance.now();
} else {
  const localDate = Date;
  const initialTime = localDate.now();
  getCurrentTime = () => localDate.now() - initialTime;
}

DOMHighResTimeStamp Performance::now() const {
  return MonotonicTimeToDOMHighResTimeStamp(tick_clock_->NowTicks());
}

function requestHostTimeout(callback, ms) {
  taskTimeoutID = setTimeout(() => {
    callback(getCurrentTime());
  }, ms);
}

function cancelHostTimeout() {
  clearTimeout(taskTimeoutID);
  taskTimeoutID = -1;
}

function handleTimeout(currentTime) {
  isHostTimeoutScheduled = false;
  // 更新 timerQueue 和 taskQueue 两个序列
  // 如果发现 timerQueue 有过期的, 就放到 taskQueue 中
  advanceTimers(currentTime);

  // 检查是否已经开始调度
  // 如果正在调度, 就什么都不做
  if (!isHostCallbackScheduled) {
    // 如果 taskQueue 中有任务, 那就先去执行已过期的任务
    if (peek(taskQueue) !== null) {
      isHostCallbackScheduled = true;
      requestHostCallback(flushWork);
    } else {
      // 如果没有过期任务, 那就接着对最高优的第一个未过期的任务
      // 继续重复这个过程, 直到它可以被放置到 taskQueue
      const firstTimer = peek(timerQueue);
      if (firstTimer !== null) {
        requestHostTimeout(handleTimeout, firstTimer.startTime - currentTime);
      }
    }
  }
}

function advanceTimers(currentTime) {
  let timer = peek(timerQueue);
  while (timer !== null) {
    if (timer.callback === null) {
      // Timer was cancelled.
      pop(timerQueue);

      // 开始时间小于等于当前时间, 说明已过期,
      // 从 taskQueue 移走, 放到 taskQueue
    } else if (timer.startTime <= currentTime) {
      pop(timerQueue);
      // taskQueue 是通过 expirationTime 判断优先级的,
      // expirationTime 越小, 说明越紧急, 它就应该放在 taskQueue 的最前面
      timer.sortIndex = timer.expirationTime;
      push(taskQueue, timer);

      if (enableProfiling) {
        markTaskStart(timer, currentTime);
        timer.isQueued = true;
      }
    } else {
      // 开始时间大于当前时间, 说明未过期, 任务仍然保留在 timerQueue
      // 任务进来的时候, 开始时间默认是当前时间, 如果进入调度的时候传了延迟时间, 开始时间则是当前时间与延迟时间的和
      // 开始时间越早, 说明会越早开始, 排在最小堆的前面
      // Remaining timers are pending.
      return;
    }
    timer = peek(timerQueue);
  }
}

let schedulePerformWorkUntilDeadline;

if (typeof setImmediate === "function") {
  schedulePerformWorkUntilDeadline = () => {
    setImmediate(performWorkUntilDeadline);
  };
} else {
  const channel = new MessageChannel();
  const port = channel.port2;

  // port1 接收调度信号, 来执行 performWorkUntilDeadline(受)
  channel.port1.onmessage = performWorkUntilDeadline;

  // port 是调度者(攻)
  schedulePerformWorkUntilDeadline = () => {
    port.postMessage(null);
  };
}

function requestHostCallback(callback) {
  scheduledHostCallback = callback;
  if (!isMessageLoopRunning) {
    isMessageLoopRunning = true;

    // postMessage, 告诉 port1 来执行 performWorkUntilDeadline 方法
    schedulePerformWorkUntilDeadline();
  }
}

//
const performWorkUntilDeadline = () => {
  if (scheduledHostCallback !== null) {
    const currentTime = getCurrentTime();
    // 时间分片
    deadline = currentTime + yieldInterval;
    const hasTimeRemaining = true;

    let hasMoreWork = true;
    try {
      // scheduledHostCallback 去执行真正的任务
      // 如果返回 true, 说明当前任务被中断了
      // 会再让调度者调度一个执行者继续执行任务
      // 下面讲 workLoop 方法时会说到中断恢复的逻辑, 先留个坑
      hasMoreWork = scheduledHostCallback(hasTimeRemaining, currentTime);
    } finally {
      if (hasMoreWork) {
        // 如果任务中断了(没执行完), 就说明 hasMoreWork 为 true
        // 这块类似于递归, 就再申请一个调度者来继续执行该任务
        schedulePerformWorkUntilDeadline();
      } else {
        // 否则当前任务就执行完了
        // 关闭 isMessageLoopRunning
        // 并将 scheduledHostCallback 置为 null
        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;
};

function flushWork(hasTimeRemaining, initialTime) {
  if (enableProfiling) {
    markSchedulerUnsuspended(initialTime);
  }

  // 由于 requestHostCallback 并不一定立即执行传入的回调函数
  // 所以 isHostCallbackScheduled 状态可能会维持一段时间
  // 等到 flushWork 开始处理任务时, 则需要释放该状态以支持其他的任务被 schedule 进来
  isHostCallbackScheduled = false;

  // 因为已经在执行 taskQueue 的任务了
  // 所以不需要等 timerQueue 中的任务过期了
  if (isHostTimeoutScheduled) {
    isHostTimeoutScheduled = false;
    cancelHostTimeout();
  }

  isPerformingWork = true;
  const previousPriorityLevel = currentPriorityLevel;
  try {
    if (enableProfiling) {
      try {
        return workLoop(hasTimeRemaining, initialTime);
      } catch (error) {
        if (currentTask !== null) {
          const currentTime = getCurrentTime();
          markTaskErrored(currentTask, currentTime);
          currentTask.isQueued = false;
        }
        throw error;
      }
    } else {
      // No catch in prod code path.
      return workLoop(hasTimeRemaining, initialTime);
    }
  } finally {
    // 执行完任务后还原这些全局状态
    currentTask = null;
    currentPriorityLevel = previousPriorityLevel;
    isPerformingWork = false;
    if (enableProfiling) {
      const currentTime = getCurrentTime();
      markSchedulerSuspended(currentTime);
    }
  }
}

function workLoop(hasTimeRemaining, initialTime) {
  let currentTime = initialTime;

  // 因为是个异步的, 需要再次调整一下 timerQueue 跟 taskQueue
  advanceTimers(currentTime);

  // 最紧急的过期任务
  currentTask = peek(taskQueue);
  while (
    currentTask !== null &&
    !(enableSchedulerDebugging && isSchedulerPaused) // 用于 debugger, 不管
  ) {
    // 任务中断!!!
    // 时间片到了, 但 currentTask 未过期, 跳出循环
    // 当前任务就被中断了, 需要放到下次 workLoop 中执行
    if (
      currentTask.expirationTime > currentTime &&
      (!hasTimeRemaining || shouldYieldToHost())
    ) {
      // This currentTask hasn't expired, and we've reached the deadline.
      break;
    }

    const callback = currentTask.callback;
    if (typeof callback === "function") {
      // 清除掉 currentTask.callback
      // 如果下次迭代 callback 为空, 说明任务执行完了
      currentTask.callback = null;

      currentPriorityLevel = currentTask.priorityLevel;

      // 已过期
      const didUserCallbackTimeout = currentTask.expirationTime <= currentTime;

      if (enableProfiling) {
        markTaskRun(currentTask, currentTime);
      }

      // 执行任务
      const continuationCallback = callback(didUserCallbackTimeout);
      currentTime = getCurrentTime();

      // 如果产生了连续回调, 说明出现了中断
      // 故将新的 continuationCallback 赋值 currentTask.callback
      // 这样下次恢复任务时, callback 就接上趟了
      if (typeof continuationCallback === "function") {
        currentTask.callback = continuationCallback;

        if (enableProfiling) {
          markTaskYield(currentTask, currentTime);
        }
      } else {
        if (enableProfiling) {
          markTaskCompleted(currentTask, currentTime);
          currentTask.isQueued = false;
        }
        // 如果 continuationCallback 不是 Function 类型, 说明任务完成!!!
        // 否则, 说明这个任务执行完了, 可以被弹出了
        if (currentTask === peek(taskQueue)) {
          pop(taskQueue);
        }
      }

      // 上面执行任务会消耗一些时间, 再次重新更新两个队列
      advanceTimers(currentTime);
    } else {
      // 上面的 if 清空了 currentTask.callback, 所以
      // 如果 callback 为空, 说明这个任务就执行完了, 可以被弹出了
      pop(taskQueue);
    }

    // 如果当前任务执行完了, 那么就把下一个最高优的任务拿出来执行, 直到清空了 taskQueue
    // 如果当前任务没执行完, currentTask 实际还是当前的任务, 只不过 callback 变成了 continuationCallback
    currentTask = peek(taskQueue);
  }

  // 任务恢复!!!
  // 上面说到 ddl 到了, 但 taskQueue 还没执行完(也就是任务被中断了)
  // 就返回 true, 这就是恢复任务的标志
  if (currentTask !== null) {
    return true;
  } else {
    // 若任务完成!!!, 去 timerQueue 中找需要最早开始执行的那个任务
    // 进行 requestHostTimeout 调度那一套
    const firstTimer = peek(timerQueue);
    if (firstTimer !== null) {
      requestHostTimeout(handleTimeout, firstTimer.startTime - currentTime);
    }
    return false;
  }
}

function shouldYieldToHost() {
  if (
    enableIsInputPending &&
    navigator !== undefined &&
    navigator.scheduling !== undefined &&
    navigator.scheduling.isInputPending !== undefined
  ) {
    const scheduling = navigator.scheduling;
    const currentTime = getCurrentTime();
    if (currentTime >= deadline) {
      // There's no time left. We may want to yield control of the main
      // thread, so the browser can perform high priority tasks. The main ones
      // are painting and user input. If there's a pending paint or a pending
      // input, then we should yield. But if there's neither, then we can
      // yield less often while remaining responsive. We'll eventually yield
      // regardless, since there could be a pending paint that wasn't
      // accompanied by a call to `requestPaint`, or other main thread tasks
      // like network events.
      // 需要绘制或者有高优先级的 I/O, 必须得让出主线程
      if (needsPaint || scheduling.isInputPending()) {
        // There is either a pending paint or a pending input.
        return true;
      }
      // There's no pending input. Only yield if we've reached the max
      // yield interval.
      return currentTime >= maxYieldInterval;
    } else {
      // There's still time left in the frame.
      return false;
    }
  } else {
    // `isInputPending` is not available. Since we have no way of knowing if
    // there's pending input, always yield at the end of the frame.

    // task 执行超过了 ddl 就应该让出主进程了
    return getCurrentTime() >= deadline;
  }
}

function unstable_cancelCallback(task) {
  if (enableProfiling) {
    if (task.isQueued) {
      const currentTime = getCurrentTime();
      markTaskCanceled(task, currentTime);
      task.isQueued = false;
    }
  }

  // Null out the callback to indicate the task has been canceled. (Can't
  // remove from the queue because you can't remove arbitrary nodes from an
  // array based heap, only the first one.)
  task.callback = null;
}

function forceFrameRate(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;
  }
}