react源码解析15.scheduler&Lane

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

function forceFrameRate(fps) {//计算时间片
  if (fps < 0 || fps > 125) {
    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 {
    yieldInterval = 5;//时间片默认5ms
  }
}

//deadline = currentTime + yieldInterval，deadline是在performWorkUntilDeadline函数中计算出来的
if (currentTime >= deadline) {
  //...
 return true
}

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

  var startTime;//开始时间
  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://优先级越高timeout越小
      timeout = IMMEDIATE_PRIORITY_TIMEOUT;//-1
      break;
    case UserBlockingPriority:
      timeout = USER_BLOCKING_PRIORITY_TIMEOUT;//250
      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;//优先级越高 过期时间越小

  var newTask = {//新建task
    id: taskIdCounter++,
    callback//回调函数
    priorityLevel,
    startTime,//开始时间
    expirationTime,//过期时间
    sortIndex: -1,
  };
  if (enableProfiling) {
    newTask.isQueued = false;
  }

  if (startTime > currentTime) {//没有过期
    newTask.sortIndex = startTime;
    push(timerQueue, newTask);//加入timerQueue
    //taskQueue中还没有过期任务，timerQueue中离过期时间最近的task正好是newTask
    if (peek(taskQueue) === null && newTask === peek(timerQueue)) {
      if (isHostTimeoutScheduled) {
        cancelHostTimeout();
      } else {
        isHostTimeoutScheduled = true;
      }
      //定时器，到了过期时间就加入taskQueue中
      requestHostTimeout(handleTimeout, startTime - currentTime);
    }
  } else {
    newTask.sortIndex = expirationTime;
    push(taskQueue, newTask);//加入taskQueue
    if (enableProfiling) {
      markTaskStart(newTask, currentTime);
      newTask.isQueued = true;
    }
    if (!isHostCallbackScheduled && !isPerformingWork) {
      isHostCallbackScheduled = true;
      requestHostCallback(flushWork);//执行过期的任务
    }
  }

  return newTask;
}

const continuationCallback = callback(didUserCallbackTimeout);//callback就是调度的callback
currentTime = getCurrentTime();
if (typeof continuationCallback === 'function') {//判断callback执行之后的返回值类型
  currentTask.callback = continuationCallback;//如果是function类型就把又赋值给currentTask.callback
  markTaskYield(currentTask, currentTime);
} else {
  if (enableProfiling) {
    markTaskCompleted(currentTask, currentTime);
    currentTask.isQueued = false;
  }
  if (currentTask === peek(taskQueue)) {
    pop(taskQueue);//如果是function类型就从taskQueue中删除
  }
}
advanceTimers(currentTime);

if (root.callbackNode === originalCallbackNode) {
  // The task node scheduled for this root is the same one that's
  // currently executed. Need to return a continuation.
  return performConcurrentWorkOnRoot.bind(null, root);
}

export function findUpdateLane(
  lanePriority: LanePriority,
  wipLanes: Lanes,
): Lane {
  switch (lanePriority) {
    //...
    case DefaultLanePriority: {
      let lane = pickArbitraryLane(DefaultLanes & ~wipLanes);//找到下一个优先级最高的lane
      if (lane === NoLane) {//上一个level的lane都占满了下降到TransitionLanes继续寻找可用的赛道
        lane = pickArbitraryLane(TransitionLanes & ~wipLanes);
        if (lane === NoLane) {//TransitionLanes也满了
          lane = pickArbitraryLane(DefaultLanes);//从DefaultLanes开始找
        }
      }
      return lane;
    }
  }
}

function ensureRootIsScheduled(root: FiberRoot, currentTime: number) {
  const existingCallbackNode = root.callbackNode;//之前已经调用过的setState的回调
  //...
 if (existingCallbackNode !== null) {
    const existingCallbackPriority = root.callbackPriority;
    //新的setState的回调和之前setState的回调优先级相等 则进入batchedUpdate的逻辑
    if (existingCallbackPriority === newCallbackPriority) {
      return;
    }
    //两个回调优先级不一致，则被高优先级任务打断，先取消当前低优先级的任务
    cancelCallback(existingCallbackNode);
  }
 //调度render阶段的起点
 newCallbackNode = scheduleCallback(
    schedulerPriorityLevel,
    performConcurrentWorkOnRoot.bind(null, root),
  );
 //...
}

function prepareFreshStack(root: FiberRoot, lanes: Lanes) {
  root.finishedWork = null;
  root.finishedLanes = NoLanes;
 //...
  //workInProgressRoot等变量重新赋值和初始化
  workInProgressRoot = root;
  workInProgress = createWorkInProgress(root.current, null);
  workInProgressRootRenderLanes = subtreeRenderLanes = workInProgressRootIncludedLanes = lanes;
  workInProgressRootExitStatus = RootIncomplete;
  workInProgressRootFatalError = null;
  workInProgressRootSkippedLanes = NoLanes;
  workInProgressRootUpdatedLanes = NoLanes;
  workInProgressRootPingedLanes = NoLanes;
 //...
}

export function markStarvedLanesAsExpired(
  root: FiberRoot,
  currentTime: number,
): void {

  const pendingLanes = root.pendingLanes;
  const suspendedLanes = root.suspendedLanes;
  const pingedLanes = root.pingedLanes;
  const expirationTimes = root.expirationTimes;

  let lanes = pendingLanes;
  while (lanes > 0) {//遍历lanes
    const index = pickArbitraryLaneIndex(lanes);
    const lane = 1 << index;

    const expirationTime = expirationTimes[index];
    if (expirationTime === NoTimestamp) {

      if (
        (lane & suspendedLanes) === NoLanes ||
        (lane & pingedLanes) !== NoLanes
      ) {
        expirationTimes[index] = computeExpirationTime(lane, currentTime);//计算过期时间
      }
    } else if (expirationTime <= currentTime) {//过期了
      root.expiredLanes |= lane;//在expiredLanes加入当前遍历到的lane
    }

    lanes &= ~lane;
  }
}

export function getNextLanes(root: FiberRoot, wipLanes: Lanes): Lanes {
  //...
  if (expiredLanes !== NoLanes) {
    nextLanes = expiredLanes;
    nextLanePriority = return_highestLanePriority = SyncLanePriority;//优先返回过期的lane
  } else {
  //...
    }
  return nextLanes;
}