//调度决策的逻辑在ensureRootIsScheduled 函数中, 任务优先级在即将调度的时候去计算，代码在ensureRootIsScheduled函数中：
  function ensureRootIsScheduled(root, currentTime) {
    var 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.
    //通过调用getNextLanes去计算在本次更新中应该处理的这批lanes（nextLanes）
    //getNextLanes会调用getHighestPriorityLanes去计算任务优先级。任务优先级计算的原理是这样：更新优先级（update的lane），
    //它会被并入root.pendingLanes，root.pendingLanes经过getNextLanes处理后，挑出那些应该处理的lanes，传入getHighestPriorityLanes，
    //根据nextLanes找出这些lanes的优先级作为任务优先级。
    var nextLanes = getNextLanes(root, root === workInProgressRoot ? workInProgressRootRenderLanes : NoLanes); // This returns the priority level computed during the `getNextLanes` call.

    var 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) {
      var 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.


    var newCallbackNode;

    if (newCallbackPriority === SyncLanePriority) {
      // Special case: Sync React callbacks are scheduled on a special
      // internal queue
      newCallbackNode = scheduleSyncCallback(performSyncWorkOnRoot.bind(null, root));
    } else if (newCallbackPriority === SyncBatchedLanePriority) {
      newCallbackNode = scheduleCallback(ImmediatePriority$1, performSyncWorkOnRoot.bind(null, root));
    } else {
      var schedulerPriorityLevel = lanePriorityToSchedulerPriority(newCallbackPriority);
      newCallbackNode = scheduleCallback(schedulerPriorityLevel, performConcurrentWorkOnRoot.bind(null, root));
    }

    root.callbackPriority = newCallbackPriority;
    root.callbackNode = newCallbackNode;
  }

function resetRenderTimer() {
    workInProgressRootRenderTargetTime = now() + RENDER_TIMEOUT_MS;
  }

function flushSyncCallbackQueue() {
    if (immediateQueueCallbackNode !== null) {
      var node = immediateQueueCallbackNode;
      immediateQueueCallbackNode = null;
      Scheduler_cancelCallback(node);
    }

    flushSyncCallbackQueueImpl();
  }

  function flushSyncCallbackQueueImpl() {
    if (!isFlushingSyncQueue && syncQueue !== null) {
      // Prevent re-entrancy.
      isFlushingSyncQueue = true;
      var i = 0;

      {
        try {
          var _isSync2 = true;
          var _queue = syncQueue;
          runWithPriority$1(ImmediatePriority$1, function () {
            for (; i < _queue.length; i++) {
              var callback = _queue[i];

              do {
                callback = callback(_isSync2);
              } while (callback !== null);
            }
          });
          syncQueue = null;
        } catch (error) {
          // If something throws, leave the remaining callbacks on the queue.
          if (syncQueue !== null) {
            syncQueue = syncQueue.slice(i + 1);
          } // Resume flushing in the next tick


          Scheduler_scheduleCallback(Scheduler_ImmediatePriority, flushSyncCallbackQueue);
          throw error;
        } finally {
          isFlushingSyncQueue = false;
        }
      }
    }
  }