react源码中的小而美函数之flushPassiveEffects

function flushPassiveEffects() {
    // Returns whether passive effects were flushed.
    // TODO: Combine this check with the one in flushPassiveEFfectsImpl. We should
    // probably just combine the two functions. I believe they were only separate
    // in the first place because we used to wrap it with
    // `Scheduler.runWithPriority`, which accepts a function. But now we track the
    // priority within React itself, so we can mutate the variable directly.
    if (rootWithPendingPassiveEffects !== null) {
      // Cache the root since rootWithPendingPassiveEffects is cleared in
      // flushPassiveEffectsImpl
      var root = rootWithPendingPassiveEffects; // Cache and clear the remaining lanes flag; it must be reset since this
      // method can be called from various places, not always from commitRoot
      // where the remaining lanes are known

      var remainingLanes = pendingPassiveEffectsRemainingLanes;
      pendingPassiveEffectsRemainingLanes = NoLanes;
      var renderPriority = lanesToEventPriority(pendingPassiveEffectsLanes);
      var priority = lowerEventPriority(DefaultEventPriority, renderPriority);
      var prevTransition = ReactCurrentBatchConfig$3.transition;
      var previousPriority = getCurrentUpdatePriority();

      try {
        ReactCurrentBatchConfig$3.transition = null;
        setCurrentUpdatePriority(priority);
        return flushPassiveEffectsImpl();
      } finally {
        setCurrentUpdatePriority(previousPriority);
        ReactCurrentBatchConfig$3.transition = prevTransition; // Once passive effects have run for the tree - giving components a
        // chance to retain cache instances they use - release the pooled
        // cache at the root (if there is one)

        releaseRootPooledCache(root, remainingLanes);
      }
    }

    return false;
  }

函数的主要步骤如下：

1. 首先检查rootWithPendingPassiveEffects是否为null，如果不为null，则表示有待处理的被动副作用。
2. 缓存rootWithPendingPassiveEffects，因为在flushPassiveEffectsImpl函数中会清除该变量。
3. 缓存并清除pendingPassiveEffectsRemainingLanes，因为在处理被动副作用时，需要知道剩余的lanes。
4. 根据pendingPassiveEffectsLanes计算出渲染优先级，并将其转换为事件优先级。
5. 设置当前更新的优先级为计算出的优先级。
6. 调用flushPassiveEffectsImpl函数来处理被动副作用。
7. 在finally块中，恢复之前的更新优先级和ReactCurrentBatchConfig$3.transition的值。
8. 一旦被动副作用在整个树上运行完毕，释放根节点上的缓存池。
9. 如果有待处理的被动副作用，则返回true，否则返回false。