随手写系列-setState 是同步还是异步

react15批量合并同步更新

react16/react17--fiber架构下 --这位老哥已经有说了zhuanlan.zhihu.com/p/350332132 不重复 走flushSyncCallbackQueue同步条件：

1.legacy模式下(concurrent模式默认都是异步的)--if (expirationTime === Sync)

2.执行上下文为空的时候去调用setState--不进入调度reconciler--if (executionContext === NoContext) 如使用异步调用如setTimeout/原生事件等

  fiber: Fiber,
  expirationTime: ExpirationTime,
) {
  const priorityLevel = getCurrentPriorityLevel();

  if (expirationTime === Sync) {
    if (
      // Check if we're inside unbatchedUpdates
      (executionContext & LegacyUnbatchedContext) !== NoContext &&
      // Check if we're not already rendering
      (executionContext & (RenderContext | CommitContext)) === NoContext
    ) {
      performSyncWorkOnRoot(root);
    } else {
      ensureRootIsScheduled(root);
      schedulePendingInteractions(root, expirationTime);
      if (executionContext === NoContext) {
        // Flush the synchronous work now, unless we're already working or inside
        // a batch. This is intentionally inside scheduleUpdateOnFiber instead of
        // scheduleCallbackForFiber to preserve the ability to schedule a callback
        // without immediately flushing it. We only do this for user-initiated
        // updates, to preserve historical behavior of legacy mode.
        flushSyncCallbackQueue();
      }
    }
  } else {
    // Schedule a discrete update but only if it's not Sync.
    if (
      (executionContext & DiscreteEventContext) !== NoContext &&
      // Only updates at user-blocking priority or greater are considered
      // discrete, even inside a discrete event.
      (priorityLevel === UserBlockingPriority ||
        priorityLevel === ImmediatePriority)
    ) {
      // This is the result of a discrete event. Track the lowest priority
      // discrete update per root so we can flush them early, if needed.
      if (rootsWithPendingDiscreteUpdates === null) {
        rootsWithPendingDiscreteUpdates = new Map([[root, expirationTime]]);
      } else {
        const lastDiscreteTime = rootsWithPendingDiscreteUpdates.get(root);
        if (
          lastDiscreteTime === undefined ||
          lastDiscreteTime > expirationTime
        ) {
          rootsWithPendingDiscreteUpdates.set(root, expirationTime);
        }
      }
    }
    // Schedule other updates after in case the callback is sync.
    ensureRootIsScheduled(root);
    schedulePendingInteractions(root, expirationTime);
  }
}

react17--批量更新--可能存在多个setStatet同批次更新情况如下：(lanes重构由react16的精确判断改为react17的是否属于同批更新区间)

Lanes把任务优先级从批量任务中分离出来, 可以更方便的判断单个任务与批量任务的优先级是否重叠

//1. 通过expirationTime判断
const isTaskIncludedInBatch = priorityOfTask >= priorityOfBatch;
//2. 通过Lanes判断
const isTaskIncludedInBatch = (task & batchOfTasks) !== 0;

// 当同时处理一组任务, 该组内有多个任务, 且每个任务的优先级不一致
// 1. 如果通过expirationTime判断. 需要维护一个范围(在Lane重构之前, 源码中就是这样比较的)
const isTaskIncludedInBatch =
  taskPriority <= highestPriorityInRange &&
  taskPriority >= lowestPriorityInRange;
//2. 通过Lanes判断
const isTaskIncludedInBatch = (task & batchOfTasks) !== 0;