unbatchedUpdates

源码在packages\react-reconciler\src\ReactFiberWorkLoop.js中：

export function unbatchedUpdates<A, R>(fn: (a: A) => R, a: A): R {
  const prevExecutionContext = executionContext; //executionContext = NoContext = 0b000000
  executionContext &= ~BatchedContext; // 0 //BatchedContext:0b000001;
  executionContext |= LegacyUnbatchedContext; // LegacyUnbatchedContext:0b001000 ,8
  try {
    return fn(a);
  } finally {
    executionContext = prevExecutionContext;
    if (executionContext === NoContext) {
      // Flush the immediate callbacks that were scheduled during this batch
      flushSyncCallbackQueue();
    }
  }
}

export function flushSyncCallbackQueue() {
  if (immediateQueueCallbackNode !== null) {
    Scheduler_cancelCallback(immediateQueueCallbackNode);
  }
  flushSyncCallbackQueueImpl();
}
function flushSyncCallbackQueueImpl() {
  if (!isFlushingSyncQueue && syncQueue !== null) {
    // Prevent re-entrancy.
    isFlushingSyncQueue = true;
    let i = 0;
    try {
      const isSync = true;
      const queue = syncQueue;
      runWithPriority(ImmediatePriority, () => {
        for (; i < queue.length; i++) {
          let callback = queue[i];
          do {
            callback = callback(isSync);
          } 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;
    }
  }
}