React按照事件的紧急程度，把它们划分成三个等级：
- 离散事件（DiscreteEvent）：click、keydown、focusin等，这些事件的触发不是连续的，优先级为0。
- 用户阻塞事件（UserBlockingEvent）：drag、scroll、mouseover等，特点是连续触发，阻塞渲染，优先级为1。
- 连续事件（ContinuousEvent）：canplay、error、audio标签的timeupdate和canplay，优先级最高，为2。
unbatchedUpdates(function () {
updateContainer(children, fiberRoot, parentComponent, callback);
});
function unbatchedUpdates(fn, a) { //非常有趣的设计，类似锁、释放内存的设计 var prevExecutionContext = executionContext; //0 executionContext &= ~BatchedContext; //1 executionContext |= LegacyUnbatchedContext; //8 try { return fn(a); //执行updateContainer.... } finally { executionContext = prevExecutionContext; //重置.... if (executionContext === NoContext) { // Flush the immediate callbacks that were scheduled during this batch resetRenderTimer(); flushSyncCallbackQueue(); } } }