<template>
 <div>
 <div ref="test">{{test}}</div>
 <button @click="handleClick">tet</button>
 </div>
</template>

export default {
 data () {
  return {
   test: 'begin'
  };
 },
 methods () {
  handleClick () {
   this.test = 'end';
   console.log(this.$refs.test.innerText);//打印“begin”
  }
 }
}

update () {
 /* istanbul ignore else */
 if (this.lazy) {
  this.dirty = true
 } else if (this.sync) {
  /*同步则执行run直接渲染视图*/
  this.run()
 } else {
  /*异步推送到观察者队列中，下一个tick时调用。*/
  queueWatcher(this)
 }
}

 /*将一个观察者对象push进观察者队列，在队列中已经存在相同的id则该观察者对象将被跳过，除非它是在队列被刷新时推送*/
export function queueWatcher (watcher: Watcher) {
 /*获取watcher的id*/
 const id = watcher.id
 /*检验id是否存在，已经存在则直接跳过，不存在则标记哈希表has，用于下次检验*/
 if (has[id] == null) {
 has[id] = true
 if (!flushing) {
  /*如果没有flush掉，直接push到队列中即可*/
  queue.push(watcher)
 } else {
  // if already flushing, splice the watcher based on its id
  // if already past its id, it will be run next immediately.
  let i = queue.length - 1
  while (i >= 0 && queue[i].id > watcher.id) {
  i--
  }
  queue.splice(Math.max(i, index) + 1, 0, watcher)
 }
 // queue the flush
 if (!waiting) {
  waiting = true
  nextTick(flushSchedulerQueue)
 }
 }
}

/**
 * Defer a task to execute it asynchronously.
 */
 /*
 延迟一个任务使其异步执行，在下一个tick时执行，一个立即执行函数，返回一个function
 这个函数的作用是在task或者microtask中推入一个timerFunc，在当前调用栈执行完以后以此执行直到执行到timerFunc
 目的是延迟到当前调用栈执行完以后执行
*/
export const nextTick = (function () {
 /*存放异步执行的回调*/
 const callbacks = []
 /*一个标记位，如果已经有timerFunc被推送到任务队列中去则不需要重复推送*/
 let pending = false
 /*一个函数指针，指向函数将被推送到任务队列中，等到主线程任务执行完时，任务队列中的timerFunc被调用*/
 let timerFunc

 /*下一个tick时的回调*/
 function nextTickHandler () {
 /*一个标记位，标记等待状态（即函数已经被推入任务队列或者主线程，已经在等待当前栈执行完毕去执行），这样就不需要在push多个回调到callbacks时将timerFunc多次推入任务队列或者主线程*/
 pending = false
 /*执行所有callback*/
 const copies = callbacks.slice(0)
 callbacks.length = 0
 for (let i = 0; i < copies.length; i++) {
  copies[i]()
 }
 }

 // the nextTick behavior leverages the microtask queue, which can be accessed
 // via either native Promise.then or MutationObserver.
 // MutationObserver has wider support, however it is seriously bugged in
 // UIWebView in iOS >= 9.3.3 when triggered in touch event handlers. It
 // completely stops working after triggering a few times... so, if native
 // Promise is available, we will use it:
 /* istanbul ignore if */

 /*
 这里解释一下，一共有Promise、MutationObserver以及setTimeout三种尝试得到timerFunc的方法
 优先使用Promise，在Promise不存在的情况下使用MutationObserver，这两个方法都会在microtask中执行，会比setTimeout更早执行，所以优先使用。
 如果上述两种方法都不支持的环境则会使用setTimeout，在task尾部推入这个函数，等待调用执行。
 */
 if (typeof Promise !== 'undefined' && isNative(Promise)) {
 /*使用Promise*/
 var p = Promise.resolve()
 var logError = err => { console.error(err) }
 timerFunc = () => {
  p.then(nextTickHandler).catch(logError)
  // in problematic UIWebViews, Promise.then doesn't completely break, but
  // it can get stuck in a weird state where callbacks are pushed into the
  // microtask queue but the queue isn't being flushed, until the browser
  // needs to do some other work, e.g. handle a timer. Therefore we can
  // "force" the microtask queue to be flushed by adding an empty timer.
  if (isIOS) setTimeout(noop)
 }
 } else if (typeof MutationObserver !== 'undefined' && (
 isNative(MutationObserver) ||
 // PhantomJS and iOS 7.x
 MutationObserver.toString() === '[object MutationObserverConstructor]'
 )) {
 // use MutationObserver where native Promise is not available,
 // e.g. PhantomJS IE11, iOS7, Android 4.4
 /*新建一个textNode的DOM对象，用MutationObserver绑定该DOM并指定回调函数，在DOM变化的时候则会触发回调,该回调会进入主线程（比任务队列优先执行），即textNode.data = String(counter)时便会触发回调*/
 var counter = 1
 var observer = new MutationObserver(nextTickHandler)
 var textNode = document.createTextNode(String(counter))
 observer.observe(textNode, {
  characterData: true
 })
 timerFunc = () => {
  counter = (counter + 1) % 2
  textNode.data = String(counter)
 }
 } else {
 // fallback to setTimeout
 /* istanbul ignore next */
 /*使用setTimeout将回调推入任务队列尾部*/
 timerFunc = () => {
  setTimeout(nextTickHandler, 0)
 }
 }

 /*
 推送到队列中下一个tick时执行
 cb 回调函数
 ctx 上下文
 */
 return function queueNextTick (cb?: Function, ctx?: Object) {
 let _resolve
 /*cb存到callbacks中*/
 callbacks.push(() => {
  if (cb) {
  try {
   cb.call(ctx)
  } catch (e) {
   handleError(e, ctx, 'nextTick')
  }
  } else if (_resolve) {
  _resolve(ctx)
  }
 })
 if (!pending) {
  pending = true
  timerFunc()
 }
 if (!cb && typeof Promise !== 'undefined') {
  return new Promise((resolve, reject) => {
  _resolve = resolve
  })
 }
 }
})()

/*Github:https://github.com/answershuto*/
/**
 * Flush both queues and run the watchers.
 */
 /*nextTick的回调函数，在下一个tick时flush掉两个队列同时运行watchers*/
function flushSchedulerQueue () {
 flushing = true
 let watcher, id

 // Sort queue before flush.
 // This ensures that:
 // 1. Components are updated from parent to child. (because parent is always
 // created before the child)
 // 2. A component's user watchers are run before its render watcher (because
 // user watchers are created before the render watcher)
 // 3. If a component is destroyed during a parent component's watcher run,
 // its watchers can be skipped.
 /*
 给queue排序，这样做可以保证：
 1.组件更新的顺序是从父组件到子组件的顺序，因为父组件总是比子组件先创建。
 2.一个组件的user watchers比render watcher先运行，因为user watchers往往比render watcher更早创建
 3.如果一个组件在父组件watcher运行期间被销毁，它的watcher执行将被跳过。
 */
 queue.sort((a, b) => a.id - b.id)

 // do not cache length because more watchers might be pushed
 // as we run existing watchers
 /*这里不用index = queue.length;index > 0; index--的方式写是因为不要将length进行缓存，因为在执行处理现有watcher对象期间，更多的watcher对象可能会被push进queue*/
 for (index = 0; index < queue.length; index++) {
 watcher = queue[index]
 id = watcher.id
 /*将has的标记删除*/
 has[id] = null
 /*执行watcher*/
 watcher.run()
 // in dev build, check and stop circular updates.
 /*
  在测试环境中，检测watch是否在死循环中
  比如这样一种情况
  watch: {
  test () {
   this.test++;
  }
  }
  持续执行了一百次watch代表可能存在死循环
 */
 if (process.env.NODE_ENV !== 'production' && has[id] != null) {
  circular[id] = (circular[id] || 0) + 1
  if (circular[id] > MAX_UPDATE_COUNT) {
  warn(
   'You may have an infinite update loop ' + (
   watcher.user
    ? `in watcher with expression "${watcher.expression}"`
    : `in a component render function.`
   ),
   watcher.vm
  )
  break
  }
 }
 }

 // keep copies of post queues before resetting state
 /**/
 /*得到队列的拷贝*/
 const activatedQueue = activatedChildren.slice()
 const updatedQueue = queue.slice()

 /*重置调度者的状态*/
 resetSchedulerState()

 // call component updated and activated hooks
 /*使子组件状态都改编成active同时调用activated钩子*/
 callActivatedHooks(activatedQueue)
 /*调用updated钩子*/
 callUpdateHooks(updatedQueue)

 // devtool hook
 /* istanbul ignore if */
 if (devtools && config.devtools) {
 devtools.emit('flush')
 }
}

<template>
 <div>
 <div>{{test}}</div>
 </div>
</template>

export default {
 data () {
  return {
   test: 0
  };
 },
 created () {
  for(let i = 0; i < 1000; i++) {
  this.test++;
  }
 }
}

<template>
 <div>
 <div ref="test">{{test}}</div>
 <button @click="handleClick">tet</button>
 </div>
</template>

export default {
 data () {
  return {
   test: 'begin'
  };
 },
 methods () {
  handleClick () {
   this.test = 'end';
   this.$nextTick(() => {
    console.log(this.$refs.test.innerText);//打印"end"
   });
   console.log(this.$refs.test.innerText);//打印“begin”
  }
 }
}