Detailed explanation of Vue.js asynchronous update DOM strategy and nextTick instance

This article mainly introduces the asynchronous update DOM strategy and nextTick from the Vue.js source code. It has certain reference value. Interested friends can refer to it. I hope it can help everyone better understand Vue.js asynchronous.

Written before

Because I am very interested in Vue.js, and the technology stack I usually work on is also Vue.js, I have spent some time researching it in the past few months. I studied the Vue.js source code and made a summary and output.

Original address of the article: https://github.com/answershuto/learnVue.

During the learning process, Chinese comments were added to Vue https://github.com/answershuto/learnVue/tree/master/vue-src. I hope it can help others who want to learn Vue source code. Friends are helpful.

There may be deviations in understanding. Welcome to raise issues and point them out to learn and make progress together.

Operation DOM

When using vue.js, sometimes you have to operate the DOM due to some specific business scenarios, such as this:

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

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

The printed result is begin. Why do we not get the innerText of the real DOM node even though we have clearly set test to "end"? What if we expected "end" but got the previous value "begin"?

Watcher Queue

With questions, we found the Watch implementation of Vue.js source code. When a certain reactive data changes, its setter function will notify Dep in the closure, and Dep will call all Watch objects it manages. Trigger the update implementation of the Watch object. Let's take a look at the implementation of update.

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)
 }
}

We found that Vue.js uses asynchronous execution of DOM updates by default.

When updating is executed asynchronously, the queueWatcher function will be called.

 /*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)
 }
 }
}

Looking at the source code of queueWatcher, we found that the Watch object did not update the view immediately, but was pushed into a queue queue. At this time, the status is in the waiting state. This From time to time, Watch objects will continue to be pushed into this queue. When waiting for the next tick, these Watch objects will be traversed and taken out to update the view. At the same time, Watchers with repeated IDs will not be added to the queue multiple times, because during the final rendering, we only need to care about the final result of the data.

So, what is the next tick?

nextTick

vue.js provides a nextTick function, which is actually the nextTick called above.

The implementation of nextTick is relatively simple. The purpose of execution is to push a function into a microtask or task, and execute nextTick after the current stack is executed (there may also be some tasks that need to be executed in front). Take a look at the source code for the function passed in:

/**
 * 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 !== &#39;undefined&#39; && 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&#39;t completely break, but
  // it can get stuck in a weird state where callbacks are pushed into the
  // microtask queue but the queue isn&#39;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 !== &#39;undefined&#39; && (
 isNative(MutationObserver) ||
 // PhantomJS and iOS 7.x
 MutationObserver.toString() === &#39;[object MutationObserverConstructor]&#39;
 )) {
 // 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, &#39;nextTick&#39;)
  }
  } else if (_resolve) {
  _resolve(ctx)
  }
 })
 if (!pending) {
  pending = true
  timerFunc()
 }
 if (!cb && typeof Promise !== &#39;undefined&#39;) {
  return new Promise((resolve, reject) => {
  _resolve = resolve
  })
 }
 }
})()

It is an immediately executed function and returns a queueNextTick interface.

The incoming cb will be pushed into callbacks and stored, and then timerFunc will be executed (pending is a status mark to ensure that timerFunc is only executed once before the next tick).

What is timerFunc?

After reading the source code, I found that timerFunc will detect the current environment and have different implementations. In fact, it is based on the priority of Promise, MutationObserver, and setTimeout. Which one exists is used. SetTimeout is used in the most unfavorable environment.

Explain here, there are three ways to try to get timerFunc: Promise, MutationObserver and setTimeout.
Use Promise first, and use MutationObserver when Promise does not exist. The callback functions of these two methods will be executed in the microtask. They will be executed earlier than setTimeout, so they are used first.
If the environment does not support the above two methods, setTimeout will be used, and this function will be pushed at the end of the task and wait for the call to be executed.

Why should you use microtask first? I learned from Gu Yiling's answer on Zhihu:

JS's event loop will distinguish tasks and microtasks when executing. After the engine completes the execution of each task, it will take a task from the queue for execution. Before, all microtasks in the microtask queue will be executed first.

The setTimeout callback will be assigned to a new task for execution, and the Promise resolver and MutationObserver callbacks will be arranged to be executed in a new microtask, which will be executed before the task generated by setTimeout.

To create a new microtask, use Promise first. If the browser does not support it, try MutationObserver.

It really doesn’t work, I can only use setTimeout to create a task.

Why use microtask?

According to HTML Standard, after each task runs, the UI will be re-rendered, then the data update is completed in the microtask, and the latest UI can be obtained when the current task ends.

On the other hand, if a new task is created to update data, the rendering will be performed twice.

Refer to Gu Yiling’s Zhihu answer

The first is Promise, (Promise.resolve()).then() can add its callback to the microtask,

MutationObserver is created A DOM object of textNode. Use MutationObserver to bind the DOM and specify a callback function. When the DOM changes, the callback will be triggered. The callback will enter the microtask, that is, when textNode.data = String(counter), the callback will be added.

setTimeout是最后的一种备选方案，它会将回调函数加入task中，等到执行。

综上，nextTick的目的就是产生一个回调函数加入task或者microtask中，当前栈执行完以后（可能中间还有别的排在前面的函数）调用该回调函数，起到了异步触发（即下一个tick时触发）的目的。

flushSchedulerQueue

/*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 !== &#39;production&#39; && has[id] != null) {
  circular[id] = (circular[id] || 0) + 1
  if (circular[id] > MAX_UPDATE_COUNT) {
  warn(
   &#39;You may have an infinite update loop &#39; + (
   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(&#39;flush&#39;)
 }
}

flushSchedulerQueue是下一个tick时的回调函数，主要目的是执行Watcher的run函数，用来更新视图

为什么要异步更新视图

来看一下下面这一段代码

<template>
 <p>
 <p>{{test}}</p>
 </p>
</template>

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

现在有这样的一种情况，created的时候test的值会被++循环执行1000次。

每次++时，都会根据响应式触发setter->Dep->Watcher->update->patch。

如果这时候没有异步更新视图，那么每次++都会直接操作DOM更新视图，这是非常消耗性能的。

所以Vue.js实现了一个queue队列，在下一个tick的时候会统一执行queue中Watcher的run。同时，拥有相同id的Watcher不会被重复加入到该queue中去，所以不会执行1000次Watcher的run。最终更新视图只会直接将test对应的DOM的0变成1000。
保证更新视图操作DOM的动作是在当前栈执行完以后下一个tick的时候调用，大大优化了性能。

访问真实DOM节点更新后的数据

所以我们需要在修改data中的数据后访问真实的DOM节点更新后的数据，只需要这样，我们把文章第一个例子进行修改。

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

使用Vue.js的global API的$nextTick方法，即可在回调中获取已经更新好的DOM实例了。

相关推荐：

jQuery中DOM节点操作方法总结

DOM简介及节点、属性、查找节点

几种jQuery查找dom的方法

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