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Vue中之nextTick函数源码分析

2017-10-17 00:36 龙恩0707 阅读(...) 评论(...) 编辑 收藏

Vue中之nextTick函数源码分析

1. 什么是Vue.nextTick()?
官方文档解释如下:
在下次DOM更新循环结束之后执行的延迟回调。在修改数据之后立即使用这个方法,获取更新后的DOM。

2. 为什么要使用nextTick?

<!DOCTYPE html>
<html>
  <head>
    <title>演示Vue</title>
    <script src="https://tugenhua0707.github.io/vue/vue1/vue.js"></script>
  </head>
  <body>
    <div id="app">
      <template>
        <div ref="list">
          {{name}}
        </div>
      </template>
    </div>
    <script>
      new Vue({
        el: '#app',
        data: {
          name: 'aa'
        },
        mounted() {
          this.updateData();
        },
        methods: {
          updateData() {
            var self = this;
            this.name = 'bb';
            console.log(this.$el.textContent); // aa
            this.$nextTick(function(){
              console.log(self.$el.textContent); // bb
            });
          }
        }
      });
    </script>
  </body>
</html>

如上代码 在页面视图上显示bb,但是当我在控制台打印的时候,获取的文本内容还是 aa,但是使用 nextTick后,获取的文本内容就是最新的内容bb了,因此在这种情况下,我们可以使用nextTick函数了。
上面的代码为什么改变this.name = 'bb';后,再使用console.log(this.$el.textContent);打印的值还是aa呢?那是因为设置name的值后,DOM还没有更新到,所以获取值还是之前的值,但是我们放到nextTick函数里面的时候,代码会在DOM更新后执行,因此DOM更新后,再去获取元素的值就可以获取到最新值了。

理解DOM更新:在VUE中,当我们修改了data中的某一个值后,并不会立即反应到该el中,vue将对更改的数据放到watcher的一个异步队列中,只有在当前任务空闲时才会执行watcher队列任务,这就有一个延迟时间,因此放到 nextTick函数后就可以获取该el的最新值了。如果我们把上面的nextTick
改成setTimeout也是可以的。

3. Vue源码详解之nextTick(源码在 vue/src/core/util/env.js)

在理解nextTick源码之前,我们先来理解下 html5中新增的 MutationObserver的API,它的作用是用来监听DOM变动的接口,它能监听一个dom对象发生的子节点删除,属性修改,文本内容修改等等,具体使用看我这边博客(http://www.cnblogs.com/tugenhua0707/articles/6849948.html).
nextTick源码如下:

export const nextTick = (function () {
  const callbacks = []
  let pending = false
  let timerFunc

  function nextTickHandler () {
    pending = false;
    /*
     之所以要slice复制一份出来是因为有的cb执行过程中又会往callbacks中加入内容,比如$nextTick的回调函数里又有$nextTick,
     那么这些应该放入到下一个轮次的nextTick去执行,所以拷贝一份,遍历完成即可,防止一直循环下去。
     */
    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 */
  /*
    nextTick行为利用了microtask队列, 先使用 Promise.resolve().then(nextTickHandler)来将异步回调
    放入到microtask中,Promise 和 MutationObserver都可以使用,但是 MutationObserver 在IOS9.3以上的
    WebView中有bug,因此如果满足第一项的话就可以执行,如果没有原生Promise就用 MutationObserver。
   */
  if (typeof Promise !== 'undefined' && isNative(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
    /*
     创建一个MutationObserver,observe监听到DOM改动之后执行的回调 nextTickHandler 
     */
    var counter = 1
    var observer = new MutationObserver(nextTickHandler)
    var textNode = document.createTextNode(String(counter));
    // 使用MutationObserver的接口,监听文本节点的字符内容
    observer.observe(textNode, {
      characterData: true
    });
    /*
     每次执行timerFunc函数都会让文本节点的内容在0/1之间切换,切换之后将新赋值到那个我们MutationObserver监听的文本节点上去。
     */
    timerFunc = () => {
      counter = (counter + 1) % 2
      textNode.data = String(counter)
    }
  } else {
    // fallback to setTimeout
    /*
     如果上面的两种都不支持的话,我们就使用setTimeout来执行
     */
    timerFunc = () => {
      setTimeout(nextTickHandler, 0)
    }
  }

  return function queueNextTick (cb?: Function, ctx?: Object) {
    let _resolve
    callbacks.push(() => {
      if (cb) {
        try {
          cb.call(ctx)
        } catch (e) {
          handleError(e, ctx, 'nextTick')
        }
      } else if (_resolve) {
        _resolve(ctx)
      }
    });
    /* 如果pending为true,表明本轮事件循环中已经执行过 timerFunc(nextTickHandler, 0) */
    if (!pending) {
      pending = true
      timerFunc()
    }
    if (!cb && typeof Promise !== 'undefined') {
      return new Promise((resolve, reject) => {
        _resolve = resolve
      })
    }
  }
})()

整体思路理解:首先 nextTick 是一个闭包函数,代码立即执行,在理解整体代码之前,我们先来看个类似的demo,如下代码:

<!DOCTYPE html>
<html>
  <head>
    <title>演示Vue</title>
  </head>
  <body>
    <div id="app">
      
    </div>
    <script>
      var nextTick = (function(){
        return function queueNextTick(cb, ctx) {
          if (cb) {
            try {
              cb.call(ctx)
            } catch (e) {
              console.log('出错了');
            }
          }
        }
      })();

      // 方法调用
      nextTick(function(){
        console.log(2);  // 打印2
      })
    </script>
  </body>
</html>

demo代码和上面的代码很类似。
我们也可以再来抽离使用nextTick做demo代码如下:

var nextTick2 = (function(){
  const callbacks = [];
  let pending = false;
  let timerFunc;

  function nextTickHandler () {
    pending = false
    const copies = callbacks.slice(0)
    callbacks.length = 0
    for (let i = 0; i < copies.length; i++) {
      copies[i]()
    }
  }
  if (typeof Promise !== 'undefined') {
    var p = Promise.resolve()
    var logError = err => { console.error(err) }
    timerFunc = () => {
      p.then(nextTickHandler).catch(logError)
    }
  } else if (typeof MutationObserver !== 'undefined' ||
    // 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
    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 */
    timerFunc = () => {
      setTimeout(nextTickHandler, 0)
    }
  }
  return function queueNextTick (cb, ctx) {
    let _resolve
    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
      })
    }
  }
})();
nextTick2(function(){
  console.log(2222);
});

如上代码是nextTick源码的抽离,为了更好的理解nextTick,做了如上的demo。
我们再来理解一下整体的代码的含义;
先定义数组 callbacks = [];来存放所有需要执行的回调函数,定义let pending = false;判断本轮事件是否执行过 timerFunc(nextTickHandler, 0)这个函数,为true说明执行过 timeFunc函数,接着定义nextTickHandler函数,该函数的作用是依次遍历数组callbacks保存的函数,依次执行;
请看源代码如下:

function nextTickHandler () {
  pending = false
  const copies = callbacks.slice(0)
  callbacks.length = 0
  for (let i = 0; i < copies.length; i++) {
    copies[i]()
  }
}

然后就是三个判断了,代码如下:

if (typeof Promise !== 'undefined' && isNative(Promise)) {
  var p = Promise.resolve();
  var logError = err => { console.error(err) }
  timerFunc = () => {
    p.then(nextTickHandler).catch(logError);
} else if (typeof MutationObserver !== 'undefined' && (
    isNative(MutationObserver) ||
    // PhantomJS and iOS 7.x
    MutationObserver.toString() === '[object MutationObserverConstructor]'
)){
  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 {
  timerFunc = () => {
    setTimeout(nextTickHandler, 0)
  }
}

首先判断是否支持Promise对象,如果支持的话,定义了timeFunc()函数,为了下一步调用做准备,然后继续判断是否支持该对象 MutationObserver,
如果支持的话,创建一个文本节点,监听该节点数据是否发生改变,如果发生改变的话,调用timerFunc函数,counter值会在0/1切换,如果值改变了的话,
把该数据值赋值到data属性上面去,那么data属性发生改变了,就会重新渲染页面(因为vue是通过Object.defineProperty来监听属性值是否发生改变),
如果上面两种情况都不满足的话,那么直接使用setTimeout来执行nextTickHandler函数了;
最后nextTick代码返回一个函数,代码如下:

return function queueNextTick (cb?: Function, ctx?: Object) {
  let _resolve
  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
    })
  }
}

代码的含义是:传入的cb是否是函数,ctx参数是否是一个对象,如果cb是一个函数的话,使用cb.call(ctx), 如果timerFunc没有执行过的话,那么pending为
false,因此执行 timerFunc()函数。基本的思路就是这样的。