vue computed 源码分析

Posted 霜末之冬

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我们来看看computed的实现。最简单的一个demo如下:

<html>
<head>
    <meta http-equiv="Content-Type" content="text/html; charset=utf-8" />
</head>
<body>
<div id="app">
  <div name="test">{{computeA}}</div>

</div>
</body>
<script src="vue.js"></script>
<script type="text/javascript">
  new Vue({
  el: ‘#app‘,
  data: function () {
    return {
      firstName: 111,
      lastName: 222
    }
  },
  computed: {
    computeA: function () {
      return this.firstName + ‘ ‘ + this.lastName
    }
  },
  created(){
    setTimeout(
      () => {
        this.firstName = 333;
      },1000
    )
  }
})
</script>
</html>

  

1在初始化实例创建响应式的时候。对options中的computed做了特殊处理:

function initComputed (vm, computed) {
  var watchers = vm._computedWatchers = Object.create(null);

  for (var key in computed) {
    var userDef = computed[key];
    var getter = typeof userDef === ‘function‘ ? userDef : userDef.get;
    {
      if (getter === undefined) {
        warn(
          ("No getter function has been defined for computed property \"" + key + "\"."),
          vm
        );
        getter = noop;
      }
    }
    // create internal watcher for the computed property.
    watchers[key] = new Watcher(vm, getter, noop, computedWatcherOptions);//为每一个computed项目订制一个watcher

    // component-defined computed properties are already defined on the
    // component prototype. We only need to define computed properties defined
    // at instantiation here.
    if (!(key in vm)) {
      defineComputed(vm, key, userDef);
    } else {
      if (key in vm.$data) {
        warn(("The computed property \"" + key + "\" is already defined in data."), vm);
      } else if (vm.$options.props && key in vm.$options.props) {
        warn(("The computed property \"" + key + "\" is already defined as a prop."), vm);
      }
    }
  

function defineComputed (target, key, userDef) {
  if (typeof userDef === ‘function‘) {
    sharedPropertyDefinition.get = createComputedGetter(key);
    sharedPropertyDefinition.set = noop;
  } else {
    sharedPropertyDefinition.get = userDef.get
      ? userDef.cache !== false
        ? createComputedGetter(key)
        : userDef.get
      : noop;
    sharedPropertyDefinition.set = userDef.set
      ? userDef.set
      : noop;
  }
  Object.defineProperty(target, key, sharedPropertyDefinition);
}

function createComputedGetter (key) {//构造该computed的get函数
  return function computedGetter () {
    var watcher = this._computedWatchers && this._computedWatchers[key];
    if (watcher) {
      if (watcher.dirty) {
        watcher.evaluate();//收集该watcher的订阅
      }
      if (Dep.target) {
        watcher.depend();//同一为这一组订阅再加上组件re-render的订阅(该订阅负责更新组件)
      }
      return watcher.value
    }
  }
}

  组件初始化的时候。computed项和data中的分别建立响应式。data中的数据直接对属性的get,set做数据拦截。而computed则建立一个新的watcher,在组件渲染的时候。先touch一下这个computed的getter函数。将这个watcher订阅起来。这里相当于这个computed的watcher订阅了firstname和lastname。touch完后。Dep.target此时又变为之前那个用于更新组件的。再通过watcher.depend()将这个组统一加上这个订阅。这样一旦firstname和lastname变了。同时会触发两个订阅更新。其中一个便是更新组件。重新re-render的函数。

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