vue之initComputed模块源码说明

摘要：要想理解原理就得看源码，最近网上也找了好多初始化方法个恶魔。。。因为也是循序渐进的理解，对计算属性的初始化有几处看得不是很明白，网上也都是含糊其辞的要想深入必须深入。。。

因为也是循序渐进的理解，对initComputed计算属性的初始化有几处看得不是很明白，网上也都是含糊其辞的（要想深入必须深入。。。），所以debug了好几天，才算是有点头绪，现在写出来即帮自己再次理下思路，也可以让大佬指出错误

首先，基本的双向绑定原理就不说了，可以去搜下相关教程，还是要先理解下简单的例子

function initComputed (vm, computed) {
  console.log("%cinitComputed","font-size:20px;border:1px solid black")
  var watchers = vm._computedWatchers = Object.create(null);
  // computed properties are just getters during SSR
  var isSSR = isServerRendering();

  for (var key in computed) {
    var userDef = computed[key];
    var getter = typeof userDef === "function" ? userDef : userDef.get;
    if ("development" !== "production" && getter == null) {
      warn(
        ("Getter is missing for computed property "" + key + ""."),
        vm
      );
    }
    //minxing---console
    console.log("%cinitComputed 定义watchers[key]=new Watcher(vm getter noop computedWatcherOptions)","color:white;padding:5px;background:black");
   
    if (!isSSR) {
      // create internal watcher for the computed property.
      /**
      * 熟悉的newWatcher，创建一个订阅者，为了之后收集依赖
      * 将例子中的num、lastNum和计算属性comNum进行绑定
      * 也就是说在一个deps中有两个dep，其中的subs分别是
      * dep1.subs:[watcher(num),watcher(comNum)]
      * dep2.subs:[watcher(lastNum),watcher(comNum)]
      * dep3........
      * 请看前面的例子，页面html中并没有渲染{{lastNum}}；按理说就不会执行lastNum的getter
      * 从而就不会和计算属性进行关联绑定，如果更改lastNum就不会触发dep2的notify()发布
      * 自然也就不会在页面看到comNum有所变化，但是运行后却不是这样，为什么呢
      * 这就引出这个initComputed的下面方法了---依赖收集(watcher.prototype.depend)！
      * 当时也是看了好久才知道这个depend方法的作用，后面再说
      * 首先先来提个头，就是下面代码中watcher中这个getter
      * 其实就是function comNum() {return this.num+"-computed-"+this.lastNum;}}
      * 而这个getter什么时候执行呢，会在Watcher.prototype.evaluate()方法中执行
      * 所以watcher中的evaluate()与depend()两个方法都与initComputed相关
      */
      watchers[key] = new Watcher(
        vm,
        getter || noop,
        noop,
        computedWatcherOptions
      );
    }

    // component-defined computed properties are already defined on the
    // component prototype. We only need to define computed properties defined
    // at instantiation here.
    // 经过判断后定义计算属性---（关联到vm的data上面）
    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);
      }
    }
  }
}

这个方法比较简单主要就是将计算属性绑定到vm上，重要的下面的createComputedGetter方法

function defineComputed (
  target,
  key,
  userDef
) {
  console.log("%cdefineComputed","font-size:20px;border:1px solid black")
  var shouldCache = !isServerRendering();
  if (typeof userDef === "function") {
    sharedPropertyDefinition.get = shouldCache
      ? createComputedGetter(key)
      : userDef;
    sharedPropertyDefinition.set = noop;
  } else {
    sharedPropertyDefinition.get = userDef.get
      ? shouldCache && userDef.cache !== false
        ? createComputedGetter(key)
        : userDef.get
      : noop;
    sharedPropertyDefinition.set = userDef.set
      ? userDef.set
      : noop;
  }
  if ("development" !== "production" &&
      sharedPropertyDefinition.set === noop) {
    sharedPropertyDefinition.set = function () {
      warn(
        ("Computed property "" + key + "" was assigned to but it has no setter."),
        this
      );
    };
  }
  Object.defineProperty(target, key, sharedPropertyDefinition);
}

function createComputedGetter (key) {
  console.log("createComputedGetter key",key);
  return function computedGetter () {
    var watcher = this._computedWatchers && this._computedWatchers[key];
    if (watcher) {
      if (watcher.dirty) {
        watcher.evaluate();
      }
      if (Dep.target) {
        watcher.depend();
      }
      return watcher.value
    }
  }
}

function createComputedGetter (key) {
  console.log("createComputedGetter key",key);
  return function computedGetter () {
    var watcher = this._computedWatchers && this._computedWatchers[key];
    if (watcher) {
      if (watcher.dirty) {
        console.log("createComputedGetter watcher evaluate===========");
        //求值
        watcher.evaluate();
      }
      if (Dep.target) {
        console.log("createComputedGetter watcher depend===========");
        //依赖收集
        watcher.depend();
      }
      console.log("%ccreateComputedGetter watcher.value is","color:blue;font-size:40px",watcher.value);
      return watcher.value
    }
  }
}

为了更好的说明下，截两张图（都是基于最上面的html配置哦）

注释掉watcher.depend()方法，此时deps中没有dep：id4
其实dep：id4在内存中已经定义好了但是没有加入到deps中（因为没有进行依赖收集）
而dep：id5和id6就是上面的数组和递归数组中元素的dep

dep:id3 就是

这下是不是很清楚了

进行依赖收集后的deps