很多面试题都会遇到， Computed的实现原理，其实在Vue中，如果能理解透三个概念： observe, watcher 和 dep, 那么对于理解响应式很有帮助；

Computed 示例

    <div id="app">{{ fulllName }}</div>

const vm = new Vue({
    data: {
        surName: '张',
        lastName: '先森'
    },
    computed: {
        fulllName () {
            return this.surName + this.lastName
        }
    }
}).$mount('#app')

学过Vue的同学，都知道 computed 实现计算属性，可以简化复杂的逻辑，就比如上面获取fulllName, 但是还有一个隐藏的技能就是，计算属性缓存，当计算属性的响应式依赖surName 和 lastName 没有发生改变的时候， fulllName 不会重新计算

问： computed 和 methods获取值的区别？

答： methods 页面重新渲染的时候，会重新调用函数计算，而computed 只要响应式依赖没有发生改变，尽管页面重新渲染，也不会重新计算值

Computed源码阅读

定位文件: src/core/instance/state.js/initComputed 源码

function initComputed (vm: Component, computed: Object) {
  // $flow-disable-line
  const watchers = vm._computedWatchers = Object.create(null)
  // computed properties are just getters during SSR
  const isSSR = isServerRendering()

  for (const key in computed) {
    const userDef = computed[key]
    const getter = typeof userDef === 'function' ? userDef : userDef.get
    if (process.env.NODE_ENV !== 'production' && getter == null) {
      warn(
        `Getter is missing for computed property "${key}".`,
        vm
      )
    }

    if (!isSSR) {
      // create internal watcher for the computed property.
      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.
    if (!(key in vm)) {
      defineComputed(vm, key, userDef)
    } else if (process.env.NODE_ENV !== 'production') {
      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 "${key}" is already defined as a prop.`, vm)
      } else if (vm.$options.methods && key in vm.$options.methods) {
        warn(`The computed property "${key}" is already defined as a method.`, vm)
      }
    }
  }
}

创建 _computedWatchers

在Vue挂载的时候，会创建一个 Render Watcher(拓展阅读#1), 而对于 computed 会创建 Computed Watcher,故initComputed会在当前Vue实例上新建一个 _computedWatchers, 用于存储所有的 Computed Watcher

const watchers = vm._computedWatchers = Object.create(null)

解析计算属性

就是遍历所有的 computed属性，对于实现计算属性的 getter方式有两种

函数

{
    fullName: function() {
        return this.surName + this.lastName
    }
}

包含get属性的对象

{
    fullName: {
        get: function() {
            return this.surName + this.lastName
        }
    }
}

所以在源码中对两种情况进行解析：

const userDef = computed[key]
const getter = typeof userDef === 'function' ? userDef : userDef.get

新建 `Computed Watcher`

对当前的计算属性，新建一个 Watcher 实例

watchers[key] = new Watcher(
    vm,
    getter || noop,
    noop,
    computedWatcherOptions // { lazy: true }
  )

因为设置了， computedWatcherOptions: { lazy: true } 所以此时此刻的计算属性还没有值

// src/core/observer/watcher.js Watcher
this.value = this.lazy
? undefined
: this.get()

`defineComputed`

当 computed的属性，不存在当前Vue实例中，则调用 defineComputed方法, 其实就是将 computed 的属性，变成 Object.defineProperty

export function defineComputed (
  target: any,
  key: string,
  userDef: Object | Function
) {
  const shouldCache = !isServerRendering()
  if (typeof userDef === 'function') {
    sharedPropertyDefinition.get = shouldCache
      ? createComputedGetter(key)
      : createGetterInvoker(userDef)
    sharedPropertyDefinition.set = noop
  } else {
    sharedPropertyDefinition.get = userDef.get
      ? shouldCache && userDef.cache !== false
        ? createComputedGetter(key)
        : createGetterInvoker(userDef.get)
      : noop
    sharedPropertyDefinition.set = userDef.set || noop
  }
  if (process.env.NODE_ENV !== '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)
}

createComputedGetter

在 defineComputed 对计算属性的获取方式进行封装，当调用计算属性的时候，就会执行返回的computedGetter方法

// createComputedGetter
function createComputedGetter (key) {
  return function computedGetter () {
    const watcher = this._computedWatchers && this._computedWatchers[key]
    if (watcher) {
      if (watcher.dirty) {
        watcher.evaluate()
      }
      if (Dep.target) {
        watcher.depend()
      }
      return watcher.value
    }
  }
}

首先判断当前计算属性的watcher是否存在, 如果存在则继续;

因为计算属性在初始化 Watcher时，传入了 {lazy: true}, 并且在 Watcher 构造函数中有这样一行代码： this.dirty = options.lazy，所以这里面就会执行到 watcher.evaluate()，注意此时的 Dep.target是当前的Computed Watcher, 也就是，会将当前的Computed Watcher追加到响应式依赖的Dep中，在本文中是 surName 和 lastName 的Dep中，这样当响应式依赖发生改变的时候，计算属性会重新计算
evaluate () {
    this.value = this.get()
    this.dirty = false
}

get () {
    pushTarget(this) // 当前的实例赋值给 Dep.target
    let value
    const vm = this.vm
    try {
      value = this.getter.call(vm, vm)
    } catch (e) {
      if (this.user) {
        handleError(e, vm, `getter for watcher "${this.expression}"`)
      } else {
        throw e
      }
    } finally {
      // "touch" every property so they are all tracked as
      // dependencies for deep watching
      if (this.deep) {
        traverse(value)
      }
      popTarget() // 释放掉当前的的 wacther
      this.cleanupDeps()
    }
    return value
  }
此时的Dep.target是 Render Watcher，当 Computed Watcher的依赖响应式发生改变的时候，不仅仅改变 Computed Watcher的值，也要通知 Render Watcher 去进行操作

至此， computed的实现原理分析完毕，希望能够抛砖引玉，在您学习Vue源码的路上贡献一点力量

拓展阅读

1.Vue源码阅读——initData

Vue2源码阅读——Computed

Computed 示例

Computed源码阅读

创建 _computedWatchers

解析 计算属性

新建 Computed Watcher

defineComputed

createComputedGetter

拓展阅读

解析计算属性

新建 `Computed Watcher`

`defineComputed`