在Vue實例初始化的過程中烟阐，initState方法會調(diào)用initComputed和initWatch來分別初始化計算屬性和偵聽屬性，那么接下來就分析這兩個方法的實現(xiàn)拧晕。

計算屬性

這兩個方法都定義在core/instance/state.js中

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)
      }
    }
  }
}

核心是首先是創(chuàng)建了vm._computedWatchers屬性并設(shè)為一個空對象隙姿，然后對每個定義的計算屬性調(diào)用創(chuàng)建一個Watcher并調(diào)用defineComputed方法：

export function defineComputed (
  target: any,
  key: string,
  userDef: Object | Function
) {
  const 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 (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)
}

計算屬性的定義可以是一個函數(shù)或者一個對象。如果是一個函數(shù)厂捞，那么它將作為這個計算屬性的getter输玷。如果是一個對象，那么那么這個對象的get屬性和set屬性分別是這個計算屬性的getter和setter靡馁，同時可以設(shè)置cache=false來禁止緩存欲鹏。
若果是需要緩存的情況，getter將被設(shè)為createComputedGetter方法的返回值臭墨。

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

計算屬性創(chuàng)建的Watcher與普通的Watcher的不同之處是赔嚎，在Watcher的構(gòu)造函數(shù)中有這么一段邏輯：

  constructor (
    vm: Component,
    expOrFn: string | Function,
    cb: Function,
    options?: ?Object,
    isRenderWatcher?: boolean
  ) {
    // ...
    if (this.computed) {
      this.value = undefined
      this.dep = new Dep()
    } else {
      this.value = this.get()
    }
  }

在組件掛載時創(chuàng)建的Watcher尤误，this.value是updateComponent函數(shù)，也就是創(chuàng)建Watcher的時候會立即做一次組件更新结缚。而計算屬性創(chuàng)建的Watcher沒有立即調(diào)用this.value，而是創(chuàng)建了一個Dep實例红竭。

當(dāng)組件渲染時訪問到計算屬性，就會調(diào)用它的getter茵宪。首先會拿到它對應(yīng)的watcher，執(zhí)行watcher.depend():

  depend () {
    if (this.dep && Dep.target) {
      this.dep.depend()
    }
  }

此時Dep.target是當(dāng)前正在渲染的組件的Watcher眉厨，也就是讓當(dāng)前的活動正在渲染的組件訂閱了這個計算屬性的變化。

然后調(diào)用了watcher.evaluate:

  evaluate () {
    if (this.dirty) {
      this.value = this.get()
      this.dirty = false
    }
    return this.value
  }

如果this.dirty為true那么就執(zhí)行this.get()憾股，返回計算屬性的值。

this.get()會調(diào)用pushTarget(this)將Dep.target設(shè)為自身服球，那么如果在執(zhí)行g(shù)etter的過程中依賴了其他的響應(yīng)式屬性茴恰，就會觸發(fā)它們的getter斩熊，這樣就會把它們的dep添加到當(dāng)前watcher中往枣，使當(dāng)前計算屬性的watcher訂閱以來的響應(yīng)式屬性的變化。

一旦計算屬性依賴的數(shù)據(jù)被修改，就會觸發(fā) setter分冈，執(zhí)行watcher.update()方法通知watcher更新圾另。

  update () {
    /* istanbul ignore else */
    if (this.computed) {
      // A computed property watcher has two modes: lazy and activated.
      // It initializes as lazy by default, and only becomes activated when
      // it is depended on by at least one subscriber, which is typically
      // another computed property or a component's render function.
      if (this.dep.subs.length === 0) {
        // In lazy mode, we don't want to perform computations until necessary,
        // so we simply mark the watcher as dirty. The actual computation is
        // performed just-in-time in this.evaluate() when the computed property
        // is accessed.
        this.dirty = true
      } else {
        // In activated mode, we want to proactively perform the computation
        // but only notify our subscribers when the value has indeed changed.
        this.getAndInvoke(() => {
          this.dep.notify()
        })
      }
    } else if (this.sync) {
      this.run()
    } else {
      queueWatcher(this)
    }
  }

如果當(dāng)前計算屬性沒有被任何組件依賴，就僅僅把this.dirty設(shè)置為true雕沉。否則重新計算屬性值集乔，然后通知組件重新渲染。

偵聽屬性

偵聽屬性的初始化是在initWatch方法中：

function initWatch (vm: Component, watch: Object) {
  for (const key in watch) {
    const handler = watch[key]
    if (Array.isArray(handler)) {
      for (let i = 0; i < handler.length; i++) {
        createWatcher(vm, key, handler[i])
      }
    } else {
      createWatcher(vm, key, handler)
    }
  }
}
···
`initWatch`方法對每個偵聽屬性的回調(diào)函數(shù)執(zhí)行`createWatcher(vm, key, handler)`坡椒。如果一個偵聽屬性有多個回調(diào)函數(shù)可以使用一個數(shù)組扰路。handler還可以是一個包含options的Object

function createWatcher (
vm: Component,
expOrFn: string | Function,
handler: any,
options?: Object
) {
if (isPlainObject(handler)) {
options = handler
handler = handler.handler
}
if (typeof handler === 'string') {
handler = vm[handler]
}
return vm.$watch(expOrFn, handler, options)
}

`createWatcehr`最終會調(diào)用`vm.$watch`方法。`vm.$watch`是Vue原型上的方法倔叼，它是在執(zhí)行`stateMixin`時添加到原型上的汗唱。

export function stateMixin (Vue: Class<Component>) {
// ...
Vue.prototype.$watch = function (
expOrFn: string | Function,
cb: any,
options?: Object
): Function {
const vm: Component = this
if (isPlainObject(cb)) {
return createWatcher(vm, expOrFn, cb, options)
}
options = options || {}
options.user = true
const watcher = new Watcher(vm, expOrFn, cb, options)
if (options.immediate) {
cb.call(vm, watcher.value)
}
return function unwatchFn () {
watcher.teardown()
}
}
}

通過`vm.$watch`方法創(chuàng)建的watcher是一個user watcher。用來觀察Vue實例上的一個響應(yīng)式屬性的變化丈攒，在變化時執(zhí)行回調(diào)函數(shù)哩罪。它可以有`immediate`或`deep`的選項。

user watcher的并沒有很大的區(qū)別肥印，只有邏輯上的不同识椰，這里就不分析了。