【Vue2.x笔记2】从源码看computed对象

更新重新看了下源码

// Watcehr 的执行函数
get () {
    pushTarget(this) // 将当前的Watcher放入队列
    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() // 将当前的Watcher从队列里清除
      this.cleanupDeps()
    }
    return value
  }

computed 属性如何做到依赖收集总结

初始化computed属性为lazy watcher， this.value = this.lazy ? undefined: this.get(),lazy watcher 不会立即执行,此时Dep.target = null

当组件挂载时，会创建一个RenderWatcher,并且立即执行，此时Dep.target = RenderWatcher，这时data定义的每个属性都会收集RenderWatcher

当组件挂载时，如果模板里用到了某一个计算机属性，计算属性会执行自己的getter函数,dirty=true执行evaluate(),与此同时计算机属性所依赖的data属性收集该计算机属性对应watcher，

此时 Dep.target = RenderWatcher，计算机属性收集RenderWatcher

所以计算机属性只有当其依赖的data属性时，才会被依赖收集watcher

<template>
  <div>
    a:{{ a }}
    c:{{c}}
    <p>b:{{ b }}</p>
		<p>d:{{ d }}</p>
    <button @click="a++">a++</button>
    <button @click="c++">c++</button>
  </div>
</template>
<script>
export default {
  data() {
    return {
      a: 1,
      c:1
    };
  },
  computed: {
    b(vm) {
      return vm.a;
    },
    d() {
      return 1;
    }
  }
};
</script>

// b依赖a,a更新b更新，d不依赖任何响应式数据，始终返回1

以上更新2020-04-12

computed 初始化函数

const computedWatcherOptions = { lazy: true }

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
      )
    }
    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)
      }
    }
  }
}

Vue会为每个computed属性创建一个lazy Watcher，在Watcher中有下面语句：

  this.dirty = this.lazy // for lazy watchers
  this.value = this.lazy ? undefined: this.get()

  update () {
    /* istanbul ignore else */
    if (this.lazy) {
      this.dirty = true
    } else if (this.sync) {
      this.run()
    } else {
      queueWatcher(this)
    }
  }

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

computed 属性的getter

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
    }
  }
}

watcher.depend()表示在当前watcher添加依赖

watcher.dirty控制是否更新，依赖没改变就不会执行

computed定义函数

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)
}

总结：

• 如果计算机属性定义后未使用，则依赖改变并不会执行
• 计算机属性是Lazy的，只有在依赖更新的时候才会触犯返回新的值