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vue中created、watch和computed的执行顺序详解

qb 人气:1

 前言

面试题:vuecreatedwatch(immediate: true)computed的执行顺序是啥?

先看个简单的例子:

// main.js
import Vue from "vue";

new Vue({
  el: "#app",
  template: `<div>
    <div>{{computedCount}}</div>
  </div>`,
  data() {
    return {
      count: 1,
    }
  },
  watch: {
    count: {
      handler() {
        console.log('watch');
      },
      immediate: true,
    }
  },
  computed: {
    computedCount() {
      console.log('computed');
      return this.count + 1;
    }
  },
  created() {
    console.log('created');
  },
});

当前例子的执行顺序为:watch --> created --> computed

为什么?

new Vue的实例化过程中,会执行初始化方法this._init,其中有代码:

Vue.prototype._init = function (options) {
    // ...
    initState(vm);
    // ...
    callHook(vm, 'created');
    // ...
    if (vm.$options.el) {
      vm.$mount(vm.$options.el);
    }
}

function initState (vm) {
  vm._watchers = [];
  var opts = vm.$options;
  if (opts.props) { initProps(vm, opts.props); }
  if (opts.methods) { initMethods(vm, opts.methods); }
  if (opts.data) {
    initData(vm);
  } else {
    observe(vm._data = {}, true /* asRootData */);
  }
  if (opts.computed) { initComputed(vm, opts.computed); }
  if (opts.watch && opts.watch !== nativeWatch) {
    initWatch(vm, opts.watch);
  }
}

猛一看代码,是不是发现先执行的initComputed(vm, opts.computed),然后执行initWatch(vm, opts.watch),再执行callHook(vm, 'created'),那为啥不是computed --> watch --> created呢?

不要着急,听我娓娓道来。

1、关于initComputed

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 (!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') {
        // ...
    }
  }
}

在通过initComputed初始化计算属性的时候,通过遍历的方式去处理当前组件中的computed。首先,在进行计算属性实例化的时候,将{ lazy: true }作为参数传入,并且实例化的Watcher中的getter就是当前例子中的computedCount函数;其次,通过defineComputed(vm, key, userDef)的方式在当前组件实例vm上为key进行userDef的处理。具体为:

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
  }
  // ...
  Object.defineProperty(target, key, sharedPropertyDefinition)
}

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
    }
  }
}

从以上可以看出,这里通过Object.defineProperty(target, key, sharedPropertyDefinition)的方式,将函数computedGetter作为get函数,只有当对key进行访问的时候,才会触发其内部的逻辑。内部逻辑watcher.evaluate()为:

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

get中有主要逻辑:

value = this.getter.call(vm, vm)

这里的this.getter就是当前例子中的:

computedCount() {
  console.log('computed');
  return this.count + 1;
}

也就是说,只有当获取computedCount的时候才会触发computed的计算,也就是在进行vm.$mount(vm.$options.el)阶段才会执行到console.log('computed')

2、关于initWatch

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)
    }
  }
}
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)
}

在通过initWatch初始化侦听器的时候,如果watch为数组,则遍历执行createWatcher,否则直接执行createWatcher。如果handler是对象或者字符串时,将其进行处理,最终作为参数传入vm.$watch中去,具体为:

Vue.prototype.$watch = function (
    expOrFn: string | Function,
    cb: any,
    options?: Object
  ): Function {
    const vm: Component = this
    if (isPlainObject(cb)) {
      return createWatcher(vm, expOrFn, cb, options)
    }
    options = options || {}
    options.user = true
    const watcher = new Watcher(vm, expOrFn, cb, options)
    if (options.immediate) {
      try {
        cb.call(vm, watcher.value)
      } catch (error) {
        handleError(error, vm, `callback for immediate watcher "${watcher.expression}"`)
      }
    }
    return function unwatchFn () {
      watcher.teardown()
    }
  }

这里获取到的options中会有immediate: true的键值,同时通过options.user = true设置usertrue,再将其作为参数传入去进行Watcher的实例化。

当前例子中options.immediatetrue,所以会执行cb.call(vm, watcher.value),也就是以vm为主体,立刻执行cb。当前例子中cb就是handler

handler() {
    console.log('watch');
},

这里就解释了当前例子中console.log('watch')是最先执行的。

然后,执行完initComputedinitWatch以后,就会通过callHook(vm, 'created')执行到生命周期中的console.log('created')了。

最后通过vm.$mount(vm.$options.el)进行页面渲染的时候,会先去创建vNode,这时就需要获取到computedCount的值,进而触发其get函数的后续逻辑,最终执行到console.log('computed')

附:为什么vue中的watch在mounted之后执行

首先,在调用mounted的时候,会进入到defineReactive函数,然后调用函数里面的set方法,将mounted中赋的新的值给传递过去,并通过调用dep.notify( )把消息发送给订阅者,继而更新订阅者的列表

后面才开始渲染watch进入Watcher的class

总结

关于vuecreatedwatch的执行顺序相对比较简单,而其中computed是通过Object.defineProperty为当前vm进行定义,再到后续创建vNode阶段才去触发执行其get函数,最终执行到计算属性computed对应的逻辑。

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