vue 数据劫持 响应式原理 Observer Dep Watcher

1、vue响应式原理流程图概览

vue 数据劫持 响应式原理 Observer Dep Watcher_第1张图片

 

2、具体流程

(1)vue示例初始化(源码位于instance/index.js)

import { initMixin } from './init'
import { stateMixin } from './state'
import { renderMixin } from './render'
import { eventsMixin } from './events'
import { lifecycleMixin } from './lifecycle'
import { warn } from '../util/index'

function Vue (options) {
  if (process.env.NODE_ENV !== 'production' &&
    !(this instanceof Vue)
  ) {
    warn('Vue is a constructor and should be called with the `new` keyword')
  }
  this._init(options)
}

initMixin(Vue)
stateMixin(Vue)
eventsMixin(Vue)
lifecycleMixin(Vue)
renderMixin(Vue)

export default Vue

响应式相关的是“stateMixin”。

 

(2)、state.js(源码位于instance/state.js)

与响应式有关的是:

function initData (vm: Component) {
  let data = vm.$options.data
  data = vm._data = typeof data === 'function'
    ? getData(data, vm)
    : data || {}
  if (!isPlainObject(data)) {
    data = {}
    process.env.NODE_ENV !== 'production' && warn(
      'data functions should return an object:\n' +
      'https://vuejs.org/v2/guide/components.html#data-Must-Be-a-Function',
      vm
    )
  }
  // proxy data on instance
  const keys = Object.keys(data)
  const props = vm.$options.props
  const methods = vm.$options.methods
  let i = keys.length
  while (i--) {
    const key = keys[i]
    if (process.env.NODE_ENV !== 'production') {
      if (methods && hasOwn(methods, key)) {
        warn(
          `Method "${key}" has already been defined as a data property.`,
          vm
        )
      }
    }
    if (props && hasOwn(props, key)) {
      process.env.NODE_ENV !== 'production' && warn(
        `The data property "${key}" is already declared as a prop. ` +
        `Use prop default value instead.`,
        vm
      )
    } else if (!isReserved(key)) {
      proxy(vm, `_data`, key)
    }
  }
  // observe data
  observe(data, true /* asRootData */)
}

在initData中实现了2个功能:

(2).1 将data中的对象代理(proxy)到_data

说明proxy函数也是使用的Object.defineProperty,

export function proxy (target: Object, sourceKey: string, key: string) {
  sharedPropertyDefinition.get = function proxyGetter () {
    return this[sourceKey][key]
  }
  sharedPropertyDefinition.set = function proxySetter (val) {
    this[sourceKey][key] = val
  }
  Object.defineProperty(target, key, sharedPropertyDefinition)
}

也就是说vm._data.变量都是响应式数据(即vm.变量)

(2).2 将data中的数据变为响应式数据,即

// observe data
  observe(data, true /* asRootData */)

 

(3)observe类

第(2)步的observe函数:

export function observe (value: any, asRootData: ?boolean): Observer | void {
  if (!isObject(value) || value instanceof VNode) {
    return
  }
  let ob: Observer | void
  if (hasOwn(value, '__ob__') && value.__ob__ instanceof Observer) {
    ob = value.__ob__
  } else if (
    shouldObserve &&
    !isServerRendering() &&
    (Array.isArray(value) || isPlainObject(value)) &&
    Object.isExtensible(value) &&
    !value._isVue
  ) {
    ob = new Observer(value)
  }
  if (asRootData && ob) {
    ob.vmCount++
  }
  return ob
}

调用了Observer类:

现在看Observer的构造函数和walk方法:

constructor (value: any) {
    this.value = value
    this.dep = new Dep()
    this.vmCount = 0
    def(value, '__ob__', this)
    if (Array.isArray(value)) {
      const augment = hasProto
        ? protoAugment
        : copyAugment
      augment(value, arrayMethods, arrayKeys)
      this.observeArray(value)
    } else {
      this.walk(value)
    }
  }

  /**
   * Walk through each property and convert them into
   * getter/setters. This method should only be called when
   * value type is Object.
   */
  walk (obj: Object) {
    const keys = Object.keys(obj)
    for (let i = 0; i < keys.length; i++) {
      defineReactive(obj, keys[i])
    }
  }

需要说明的是,并不是data中的所有数据都会变成响应式的。

请看例子:

new Vue({
    template: 
        `
text1: {{text1}} text2: {{text2}}
`, data: { text1: 'text1', text2: 'text2', text3: 'text3' } });

data中text3并没有被模板实际用到,为了提高代码执行效率,我们没有必要对其进行响应式处理,因此,依赖收集简单点理解就是收集只在实际页面中用到的data数据,即text1和text2

上面的defineReactive方法将数据变为响应式,核心代码:
Object.defineProperty(obj, key, {
    enumerable: true,
    configurable: true,
    get: function reactiveGetter () {
      const value = getter ? getter.call(obj) : val
      if (Dep.target) {
        dep.depend()
        if (childOb) {
          childOb.dep.depend()
          if (Array.isArray(value)) {
            dependArray(value)
          }
        }
      }
      return value
    },
    set: function reactiveSetter (newVal) {
      const value = getter ? getter.call(obj) : val
      /* eslint-disable no-self-compare */
      if (newVal === value || (newVal !== newVal && value !== value)) {
        return
      }
      /* eslint-enable no-self-compare */
      if (process.env.NODE_ENV !== 'production' && customSetter) {
        customSetter()
      }
      if (setter) {
        setter.call(obj, newVal)
      } else {
        val = newVal
      }
      childOb = !shallow && observe(newVal)
      dep.notify()
    }

defineReactive函数中会用到Dep类来收集依赖(dep.depend())以及当数据变化时触发更新(dep.notify())。

 

(4)Dep类

export default class Dep {
  static target: ?Watcher;
  id: number;
  subs: Array<Watcher>;

  constructor () {
    this.id = uid++
    this.subs = []
  }

  addSub (sub: Watcher) {
    this.subs.push(sub)
  }

  removeSub (sub: Watcher) {
    remove(this.subs, sub)
  }

  depend () {
    if (Dep.target) {
      Dep.target.addDep(this)
    }
  }

  notify () {
    // stabilize the subscriber list first
    const subs = this.subs.slice()
    if (process.env.NODE_ENV !== 'production' && !config.async) {
      // subs aren't sorted in scheduler if not running async
      // we need to sort them now to make sure they fire in correct
      // order
      subs.sort((a, b) => a.id - b.id)
    }
    for (let i = 0, l = subs.length; i < l; i++) {
      subs[i].update()
    }
  }
}

Dep类的构造函数中的subs是Watcher(观察者)类。vue实例中data的一个值,可以添加多个Watcher,同时这个值变化的时候也是触发这多个Watcher的更新。

 

(5)Watcher类

Watcher类主要用来收集依赖和触发更新。

Watcher类也是实现了$watch(),即:https://cn.vuejs.org/v2/api/#watch

(6)Observer、Dep和Watcher类关系

vue 数据劫持 响应式原理 Observer Dep Watcher_第2张图片

Observer类是书店(vue实例的data对象),里面有好多书(Dep类),每本书可以被订阅(Watcher类)。

当某一本书更新时,订阅的Watcher类会收到通知,进而更新书店内容(vue实例的data对象)。

Dep类是Observer类和Watcher类链接的桥梁。

 

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