vue.js 源代码学习笔记 ----- observe

参考 vue 2.2.6版本

/* @flow */
//引入订阅者模式
import Dep from './dep'

import { arrayMethods } from './array'
import {
  def,
  isObject,
  isPlainObject,
  hasProto,
  hasOwn,
  warn,
  isServerRendering
} from '../util/index'

const arrayKeys = Object.getOwnPropertyNames(arrayMethods)

/**
 * By default, when a reactive property is set, the new value is
 * also converted to become reactive. However when passing down props,
 * we don't want to force conversion because the value may be a nested value
 * under a frozen data structure. Converting it would defeat the optimization.

  默认新的值也会被definePropty, 但是有些情况是不希望被转换的, 比如 frozen的属性, 实际不需要监听, 如果强制转换, 会破坏做的性能优化, 所以这里监听做成可配置
 */
export const observerState = {
  shouldConvert: true,
  isSettingProps: false
}

/**
 * Observer class that are attached to each observed
 * object. Once attached, the observer converts target
 * object's property keys into getter/setters that
 * collect dependencies and dispatches updates.

   每个监听对象在Observer里面被监听, 对象属性进入 getter setter方法后, 会收集依赖项 和 触发更新也的回调方法
 */
export class Observer {
  value: any;
  dep: Dep;
  vmCount: number; // number of vms that has this object as root $data

  constructor (value: any) {
    this.value = value
　

　　// 监听的是 obj = {a:{}}, 这里的观察是整个对象 obj, 而不是某个属性的变化

   /*
　　　　举个列子

       data: { a:{b:{c:1}} }  var w1 = this.$watch('a.b.c', fn1);  

       初始化的时候, 会把这个data生成 6 个dep, 其中三个是这个方法生成的, 另外三个是下面那个defineReact方法生成的
　　　　前三个对应的值分别是 {a:{b:{c:1}}} {b:{c:1}}  {c:1}
　　　　后三个对应的值分别是  data.a  a.b  b.c

　　　　上面的 w1 这个watcher收集依赖的时候, 会有5个依赖, 分别是 data.a || {b:{c:1}} ||  a.b ||  {c:1} || b.c 对应的dep
　　　　 
       在wathc a.b.c 的时候就会依次去取值 data.a => a.b => b.c, 收集这个三个属性依赖就可以了, 至于为什么要收集前三个对象依赖,
       还没有搞清楚, 可能在一些地方会在这三个对象ob中挂载一些东西, 以便发布一些东西. 
   */ 

　　this.dep = new Dep() 
　　this.vmCount = 0

    //value增加一个 属性 __ob__ , 值就是 observer
    def(value, '__ob__', this)
    if (Array.isArray(value)) {

　　  //有__proto__的浏览器才能监控数组的push pop 方法, 如果没有, 则不能监控
      const augment = hasProto
        ? protoAugment
        : copyAugment
      augment(value, arrayMethods, arrayKeys)
　　　 
      //监控数组的每一项　
      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.
     循环每一个对象属性, 逐个监控
   */
  walk (obj: Object) {
    const keys = Object.keys(obj)
    for (let i = 0; i < keys.length; i++) {
      defineReactive(obj, keys[i], obj[keys[i]])
    }
  }

  /**
   * Observe a list of Array items.
     监控数组每个元素, 如果元素也是数组, 又走数组监控的分支
   */
  observeArray (items: Array<any>) {
    for (let i = 0, l = items.length; i < l; i++) {
      observe(items[i])
    }
  }
}

// helpers

/**
 * Augment an target Object or Array by intercepting
 * the prototype chain using __proto__
   利用 __proto__ 截断原有的原型链
　　相当于
   arrA.__proto__ = arrayMethods
   这样的话 , 用户使用 arrA.push('haha');
   就会沿着 __proto__找到 arrayMethods 属性 __proto__ 的push方法, 因为这个方法被检测了, 所以会触发set方法, 最终触发 notify()
 */
function protoAugment (target, src: Object) {
  /* eslint-disable no-proto */
  target.__proto__ = src
  /* eslint-enable no-proto */
}

/**
 * Augment an target Object or Array by defining
 * hidden properties.
 *通过定义隐藏属性来加强一个目标对象 或者 数组
/* istanbul ignore next */
function copyAugment (target: Object, src: Object, keys: Array<string>) {
  for (let i = 0, l = keys.length; i < l; i++) {
    const key = keys[i]
    def(target, key, src[key])
  }
}

/**
 * Attempt to create an observer instance for a value,
 * returns the new observer if successfully observed,
 * or the existing observer if the value already has one.
   如果监控的值已经存在, 返回这个订阅者
 */
export function observe (value: any, asRootData: ?boolean): Observer | void {
  if (!isObject(value)) {
    return
  }
  let ob: Observer | void

  //如果这个值有__ob__, 直接返回
  if (hasOwn(value, '__ob__') && value.__ob__ instanceof Observer) {
    ob = value.__ob__
  } else if (
　　 //如果不是服务器 而且 能被监控, 而且是一个数组或者对象, 而且这个对象不是vue实例
    observerState.shouldConvert &&
    !isServerRendering() &&
    (Array.isArray(value) || isPlainObject(value)) &&
    Object.isExtensible(value) &&
    !value._isVue
  ) {
    //根据值来创建ob
    ob = new Observer(value)
  }
  if (asRootData && ob) {
    ob.vmCount++
  }
  return ob
}

/**
 * Define a reactive property on an Object.
   为属性添加监控主方法
 */
export function defineReactive (
  obj: Object,
  key: string,
  val: any,
  customSetter?: Function
) {

  //而这个dep是 observe下的某一个属性,  比如监听的是 obj = {a:{}}, 这里的dep是obj的属性 a
  const dep = new Dep()
　
  //获取Object的get  set函数　
  const property = Object.getOwnPropertyDescriptor(obj, key)
  if (property && property.configurable === false) {
    return
  }

  // cater for pre-defined getter/setters
  const getter = property && property.get
  const setter = property && property.set
  
  //如果val也是一个对象, 而且监控成功, 返回 一个observe实例
  let childOb = observe(val)
  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 = observe(newVal)
      dep.notify()
    }
  })
}

/**
 * Set a property on an object. Adds the new property and
 * triggers change notification if the property doesn't
 * already exist.
   对象添加一个属性, 如果新的属性, 触发通知,  数组的$set 方法 其实就是通过splice方法触发的页面变化
 */
export function set (target: Array<any> | Object, key: any, val: any): any {
  if (Array.isArray(target) && typeof key === 'number') {
    target.length = Math.max(target.length, key)
    target.splice(key, 1, val)
    return val
  }
  if (hasOwn(target, key)) {
    target[key] = val
    return val
  }
  const ob = (target : any).__ob__
  if (target._isVue || (ob && ob.vmCount)) {
    process.env.NODE_ENV !== 'production' && warn(
      'Avoid adding reactive properties to a Vue instance or its root $data ' +
      'at runtime - declare it upfront in the data option.'
    )
    return val
  }
  if (!ob) {
    target[key] = val
    return val
  }
  defineReactive(ob.value, key, val)
  ob.dep.notify()
  return val
}

/**
 * Delete a property and trigger change if necessary.
   Vue.$remove 方法触发页面变化的来源
 */
export function del (target: Array<any> | Object, key: any) {
  //判断如果是数组
  if (Array.isArray(target) && typeof key === 'number') {
    target.splice(key, 1)
    return
  }
  const ob = (target : any).__ob__
  //避免删除vue实例
  if (target._isVue || (ob && ob.vmCount)) {
    process.env.NODE_ENV !== 'production' && warn(
      'Avoid deleting properties on a Vue instance or its root $data ' +
      '- just set it to null.'
    )
    return
  }
  if (!hasOwn(target, key)) {
    return
  }
  //如果是对象
  delete target[key]
  if (!ob) {
    return
  }
  //触发通知
  ob.dep.notify()
}

/**
 * Collect dependencies on array elements when the array is touched, since
 * we cannot intercept array element access like property getters.
 */
function dependArray (value: Array<any>) {
  for (let e, i = 0, l = value.length; i < l; i++) {
    e = value[i]
    e && e.__ob__ && e.__ob__.dep.depend()
    if (Array.isArray(e)) {
      dependArray(e)
    }
  }
}