Vue进阶——源码解读 课题1 Vue数据响应式原理

目的：
数据驱动视图，即数据变化引起视图变化

怎么做：
首先，要监听到数据的变化，简称变化侦测
其次，要收集依赖，即实现数据变化后，‘对应视图’的刷新

1 变化侦测：通过Object.defineProperty()方法使属性变的可观测
代码地址：src\core\observer\index.js (Vue2.x源码)
1.1 对象(Object)的变化侦测
关键类：Observer

export class Observer {
  value: any; //flow类型检查
  dep: Dep;
  vmCount: number; // number of vms that have this object as root $data

  constructor (value: any) {
    this.value = value
    this.dep = new Dep()
    this.vmCount = 0
    def(value, '__ob__', this) //为value打上标记，表示它已经被转化成响应式了，避免重复操作
    if (Array.isArray(value)) {
      //...
      //下节课分析
    } else {
      //为对象属性
      this.walk(value)
    }
  }

  /**
   * Walk through all properties 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) //获取key组成的数组
    for (let i = 0; i < keys.length; i++) {
      defineReactive(obj, keys[i]) //可观测化
    }
  }
}

export function defineReactive (
  obj: Object,
  key: string,
  val: any,
  customSetter?: ?Function,
  shallow?: boolean
) {
  const dep = new Dep()

  const property = Object.getOwnPropertyDescriptor(obj, key)
  //获取预定义的属性描述符(https://developer.mozilla.org/zh-CN/docs/Web/JavaScript/Reference/Global_Objects/Object/getOwnPropertyDescriptor)
  if (property && property.configurable === false) {
    return
  }

  // cater for pre-defined getter/setters
  const getter = property && property.get
  const setter = property && property.set
  if ((!getter || setter) && arguments.length === 2) {
    val = obj[key]
  }

  let childOb = !shallow && 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()
      }
      // #7981: for accessor properties without setter
      if (getter && !setter) return
      if (setter) {
        setter.call(obj, newVal)
      } else {
        val = newVal
      }
      childOb = !shallow && observe(newVal)
      dep.notify()
    }
  })
}

2 依赖收集
核心思路：在getter中收集依赖，在setter中通知依赖更新
谁用到了某个数据谁就是这个数据的一个“依赖”，将数据与试图之间建立起来联系
依赖管理器类：Dep

let uid = 0

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

  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()
    }
  }
}

// The current target watcher being evaluated.
// This is globally unique because only one watcher
// can be evaluated at a time.
Dep.target = null
const targetStack = []

export function pushTarget (target: ?Watcher) {
  targetStack.push(target)
  Dep.target = target
}

export function popTarget () {
  targetStack.pop()
  Dep.target = targetStack[targetStack.length - 1]
}

由以上源码，也可以看出，有一个关键类Wather，它就是在依赖集合中的每一个依赖于某一个数据的“局部视图”

//源码位置：/src/core/observer/watcher.js