Vue.prototype.$watch = function (expOrFn, cb, options) {
  const vm = this
  options = options || {}
  const watcher = new Watcher(vm, expOrFn, cb, options) 
  if(options.immediate) {
    cb.call(vm, watcher,.value)
  }
  return function unwatchFn() {
    watcher.teardown()
  }
}

class Watcher {
  constructor (vm, expOrFn, cb) {
    this.vm = vm
    this.deps = []
    this.depIds = new Set()
    if(typeof expOrFn === 'function') {
      this.getter = expOrFn
    }else {
      this.getter = parsePath(expOrFn)
    }
    this.cb = cb
    this.value = this.get()
  }
  ....
  addDep (dep) {
    const id = dep.id       //参数dep是Dep实例对象
    if(!this.depIds.has(id)) {  //判断是否存在避免重复添加
      this.depIds.add(id)    
      this.deps.push(dep)
      dep.addSub(this)     //this 是依赖
    }
  }
  teardown () {
    let i = this.deps.length
    while (i--) {
      this.deps[i].removeSub(this)
    }
  }
}
let uid = 0
class Dep {
  constructor () {
    this.id = uid++
    ...
  }
  ...
  depend () {
    if(window.target) {
      window.target.addDep(this)  //将this即当前dep对象加入到watcher对象上
    }
  }
  removeSub (sub) {
    const index = this.subs.indexOf(sub)
    if(index > -1) {
      return this.subs.splice(index, 1)
    }
  }
}

class Watcher {
  constructor (vm, expOrFn, cb, options) {
    this.vm = vm
    this.deps = []
    this.depIds = new Set()
    if(typeof expOrFn === 'function') {
      this.getter = expOrFn
    }else {
      this.getter = parsePath(expOrFn)
    }
    if(options) {          //取值
      this.deep = !!options.deep
    }else {
      this.deep = false
    }
    this.cb = cb
    this.value = this.get()
  }
  get () {
    window.target = this
    let value = this.getter.call(vm, vm)
    if(this.deep) {
      traverse(value)
    }
    window.target = undefined
    return value
  }
  ...
}
const seenObjects = new Set()
function traverse (val) {
  _traverse(val, seenObjects)
  seenObjects.clear()
}
function _traverse(val, seen) {
  let i, keys
  const isA = Array.isArray(val)
  if((!isA && isObject(val)) || Object.isFrozen(val)) { //判断val是否是对象或者数组以及是否被冻结
    return
  }
  if(val._ob_) {
    const depId = val._ob_.dep.id   //可以看前面一篇我们对Observer类添加了this.dep = new Dep()，所以能访问其dep.id
    if(seen.has(depId)) {
      return
    }
    seen.add(depId)
  }
  if(isA) {
    i = val.length
    while (i--) _traverse(val[i], seen)
  } else {
    keys = Object.keys(val)
    i = keys.length
    while (i--) _traverse(val[i], seen)
  }
}

Vue.prototype.$set = function (target, key, val) {
  if(Array.isArray(target) && isValidArrayIndex(key)) {  //是数组并且key有效
    target.length = Math.max(target.length, key)  //处理key > target.length
    target.splice(key, 1, val)  //添加新元素，并输出依赖更新同时新元素也会进行`Obsever`处理
    return val
  }
  if(key in targert && !(key in Object.prototype) { //能遍历并且是自身key
    target[key] = val  //触发set，执行依赖更新
    return val
  }
  const ob = target._ob_
  if(target.isVue || (ob && ob.vm.Count) { //不是vue实例也不是vue实例的根对象(即不是this.$data跟对象)
    //触发警告
    return
  }
  if(!ob) {  //只添加
    target[key] = val
    return val
  }
  defineReactive(ob.value, key, val) //进行响应式处理
  ob.dep.notify() //触发依赖更新
  returnv val
}

Vue.prototype.$delete = function (target, key) {
  if(Array.isArray(target) && isValidArrayIndex(key)) {
    target.splice(key, 1)
    return
  }
  const ob = target._ob_
  if(target.isVue || (ob && ob.vm.Count) { //不是vue实例也不是vue实例的根对象(即不是this.$data跟对象)
    //触发警告
    return
  }
  if(!hasOwn(target, key)) {
    return
  }
  delete target[key]
  if(!ob) {
    return
  }
  ob.dep.notify()
}