var vm = new Vue({
 data () {
  return {
   a: 'hello vue'
  }
 }
})

walk (obj: Object) {
 const keys = Object.keys(obj)
 // 遍历将其变成 vue 的访问器属性
 for (let i = 0; i < keys.length; i++) {
  defineReactive(obj, keys[i], obj[keys[i]])
 }
}

// 设置为访问器属性，并在其 getter 和 setter 函数中，使用发布/订阅模式，互相监听。
export function defineReactive (
 obj: Object,
 key: string,
 val: any
) {
 // 这里用到了观察者（发布/订阅）模式进行了劫持封装，它定义了一种一对多的关系，让多个观察者监听一个主题对象，这个主题对象的状态发生改变时会通知所有观察者对象，观察者对象就可以更新自己的状态。
 // 实例化一个主题对象，对象中有空的观察者列表
 const dep = new Dep() 
 // 获取属性描述符对象(更多的为了 computed 里面的自定义 get 和 set 进行的设计)
 const property = Object.getOwnPropertyDescriptor(obj, key)
 if (property && property.configurable === false) {
  return
 }
 const getter = property && property.get
 const setter = property && property.set 
 let childOb = observe(val)
 Object.defineProperty(obj, key, {
  enumerable: true,
  configurable: true,
  // 收集依赖，建立一对多的的关系，让多个观察者监听当前主题对象
  get: function reactiveGetter () {
   const value = getter ? getter.call(obj) : val
   if (Dep.target) {
    dep.depend()
    if (childOb) {
     childOb.dep.depend()
     // 这里是对数组进行劫持
     if (Array.isArray(value)) {
      dependArray(value)
     }
    }
   }
   return value
  },
  // 劫持到数据变更，并发布消息进行通知
  set: function reactiveSetter (newVal) {
   const value = getter ? getter.call(obj) : val
   if (newVal === value || (newVal !== newVal && value !== value)) {
    return
   }
   if (setter) {
    setter.call(obj, newVal)
   } else {
    val = newVal
   }
   childOb = observe(newVal)
   dep.notify()
  }
 })
}

return new Observer(value)

let uid = 0

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

 constructor () {
  // 用来给每个订阅者 Watcher 做唯一标识符，防止重复收集
  this.id = uid++
  // 定义subs数组，用来做依赖收集(收集所有的订阅者 Watcher)
  this.subs = []
 }

 // 收集订阅者
 addSub (sub: Watcher) {
  this.subs.push(sub)
 }

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

 notify () {
  // stabilize the subscriber list first
  const subs = this.subs.slice()
  for (let i = 0, l = subs.length; i < l; i++) {
   subs[i].update()
  }
 }
}

// the current target watcher being evaluated.
// this is globally unique because there could be only one
// watcher being evaluated at any time.
Dep.target = null

// 定义收集目标栈
const targetStack = []

export function pushTarget (_target: Watcher) {
 if (Dep.target) targetStack.push(Dep.target)
 // 改变目标指向
 Dep.target = _target
}

export function popTarget () {
 // 删除当前目标，重算指向
 Dep.target = targetStack.pop()
}

export default class Watcher {
 vm: Component;
 expression: string;
 cb: Function;

 constructor (
  vm: Component,
  expOrFn: string | Function,
  cb: Function,
  options?: Object
 ) {
  this.vm = vm
  vm._watchers.push(this)
  this.cb = cb
  // parse expression for getter
  if (typeof expOrFn === 'function') {
   this.getter = expOrFn
  } else {
   // 解析表达式
   this.getter = parsePath(expOrFn)
   if (!this.getter) {
    this.getter = function () {}
   }
  }
  this.value = this.get()
 }

 get () {
  // 将目标收集到目标栈
  pushTarget(this)
  const vm = this.vm
  
  let value = this.getter.call(vm, vm)
  // 删除目标
  popTarget()
  
  return value
 }

 // 订阅 Dep，同时让 Dep 知道自己订阅着它
 addDep (dep: Dep) {
  const id = dep.id
  if (!this.newDepIds.has(id)) {
   this.newDepIds.add(id)
   this.newDeps.push(dep)
   if (!this.depIds.has(id)) {
    // 收集订阅者
    dep.addSub(this)
   }
  }
 }

 // 订阅者'消费'动作，当接收到变更时则会执行
 update () {
  this.run()
 }

 run () {
  const value = this.get()
  const oldValue = this.value
  this.value = value
  this.cb.call(this.vm, value, oldValue)
 }
}

Vue.prototype.$on = function (event: string | Array, fn: Function): Component {
 const vm: Component = this
 if (Array.isArray(event)) {
  for (let i = 0, l = event.length; i < l; i++) {
   this.$on(event[i], fn)
  }
 } else {
  (vm._events[event] || (vm._events[event] = [])).push(fn)
 }
 return vm
}

Vue.prototype.$emit = function (event: string): Component {
 const vm: Component = this
 let cbs = vm._events[event]
 if (cbs) {
  cbs = cbs.length > 1 ? toArray(cbs) : cbs
  const args = toArray(arguments, 1)
  for (let i = 0, l = cbs.length; i < l; i++) {
   cbs[i].apply(vm, args)
  }
 }
 return vm
}