// 创建 vm
let vm = new Vue({
 data: 'a'
})

// 键路径
vm.$watch('a.b.c', function () {
 // 做点什么
})

export function observe (val) {
 if (!val || typeof val !== 'object') {
  return
 }
 return new Observer(val)
}

function defineReactive (obj, key, val) {
 var dep = new Dep()

 var property = Object.getOwnPropertyDescriptor(obj, key)
 // 是否允许修改
 if (property && property.configurable === false) {
  return
 }

 // 获取定义好的 get set 函数
 var getter = property && property.get
 var setter = property && property.set

 var childOb = observe(val)
 Object.defineProperty(obj, key, {
  enumerable: true,
  configurable: true,
  get: () => {
   var value = getter ? getter.call(obj) : val
   // 说明是 Watcher 初始化时获取的, 就添加订阅者
   if (Dep.target) {
    dep.depend()
    if (childOb) {
     childOb.dep.depend()
    }
    // if isArray do some....
   }
   return value
  },
  set: (newVal) => {
   var value = getter ? getter.call(obj) : val
   if (value === newVal) {
    return
   }
   if (setter) {
    setter.call(obj, newVal)
   } else {
    val = newVal
   }
   childOb = observe(newVal)
   dep.notify()
  }
 })
}

export default function Dep () {
 this.subs = []
}

// 就是你！！~
Dep.target = null 

// 添加订阅者
Dep.prototype.addSub = function (sub) {
 this.subs.push(sub)
}

// 添加依赖
Dep.prototype.depend = function () {
 Dep.target.addDep(this)
}

// 通知订阅者：要更新啦~
Dep.prototype.notify = function () {
 this.subs.forEach(sub => sub.update())
}

export default function Watcher (vm, expOrFn, cb) {
 this.cb = cb
 this.vm = vm
 this.expOrFn = expOrFn
 this.value = this.get()
}

Watcher.prototype.get = function () {
 Dep.target = this
 const value = this.vm._data[this.expOrFn]
 // 此时 target 有值，此时执行到了上面的 defineReactive 函数中 get 函数。就添加订阅者
 Dep.target = null
 // 为了不重复添加 就设置为 null
 return value
}

// Vue 实例
export default function Vue(options) {
 this.$options = options
 this._initState()
}

// 初始化State
Vue.prototype._initState = function () {
 let data = this._data = this.$options.data
 Object.keys(data).forEach(key => this._proxy(key))
 observe(data, this)
}

// $watch 函数,
Vue.prototype.$watch = function (expOrFn, fn, options) {
 new Watcher(this, expOrFn, fn)
}