new Vue({
  el: "#app",
  template: `<div>
    <div>{{count}}</div>
  </div>`,
  data() {
    return {
      count: 1,
    }
  },
  created() {
    this.count = 2;
    this.count = 3;
  },
});

export function defineReactive (
  obj: Object,
  key: string,
  val: any,
  customSetter?: ?Function,
  shallow?: boolean
) {
  // 重點：創(chuàng)建一個發(fā)布者實例
  const dep = new Dep()
  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
  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) {
        // 重點：進行當前正在計算的渲染W(wǎng)atcher的收集
        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)
      // 重點：當數(shù)據(jù)發(fā)生變化時，發(fā)布者實例dep會通知收集到的watcher進行更新
      dep.notify()
    }
  })
}

Vue.prototype._init = function (options) {
    // ...
    initState(vm);
    // ...
    callHook(vm, 'created');
    // ...
    if (vm.$options.el) {
      vm.$mount(vm.$options.el);
    }
}

new Vue({
  el: "#app",
  template: `<div>
    <div>{{count}}</div>
  </div>`,
  data() {
    return {
      count: 1,
    }
  },
  created() {
    setTimeout(() => {
      this.count = 2;
    }, 0)
    setTimeout(() => {
      this.count = 3;
    }, 0)
  },
});

new Vue({
  el: "#app",
  template: `<div>
    <div>{{count}}</div>
  </div>`,
  data() {
    return {
      count: 1,
    }
  },
  created() {
    setTimeout(() => {
      this.count = 1;
    }, 0)
  },
});