function initData(vm) { //將data上數據復制到_data并遍歷所有屬性添加代理
 vm._data = vm.$options.data;
 const keys = Object.keys(vm._data); 
 let i = keys.length;
 while(i--) { 
  const key = keys[i];
  proxy(vm, `_data`, key);
 }
 observe(data, true /* asRootData */) //對data進行監(jiān)聽
}

export function observe (value) {
 //遞歸子屬性時的判斷
 if (!isObject(value) || value instanceof VNode) {
  return
 }
 ...
 ob = new Observer(value)
}
export class Observer {
 constructor (value) {
  ... //此處省略對數組的處理
  this.walk(value)
 }

 walk (obj: Object) {
  const keys = Object.keys(obj)
  for (let i = 0; i < keys.length; i++) {
   defineReactive(obj, keys[i]) //為每個屬性創(chuàng)建setter/getter
  }
 }
 ...
}

//設置set/get
export function defineReactive (
 obj: Object,
 key: string,
 val: any
) {
 //利用閉包存儲每個屬性關聯的watcher隊列，當setter觸發(fā)時依然能訪問到
 const dep = new Dep()
 ...
 //如果屬性為對象也創(chuàng)建相應observer
 let childOb = observe(val)
 Object.defineProperty(obj, key, {
  enumerable: true,
  configurable: true,
  get: function reactiveGetter () {
   if (Dep.target) {
    dep.depend() //將當前dep傳到對應watcher中再執(zhí)行watcher.addDep將watcher添加到當前dep.subs中
    if (childOb) { //如果屬性是對象則繼續(xù)收集依賴
     childOb.dep.depend()
     ...
    }
   }
   return value
  },
  set: function reactiveSetter (newVal) {
   ...
   childOb = observe(newVal) //如果設置的新值是對象，則為其創(chuàng)建observe
   dep.notify() //通知隊列中的watcher進行更新
  }
 })
}

export default class Dep {
 static target: ?Watcher; //指向當前watcher
 constructor () {
  this.subs = []
 }
 //添加watcher
 addSub (sub: Watcher) {
  this.subs.push(sub)
 }
 //移除watcher
 removeSub (sub: Watcher) {
  remove(this.subs, sub)
 }
 //通過watcher將自身添加到dep中
 depend () {
  if (Dep.target) {
   Dep.target.addDep(this)
  }
 }
 //派發(fā)更新信息
 notify () {
  ...
  for (let i = 0, l = subs.length; i < l; i++) {
   subs[i].update()
  }
 }
}

//解析表達式（a.b）,返回一個函數
export function parsePath (path: string): any {
 if (bailRE.test(path)) {
  return
 }
 const segments = path.split('.')
 return function (obj) {
  for (let i = 0; i < segments.length; i++) {
   if (!obj) return
   obj = obj[segments[i]]  //遍歷得到表達式所代表的屬性
  }
  return obj
 }
}
export default class Watcher {
 constructor (
  vm: Component,
  expOrFn: string | Function,
  cb: Function,
  options?: ?Object,
  isRenderWatcher?: boolean
 ) {
  this.vm = vm
  if (isRenderWatcher) {
   vm._watcher = this
  } 
  //對創(chuàng)建的watcher進行收集，destroy時對這些watcher進行銷毀
  vm._watchers.push(this)
  // options
  if (options) {
   ...
   this.before = options.before
  }
  ...
  //上一輪收集的依賴集合Dep以及對應的id
  this.deps = []
  this.depIds = new Set()
  //新收集的依賴集合Dep以及對應的id
  this.newDeps = []
  this.newDepIds = new Set()
  this.expression = process.env.NODE_ENV !== 'production'
   ? expOrFn.toString()
   : ''
  // parse expression for getter
  if (typeof expOrFn === 'function') {
   this.getter = expOrFn
  } else {
   this.getter = parsePath(expOrFn)
   ...
  }
  ...
  this.value = this.get()
 }

 /** * Evaluate the getter, and re-collect dependencies. */
 get () {
  pushTarget(this)
  let value
  const vm = this.vm
  try {
   value = this.getter.call(vm, vm)
  } catch (e) {
   if (this.user) {
    handleError(e, vm, `getter for watcher "${this.expression}"`)
   } else {
    throw e
   }
  } finally {
   // "touch" every property so they are all tracked as
   // dependencies for deep watching
   if (this.deep) {
    traverse(value)
   }
   popTarget()
   this.cleanupDeps() //清空上一輪的依賴
  }
  return value
 }

 /** * Add a dependency to this directive. */
 addDep (dep: Dep) {
  const id = dep.id
  if (!this.newDepIds.has(id)) { //同一個數據只收集一次
   this.newDepIds.add(id)
   this.newDeps.push(dep)
   if (!this.depIds.has(id)) {
    dep.addSub(this)
   }
  }
 }

 //每輪收集結束后去除掉上輪收集中不需要跟蹤的依賴
 cleanupDeps () {
  let i = this.deps.length
  while (i--) {
   const dep = this.deps[i]
   if (!this.newDepIds.has(dep.id)) {
    dep.removeSub(this)
   }
  }
  let tmp = this.depIds
  this.depIds = this.newDepIds
  this.newDepIds = tmp
  this.newDepIds.clear()
  tmp = this.deps
  this.deps = this.newDeps
  this.newDeps = tmp
  this.newDeps.length = 0
 },
 update () {
  ...
  //經過一些優(yōu)化處理后，最終執(zhí)行this.get
  this.get();
 }
 // ...
}

updateComponent = () => {
 vm._update(vm._render(), hydrating) //執(zhí)行render生成VNode并更新dom
}
new Watcher(vm, updateComponent, noop, {
 before () {
  if (vm._isMounted) {
   callHook(vm, 'beforeUpdate')
  }
 }
}, true /* isRenderWatcher */)

while (i--) {
   const dep = this.deps[i]
   if (!this.newDepIds.has(dep.id)) {
    dep.removeSub(this)
   }
  }
  let tmp = this.depIds
  this.depIds = this.newDepIds
  this.newDepIds = tmp
  this.newDepIds.clear()
  tmp = this.deps
  this.deps = this.newDeps
  this.newDeps = tmp
  this.newDeps.length = 0

<body>
 <div id="app">
  <div v-if='flag'> </div>   
  <div v-else> </div> 
  <button @click="msg1 += '1'">change</button>   
  <button @click="flag = !flag">toggle</button>  
 </div> 
  <script type="text/javascript">
  var vm = new Vue({
   el: '#app',
   data: {
    flag: true,
    msg1: 'msg1',
    msg2: 'msg2'
   }
  })
  </script> 
</body>