<div id="app">
  <input id="input" type="text" v-model="text">
  <div id="text">输入的值为：{{text}}</div>
</div>
<script>
  var vm = new Vue({
      el: '#app',
      data: {
        text: 'hello world'
      }
  })
</script>

export class Observer {
  value: any;
  dep: Dep;
  vmCount: number;

  constructor (value: any) {
    this.value = value
    this.dep = new Dep()
    this.vmCount = 0
    // 添加__ob__来标示value有对应的Observer
    def(value, '__ob__', this)
    if (Array.isArray(value)) { // 处理数组
      if (hasProto) { // 实现是'__proto__' in {}
        protoAugment(value, arrayMethods)
      } else {
        copyAugment(value, arrayMethods, arrayKeys)
      }
      this.observeArray(value)
    } else { // 处理对象
      this.walk(value)
    }
  }

  // 给对象每个属性添加getter/setters
  walk (obj: Object) {
    const keys = Object.keys(obj)
    for (let i = 0; i < keys.length; i++) {
      defineReactive(obj, keys[i]) // 重点
    }
  }

  // 循环观察数组的每一项
  observeArray (items: Array<any>) {
    for (let i = 0, l = items.length; i < l; i++) {
      observe(items[i]) // 重点
    }
  }
}

export function defineReactive (
  obj: Object,
  key: string,
  val: any,
  customSetter?: ?Function,
  shallow?: boolean
) {
  const dep = new Dep()

  const property = Object.getOwnPropertyDescriptor(obj, key)
  // 带有不可配置的属性直接跳过
  if (property && property.configurable === false) {
    return
  }

  // cater for pre-defined getter/setters
  // 保存对象属性上自有的getter和setter
  const getter = property && property.get
  const setter = property && property.set
  // 如果属性上之前没有定义getter，并且没有传入初始val值，就把属性原有的值赋值给val
  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 () {
      // 给属性设置getter
      const value = getter ? getter.call(obj) : val
      if (Dep.target) {
        // 给每个属性创建一个dep
        dep.depend()
        if (childOb) {
          childOb.dep.depend()
          // 如果是数组，就递归创建
          if (Array.isArray(value)) {
            dependArray(value)
          }
        }
      }
      return value
    },
    set: function reactiveSetter (newVal) {
      // 给属性设置setter
      const value = getter ? getter.call(obj) : val
      // 值未变化，就跳过
      if (newVal === value || (newVal !== newVal && value !== value)) {
        return
      }
      if (process.env.NODE_ENV !== 'production' && customSetter) {
        customSetter() // 非生产环境自定义调试用，这里忽略
      }
      if (getter && !setter) return
      if (setter) {
        setter.call(obj, newVal)
      } else {
        val = newVal
      }
      childOb = !shallow && observe(newVal)
      // 值发生变化进行通知
      dep.notify()
    }
  })
}

export function observe (value: any, asRootData: ?boolean): Observer | void {
  // 如果不是对象就跳过
  if (!isObject(value) || value instanceof VNode) {
    return
  }
  let ob: Observer | void
  if (hasOwn(value, '__ob__') && value.__ob__ instanceof Observer) {
    // 如果已有observer，就直接返回，上面讲到过会用`__ob__`属性来记录
    ob = value.__ob__
  } else if (
    shouldObserve &&
    !isServerRendering() &&
    (Array.isArray(value) || isPlainObject(value)) &&
    Object.isExtensible(value) &&
    !value._isVue
  ) {
    // 如果没有，就创建一个
    ob = new Observer(value)
  }
  if (asRootData && ob) {
    ob.vmCount++
  }
  return ob
}

// augment在环境支持__proto__时
function protoAugment (target, src: Object) {
  target.__proto__ = src
}
// augment在环境不支持__proto__时
function copyAugment (target: Object, src: Object, keys: Array<string>) {
  for (let i = 0, l = keys.length; i < l; i++) {
    const key = keys[i]
    def(target, key, src[key])
  }
}

const arrayProto = Array.prototype
export const arrayMethods = Object.create(arrayProto)

const methodsToPatch = [
  'push',
  'pop',
  'shift',
  'unshift',
  'splice',
  'sort',
  'reverse'
]

methodsToPatch.forEach(function (method) {
  const original = arrayProto[method]
  def(arrayMethods, method, function mutator (...args) {
    const result = original.apply(this, args)
    const ob = this.__ob__
    let inserted
    switch (method) {
      case 'push':
      case 'unshift':
        inserted = args
        break
      case 'splice':
        inserted = args.slice(2)
        break
    }
    // 是push、unshift、splice时，重新观察数组，因为这三个方法都是像数组中添加新的元素
    if (inserted) ob.observeArray(inserted)
    // 通知变化
    ob.dep.notify()
    return result
  })
})

let uid = 0
export default class Dep {
  static target: ?Watcher;
  id: number;
  subs: Array<Watcher>;

  constructor () {
    this.id = uid++
    this.subs = []
  }

  // 添加订阅者
  addSub (sub: Watcher) {
    this.subs.push(sub)
  }

  // 移除订阅者
  removeSub (sub: Watcher) {
    remove(this.subs, sub)
  }

  // 添加到订阅管理器
  depend () {
    if (Dep.target) {
      Dep.target.addDep(this)
    }
  }

  // 通知变化
  notify () {
    const subs = this.subs.slice()
    if (process.env.NODE_ENV !== 'production' && !config.async) {
      subs.sort((a, b) => a.id - b.id)
    }
    // 遍历所有的订阅者，通知更新
    for (let i = 0, l = subs.length; i < l; i++) {
      subs[i].update()
    }
  }
}

// 注，我删除了源码中一些不太重要或与双向绑定关系不太大的逻辑，删除的代码用// ... 表示
let uid = 0
export default class Watcher {
  vm: Component;
  expression: string;
  cb: Function;
  id: number;
  deep: boolean;
  user: boolean;
  lazy: boolean;
  sync: boolean;
  dirty: boolean;
  active: boolean;
  deps: Array<Dep>;
  newDeps: Array<Dep>;
  depIds: SimpleSet;
  newDepIds: SimpleSet;
  before: ?Function;
  getter: Function;
  value: any;

  constructor (
    vm: Component,
    expOrFn: string | Function,
    cb: Function,
    options?: ?Object,
    isRenderWatcher?: boolean
  ) {
    this.vm = vm
    if (isRenderWatcher) {
      vm._watcher = this
    }
    vm._watchers.push(this)
    // ...
    this.cb = cb
    this.id = ++uid
    // ...
    this.expression = process.env.NODE_ENV !== 'production'
      ? expOrFn.toString()
      : ''
    if (typeof expOrFn === 'function') {
      this.getter = expOrFn
    } else {
      this.getter = parsePath(expOrFn)
      if (!this.getter) {
        this.getter = noop
        process.env.NODE_ENV !== 'production' && warn(
          `Failed watching path: "${expOrFn}" ` +
          'Watcher only accepts simple dot-delimited paths. ' +
          'For full control, use a function instead.',
          vm
        )
      }
    }
    this.value = this.lazy
      ? undefined
      : this.get()
  }

  get () {
    pushTarget(this)
    let value
    const vm = this.vm
    // ...
    if (this.deep) {
      traverse(value)
    }
    popTarget()
    this.cleanupDeps()
    return value
  }

  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)) {
        dep.addSub(this)
      }
    }
  }

  cleanupDeps () {
    // ...
  }

  update () {
    // 更新三种模式吧，lazy延迟更新，sync同步更新直接执行，默认异步更新添加到处理队列
    if (this.lazy) {
      this.dirty = true
    } else if (this.sync) {
      this.run()
    } else {
      queueWatcher(this)
    }
  }

  run () {
    // 触发更新，在这里调用cb函数
    if (this.active) {
      const value = this.get()
      if (
        value !== this.value ||
        isObject(value) ||
        this.deep
      ) {
        const oldValue = this.value
        this.value = value
        if (this.user) {
          try {
            this.cb.call(this.vm, value, oldValue)
          } catch (e) {
            handleError(e, this.vm, `callback for watcher "${this.expression}"`)
          }
        } else {
          this.cb.call(this.vm, value, oldValue)
        }
      }
    }
  }

  evaluate () {
    // ...
  }

  depend () {
    let i = this.deps.length
    while (i--) {
      this.deps[i].depend()
    }
  }

  teardown () {
    // ...
  }
}

vm._watcher = new Watcher(vm, updateComponent, noop)