export function initState (vm: Component) {
  vm._watchers = []
  const opts = vm.$options
  if (opts.props) initProps(vm, opts.props)
  if (opts.methods) initMethods(vm, opts.methods)
  if (opts.data) {
    initData(vm)
  } else {
    observe(vm._data = {}, true /* asRootData */)
  }
  if (opts.computed) initComputed(vm, opts.computed)
  if (opts.watch && opts.watch !== nativeWatch) {
    initWatch(vm, opts.watch)
  }
}

/**
 * 嘗試給這個(gè)值去創(chuàng)建一個(gè) observer 實(shí)例，如果創(chuàng)建成功，返回新的 observer 
 * 或者如果值已經(jīng)有了，返回一個(gè)現(xiàn)有的 observer
 * @param {*} value 
 * @param {boolean} asRootData 
 * @returns Observer | void
 */
export function observe (value: any, asRootData: ?boolean): Observer | void {
  if (!isObject(value) || value instanceof VNode) {
    return
  }
  let ob: Observer | void
  // 如果 value 已經(jīng)有 observer，就返回現(xiàn)有的 observer
  // 否則如果不是服務(wù)器渲染，value是數(shù)組或者對(duì)象，value 是可擴(kuò)展的，value 不是 vue 實(shí)例，就創(chuàng)建一個(gè)新的 observer
  if (hasOwn(value, '__ob__') && value.__ob__ instanceof Observer) {
    ob = value.__ob__
  } else if (
    shouldObserve &&
    !isServerRendering() &&
    (Array.isArray(value) || isPlainObject(value)) &&
    Object.isExtensible(value) &&
    !value._isVue
  ) {
    ob = new Observer(value)
  }
  // 如果是根組件，vmCount 不為0
  if (asRootData && ob) {
    ob.vmCount++
  }
  return ob
}

/**
 * Observer 類和每個(gè)響應(yīng)式對(duì)象關(guān)聯(lián)。
 * observer 會(huì)轉(zhuǎn)化對(duì)象的屬性值的 getter/setters 方法收集依賴和派發(fā)更新。
 */
export class Observer {
  value: any;
  dep: Dep;
  vmCount: number; // number of vms that have this object as root $data

  constructor(value: any) {
    this.value = value
    this.dep = new Dep() // 存放 Observer 的 watcher 列表
    this.vmCount = 0
    def(value, '__ob__', this) // __ob__ 指向自身 observe 實(shí)例，存在 __ob__ 屬性意味著已經(jīng)被觀察過(guò)
    // 如果是數(shù)組
    if (Array.isArray(value)) {
      // hasProto = '__proto__' in {} 判斷對(duì)象是否存在 __proto__ 屬性
      if (hasProto) {
        // 如果有 __proto__，就將 value.__proto__ 指向 arrayMethods
        protoAugment(value, arrayMethods)
      } else {
        // 否則，就遍歷 arrayMethods，將值復(fù)制到 value 上
        copyAugment(value, arrayMethods, arrayKeys)
      }
      this.observeArray(value) // 數(shù)組項(xiàng)遍歷，給數(shù)組的每一項(xiàng)創(chuàng)建一個(gè) observe 實(shí)例
    } else {
      this.walk(value) // 遍歷所有的屬性，修改 getter/setters
    }
  }

  // 遍歷所有的屬性，修改 getter/setters，這個(gè)方法只有在 value 是object時(shí)調(diào)用
  walk (obj: Object) {
    const keys = Object.keys(obj)
    for (let i = 0; i < keys.length; i++) {
      defineReactive(obj, keys[i])
    }
  }

  // 數(shù)組項(xiàng)遍歷，給數(shù)組的每一項(xiàng)創(chuàng)建一個(gè) observe 實(shí)例
  observeArray (items: Array<any>) {
    for (let i = 0, l = items.length; i < l; i++) {
      observe(items[i])
    }
  }
}

/**
 * 將 target.__proto__ 指向 src
 * 攔截原型鏈__proto__，來(lái)增強(qiáng)目標(biāo)對(duì)象或數(shù)組
 * @param {*} target 
 * @param {Object} src 
 */
function protoAugment (target, src: Object) {
  /* eslint-disable no-proto */
  target.__proto__ = src
  /* eslint-enable no-proto */
}

/**
 * 遍歷 key 屬性值列表，將 src 中的 key 屬性值逐一定義到 target 的屬性中
 * 通過(guò)定義隱藏屬性，來(lái)增強(qiáng)目標(biāo)對(duì)象或數(shù)組
 * @param {Object} target 
 * @param {Object} src 
 * @param {Array<string>} keys 
 */
/* istanbul ignore next */
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]) // // 為 target 定義 key 和值
  }
}

/**
 * 在對(duì)象上定義一個(gè)響應(yīng)式的屬性。
 * @param {Object} obj 
 * @param {string} key 
 * @param {*} val 
 * @param {*} customSetter 
 * @param {*} shallow 
 * @returns 
 */
export function defineReactive (
  obj: Object,
  key: string,
  val: any,
  customSetter?: ?Function,
  shallow?: boolean
) {
  const dep = new Dep() // 為 Object 的 key 創(chuàng)建一個(gè)依賴類，會(huì)幫這個(gè)key 定義一個(gè) id 和 subs(watcher 訂閱者列表)

  const property = Object.getOwnPropertyDescriptor(obj, key)
  // 獲取 obj[key] 的屬性描述符，發(fā)現(xiàn)它是不可配置對(duì)象的話直接 return
  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]
  }
  // 對(duì) obj[key] 進(jìn)行觀察，保證對(duì)象中的所有 key 都被觀察
  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) {
        dep.depend()
        if (childOb) {
          childOb.dep.depend()
          if (Array.isArray(value)) {
            dependArray(value)
          }
        }
      }
      return value
    },
    set: function reactiveSetter (newVal) {
      // 舊的 obj[key]
      const value = getter ? getter.call(obj) : val
      // 如果新老值一樣，則直接 return，不跟新更不觸發(fā)響應(yīng)式更新過(guò)程
      /* 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()
      }
      // setter 不存在說(shuō)明該屬性是一個(gè)只讀屬性，直接 return
      // #7981: for accessor properties without setter
      if (getter && !setter) return
      // 設(shè)置新值
      if (setter) {
        setter.call(obj, newVal)
      } else {
        val = newVal
      }
      // 對(duì)新值進(jìn)行觀察，讓新值也是響應(yīng)式的
      childOb = !shallow && observe(newVal)
      dep.notify() // 通知依賴的觀察者更新
    }
  })
}

/**
 * 一個(gè) dep 對(duì)應(yīng)一個(gè) object.key，每次 key 更新時(shí)調(diào)用 dep.notify()，
 * dep 下的 subs 存放 Watcher 列表，可以調(diào)用 dep.notify() 觸發(fā) watcher.update() 使 Watcher 列表更新。
 */
export default class Dep {
  static target: ?Watcher; // Dep 類的靜態(tài)屬性，可以使用 Dep.target 訪問(wèn)，內(nèi)容是 Watcher
  id: number;
  subs: Array<Watcher>; // Watcher 組成的訂閱列表

  constructor() {
    this.id = uid++
    this.subs = [] // watcher 訂閱者列表
  }

  // 向訂閱者列表中添加一個(gè)訂閱者 Watcher
  addSub (sub: Watcher) {
    this.subs.push(sub)
  }

  // 從訂閱者列表中刪掉一個(gè) Watcher
  removeSub (sub: Watcher) {
    remove(this.subs, sub)
  }

  // 讓全局唯一的 watcher 添加當(dāng)前的依賴
  depend () {
    if (Dep.target) {
      Dep.target.addDep(this)
    }
  }

  // 通知訂閱者列表觸發(fā)更新
  notify () {
    // 用 slice() 方法拷貝一個(gè) subs，不影響 this.subs
    const subs = this.subs.slice()
    if (process.env.NODE_ENV !== 'production' && !config.async) {
      // 如果不是運(yùn)行異步，Watcher 列表不會(huì)在調(diào)度器中排序，我們需要去對(duì)他們進(jìn)行排序以確保他們按順序正確的調(diào)度
      subs.sort((a, b) => a.id - b.id)
    }
    // 依次觸發(fā) Watcher.update()
    for (let i = 0, l = subs.length; i < l; i++) {
      subs[i].update()
    }
  }
}

Dep.target = null
const targetStack = []

// 在需要進(jìn)行依賴收集的時(shí)候調(diào)用，設(shè)置 Dep.target = watcher
export function pushTarget (target: ?Watcher) {
  targetStack.push(target)
  Dep.target = target
}

// 依賴收集結(jié)束調(diào)用，設(shè)置 Dep.target 為對(duì)堆棧中前一個(gè) watcher
export function popTarget () {
  targetStack.pop()
  Dep.target = targetStack[targetStack.length - 1]
}

/**
 * Watcher 解析一個(gè)表達(dá)式，收集依賴，當(dāng)表達(dá)式發(fā)生改變時(shí)觸發(fā)調(diào)度。Watcher 類用于 $watch() api 和指令。
 */
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>; // deps 表示上一次添加的 Dep 實(shí)例數(shù)組
  newDeps: Array<Dep>; // newDeps 表示新添加的 Dep 實(shí)例數(shù)組
  depIds: SimpleSet;
  newDepIds: SimpleSet;
  before: ?Function;
  getter: Function;
  value: any;

  constructor( // 類實(shí)例化時(shí)傳入的參數(shù)會(huì)用作構(gòu)造函數(shù)的參數(shù)
    vm: Component,
    expOrFn: string | Function,
    cb: Function,
    options?: ?Object,
    isRenderWatcher?: boolean
  ) {
    this.vm = vm
    if (isRenderWatcher) {
      vm._watcher = this
    }
    vm._watchers.push(this)
    // options
    if (options) {
      this.deep = !!options.deep
      this.user = !!options.user
      this.lazy = !!options.lazy
      this.sync = !!options.sync
      this.before = options.before
    } else {
      this.deep = this.user = this.lazy = this.sync = false
    }
    this.cb = cb
    this.id = ++uid // uid for batching
    this.active = true
    this.dirty = this.lazy // for lazy watchers
    this.deps = []
    this.newDeps = []
    this.depIds = new Set()
    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)
      if (!this.getter) {
        this.getter = noop
        process.env.NODE_ENV !== 'production' && warn(
          `Failed watching path: "${expOrFn}" ` +
          'Watcher only accepts simple dot-delimited paths. ' +
          'For full control, use a function instead.',
          vm
        )
      }
    }
    this.value = this.lazy
      ? undefined
      : this.get()
  }
  // 一些原型方法
  // 以下是定義在 watcher 類原型對(duì)象上的方法，用 Watcher.prototype.get() 訪問(wèn)
  /**
   * Evaluate the getter, and re-collect dependencies.
   */
  get () {
    // 將 Dep.target 指向當(dāng)前 watcher
    pushTarget(this)
    let value
    const vm = this.vm
    try {
      // 讓 vm 調(diào)用 this.getter，并傳入 vm 作為參數(shù)
      // this.getter = expOrFn
      value = this.getter.call(vm, vm)
    } catch (e) {
      if (this.user) {
        handleError(e, vm, `getter for watcher "${this.expression}"`)
      } else {
        throw e
      }
    } finally {
      // 如果需要監(jiān)聽(tīng)對(duì)象內(nèi)部值的變化，那么調(diào)用 traverse 方法
      if (this.deep) {
        traverse(value) // 遞歸遍歷 value 的每個(gè)屬性， 確保每個(gè)屬性都被監(jiān)聽(tīng)
      }
      // 當(dāng)前 vm 的數(shù)據(jù)依賴收集已經(jīng)完成，恢復(fù) Dep.target
      popTarget()
      this.cleanupDeps()
    }
    return value
  }

  /**
   * Add a dependency to this directive.
   * 添加一個(gè)依賴：如果dep數(shù)組中沒(méi)有dep.id，那么觸發(fā) dep 訂閱當(dāng)前 watcher
   */
  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)
      }
    }
  }

  /**
   * Clean up for dependency collection.
   * 清除依賴收集
   */
  cleanupDeps () {
    // 先保持 deps 和 newDepIds數(shù)量相同
    let i = this.deps.length
    while (i--) {
      const dep = this.deps[i]
      if (!this.newDepIds.has(dep.id)) {
        dep.removeSub(this) // 如果當(dāng)前 dep 中沒(méi)有 newDepIds，就移除它的訂閱者列表
      }
    }
    // 更新 depIds、deps 為當(dāng)前的 deps，然后清除 newDepIds 和 newDeps
    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
  }

  /**
   * Subscriber interface.
   * Will be called when a dependency changes.
   */
  // 訂閱者接口，當(dāng)依賴改變時(shí)將會(huì)被調(diào)用
  update () {
    /* istanbul ignore else */
    if (this.lazy) {
      this.dirty = true
    } else if (this.sync) {
      this.run()
    } else {
      queueWatcher(this)
    }
  }

  /**
   * Scheduler job interface.
   * Will be called by the scheduler.
   */
  // 調(diào)度器工作接口，將會(huì)被調(diào)度器調(diào)用
  run () {
    if (this.active) {
      const value = this.get()
      if (
        value !== this.value ||
        // Deep watchers and watchers on Object/Arrays should fire even
        // when the value is the same, because the value may
        // have mutated.
        isObject(value) ||
        this.deep
      ) {
        // set new value
        const oldValue = this.value
        this.value = value
        if (this.user) {
          const info = `callback for watcher "${this.expression}"`
          invokeWithErrorHandling(this.cb, this.vm, [value, oldValue], this.vm, info)
        } else {
          this.cb.call(this.vm, value, oldValue)
        }
      }
    }
  }

  /**
   * Evaluate the value of the watcher.
   * This only gets called for lazy watchers.
   */
  evaluate () {
    this.value = this.get()
    this.dirty = false
  }

  /**
   * Depend on all deps collected by this watcher.
   */
  depend () {
    let i = this.deps.length
    while (i--) {
      this.deps[i].depend()
    }
  }

  /**
   * Remove self from all dependencies' subscriber list.
   * 從所有依賴項(xiàng)的訂閱者列表中刪除 self
   */
  teardown () {
    if (this.active) {
      // remove self from vm's watcher list
      // this is a somewhat expensive operation so we skip it
      // if the vm is being destroyed.
      // 如果組件不是正在被銷毀
      if (!this.vm._isBeingDestroyed) {
        remove(this.vm._watchers, this) // 從數(shù)組中刪除一個(gè)項(xiàng)目。
      }
      let i = this.deps.length
      while (i--) {
        this.deps[i].removeSub(this)
      }
      this.active = false
    }
  }
}

callHook(vm, 'beforeMount')

updateComponent = () => {
  vm._update(vm._render(), hydrating)
}

new Watcher(vm, updateComponent, noop, {
  before () {
    if (vm._isMounted && !vm._isDestroyed) {
      callHook(vm, 'beforeUpdate')
    }
  }
}, true /* isRenderWatcher */)

// this.getter = expOrFn = updateComponent()
value = this.getter.call(vm, vm)

get: function reactiveGetter () {
  const value = getter ? getter.call(obj) : val
  if (Dep.target) {
    dep.depend()
    if (childOb) {
      childOb.dep.depend()
      if (Array.isArray(value)) {
        dependArray(value)
      }
    }
  }
  return value
},

popTarget()
this.cleanupDeps()