/* @flow */

import { queueWatcher } from './scheduler'

import Dep, { pushTarget, popTarget } from './dep'

import {

  warn,

  remove,

  isObject,

  parsePath,

  _Set as Set,

  handleError

} from '../util/index'

let uid = 0

/**

 * A watcher parses an expression, collects dependencies,

 * and fires callback when the expression value changes.

 * This is used for both the $watch() api and directives.

   一个 watcher编译成 一个表达式 和 依赖集合, 还能出发回调当 v-model的值改变
    这也能用到wathcer api中, 如果 vm.$watch('abc', fn);

 */

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: Set;

  newDepIds: Set;

  getter: Function;

  value: any;

  constructor (

    vm: Component,

    expOrFn: string | Function,

    cb: Function,

    options?: Object

  ) {
    //这个watcher所属的vm

    this.vm = vm

    // 一个vm 有多个watchers , vm.$watch(a,fn1)  vm.$watch(b,fn2) {{b.UpperCase()}} 等等

    vm._watchers.push(this)

    // options

    if (options) {
      //是否深度检测依赖

      this.deep = !!options.deep

      this.user = !!options.user

      this.lazy = !!options.lazy

      this.sync = !!options.sync

    } 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') {
      //监控函数

      this.getter = expOrFn

    } else {
      //监控表达式, 注意 这里也会返回一个路径的函数

      this.getter = parsePath(expOrFn)

      if (!this.getter) {

        this.getter = function () {}

        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()

  }

  /**

   * Evaluate the getter, and re-collect dependencies.

   */

  get () {
    //Dep.target = this

    pushTarget(this)

    let value

    const vm = this.vm

    if (this.user) {

      try {
        // 因为传入了 vm, 所以在methods中可以取得this.data.abc
        // 比如 this.getter 是 fucntion(){ return this.price * this.nums; }; 因为执行这个函数, 也就会去获取 this.price 和 this.nums的值
        // 就会分别触发这两个数据的get方法, 然后这个watcher就会加入这两个数据的dep, this.push(depq) this.push(dep2); 通过这一步就收集了这个函数的两个依赖

        value = this.getter.call(vm, vm)

      } catch (e) {

        handleError(e, vm, `getter for watcher "${this.expression}"`)

      }

    } else {

      value = this.getter.call(vm, vm)

    }

    // "touch" every property so they are all tracked as

    // dependencies for deep watching
    //“触摸”每一个属性，这样它们都被跟踪为 深度观察依赖

    if (this.deep) {

      traverse(value)

    }
 

    popTarget()

    this.cleanupDeps()

    return value

  }

  /**

   * Add a dependency to this directive.
     添加一个依赖到这个指令

   */

  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.

   */

  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

  }

  /**

   * Subscriber interface.

   * Will be called when a dependency changes.

   */

  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.

   */

  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) {

          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 the value of the watcher.

   * This only gets called for lazy watchers.

     计算一个watcher的值, 这个只会被 延迟watchers调用

   */

  evaluate () {

    this.value = this.get()

    this.dirty = false

  }

  /**

   * Depend on all deps collected by this watcher.
     通过这个watcher, 收集所有的依赖?

   */

  depend () {

    let i = this.deps.length

    while (i--) {

      this.deps[i].depend()

    }

  }

  /**

   * Remove self from all dependencies' subscriber list.

     从所有的依赖订阅清单中移除自己

   */

  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.
      // 移除依赖是一个昂贵的操作, 所有如果这个vm已经销毁le, 我们就跳一跳

      if (!this.vm._isBeingDestroyed) {

        remove(this.vm._watchers, this)

      }

      let i = this.deps.length

      while (i--) {

        this.deps[i].removeSub(this)

      }

      this.active = false

    }

  }

}

/**

 * Recursively traverse an object to evoke all converted

 * getters, so that every nested property inside the object

 * is collected as a "deep" dependency.

   递归追踪一个对象去唤起所有的已经转换的getters,
   这样在一个对象的所以嵌套的熟悉都被收集成为一个深层的依赖

 */

const seenObjects = new Set()

function traverse (val: any) {

  seenObjects.clear()

  _traverse(val, seenObjects)

}

function _traverse (val: any, seen: Set) {

  let i, keys

  const isA = Array.isArray(val)

  if ((!isA && !isObject(val)) || !Object.isExtensible(val)) {

    return

  }

  if (val.__ob__) {

    const depId = val.__ob__.dep.id

    if (seen.has(depId)) {

      return

    }

    seen.add(depId)

  }

  if (isA) {

    i = val.length

    while (i--) _traverse(val[i], seen)

  } else {
    //for in 都不用了, 加快速度

    keys = Object.keys(val)

    i = keys.length

    while (i--) _traverse(val[keys[i]], seen)

  }

}