vue.js 源代码学习笔记 ----- $watcher

/* @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)
  }
}