前言

这篇文章和上篇文章一样，简单的介绍一下Watcher这个class，大致浏览一番即可，此处无需看源码，可略过

--以下vm都代指vue的实例

--以下所有代码来自Vue 2.6.11 版本

贴源码

Watcher对象定义在 src/core/observer/watcher.js 中，未删注释，代码略长。

Watcher，正如其名称一样：观察者。那被观察的东西是什么？就是 Dep对象，也是 data props computed 的值，除了能观测这些，还可以监测Vuex中的 getters ，Vue-Router中的 $route.query

在 defineReactive 源码中我们可以看 const dep = new Dep()，这个dep 会和每个字段的 value 一起通过闭包保存在内存中，并且是一对一绑定的。当 Watcher 观测的value 触发了get逻辑，就会通过dep完成 依赖收集，当触发 set的时候，就会通过dep完成 派发更新。

使用Watcher的地方有这么几个

1：computed，计算属性，这里称之为 computed-watcher

在代码中找到 evaluate 这个方法（译为：估计，评估），这里有求值的意思，这个方法其实是专门给 computed（计算属性）使用的，方法上面的注释也说明了，配合this.lazy字段使用，在后面单独讲 initComputed 逻辑的时候就会知道这玩意是怎么用的了

2：watch，监听器，这里称之为 user-watcher

{
  ...............
  watch:{
    xxxx:function(val){}
  }
}

watch 怎么用的就不说了，就是一个数据观测，在 initWatch 的过程中也是会创建Watcher对象（后面单独讲 initWatch 的时候再细说）

3：组件的Watcher，这里称之为 render-watcher

每一个vm中，有且只有一个 render-watcher ，这个watcher是用来触发视图更新的。被render-watcher观测的数据发生变化时，就会由它触发视图的更新，它就是 data 和 view 连接的中间人，触发的方法叫 update 源码中能看到，方法上的注释也说了，当监听的数据发生了变化，就会执行这个方法。

在控制台中打印一个vm可以看到，_watcher就是那唯一一个render-watcher，_watchers是一个数组，会把vm中所有的watcher都装在里面

我们在Watcher的构造函数的逻辑中也可以看到对 _watcher 的赋值，以及对 _watchers 的添加

至于其它方法的作用，单独在这里说没啥意义，下一篇文章开始将会细说响应式的完整流程

/* @flow */

import {
  warn,
  remove,
  isObject,
  parsePath,
  _Set as Set,
  handleError,
  noop
} from '../util/index'

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

import type { SimpleSet } 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.
 */
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)
    // 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()
  }

  /**
   * 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()
    }
    return value
  }

  /**
   * Add a dependency to this directive.
   */
  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.
   */
  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.
   */
  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.
      if (!this.vm._isBeingDestroyed) {
        remove(this.vm._watchers, this)
      }
      let i = this.deps.length
      while (i--) {
        this.deps[i].removeSub(this)
      }
      this.active = false
    }
  }
}

总结

最后放一张图，辅助理解整个响应原理的流程，后面的文章我会时不时的回头看这张图