关于Vue源码vm.$watch()内部原理详解

<script>
  const app = new Vue({
    el: "#app",
    data: {
      a: {
        b: {
          c: 'c'
        }
      }
    },
    mounted () {
      this.$watch(function () {
        return this.a.b.c
      }, this.handle, {
        deep: true,
        immediate: true // 默认会初始化执行一次handle
      })
    },
    methods: {
      handle (newVal, oldVal) {
        console.log(this.a)
        console.log(newVal, oldVal)
      },
      changeValue () {
        this.a.b.c = 'change'
      }
    }
  })
</script>

// 判断是否是对象
export function isPlainObject (obj: any): boolean {
 return _toString.call(obj) === '[object Object]'
}

// $watch 方法允许我们观察数据对象的某个属性，当属性变化时执行回调
// 接受三个参数: expOrFn(要观测的属性), cb, options(可选的配置对象)
// cb即可以是一个回调函数, 也可以是一个纯对象(这个对象要包含handle属性。)
// options: {deep, immediate}, deep指的是深度观测, immediate立即执行回掉
// $watch()本质还是创建一个Watcher实例对象。

Vue.prototype.$watch = function (
  expOrFn: string | Function,
  cb: any,
  options"${watcher.expression}"`)
   }
  }
  // 返回一个函数，这个函数的执行会解除当前观察者对属性的观察
  return function unwatchFn () {
   // 执行teardown()
   watcher.teardown()
  }
 }

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: "${expOrFn}" ` +
     'Watcher only accepts simple dot-delimited paths. ' +
     'For full control, use a function instead.',
     vm
    )
   }
  }
  // 求值
  this.value = this.lazy
   "${this.expression}"`)
   } else {
    throw e
   }
  } finally {
   // "touch" every property so they are all tracked as
   // dependencies for deep watching
   if (this.deep) {
    // 递归地读取被观察属性的所有子属性的值
    // 这样被观察属性的所有子属性都将会收集到观察者，从而达到深度观测的目的。
    traverse(value)
   }
   popTarget()
   this.cleanupDeps()
  }
  return value
 }

 /**
  * 记录自己都订阅过哪些Dep
  */
 addDep (dep: Dep) {
  const id = dep.id
  // newDepIds: 避免在一次求值的过程中收集重复的依赖
  if (!this.newDepIds.has(id)) {
   this.newDepIds.add(id) // 记录当前watch订阅这个dep
   this.newDeps.push(dep) // 记录自己订阅了哪些dep
   if (!this.depIds.has(id)) {
    // 把自己订阅到dep
    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)
   }
  }
  //newDepIds 属性和 newDeps 属性被清空
  // 并且在被清空之前把值分别赋给了 depIds 属性和 deps 属性
  // 这两个属性将会用在下一次求值时避免依赖的重复收集。
  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) {
   // 指定同步更新
   this.run()
  } else {
   // 异步更新队列
   queueWatcher(this)
  }
 }

 /**
  * Scheduler job interface.
  * Will be called by the scheduler.
  */
 run () {
  if (this.active) {
   const value = this.get()
   // 对比新值 value 和旧值 this.value 是否相等
   // 是对象的话即使值不变(引用不变)也需要执行回调
   // 深度观测也要执行
   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) {
     // 意味着这个观察者是开发者定义的，所谓开发者定义的是指那些通过 watch 选项或 $watch 函数定义的观察者
     try {
      this.cb.call(this.vm, value, oldValue)
     } catch (e) {
      // 回调函数在执行的过程中其行为是不可预知, 出现错误给出提示
      handleError(e, this.vm, `callback for watcher "${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()
  }
 }

 /**
  * 把Watcher实例从从当前正在观测的状态的依赖列表中移除
  */
 teardown () {
  if (this.active) {
   // 该观察者是否激活状态 
   if (!this.vm._isBeingDestroyed) {
    // _isBeingDestroyed一个标识，为真说明该组件实例已经被销毁了，为假说明该组件还没有被销毁
    // 将当前观察者实例从组件实例对象的 vm._watchers 数组中移除
    remove(this.vm._watchers, this)
   }
   // 当一个属性与一个观察者建立联系之后，属性的 Dep 实例对象会收集到该观察者对象
   let i = this.deps.length
   while (i--) {
    this.deps[i].removeSub(this)
   }
   // 非激活状态
   this.active = false
  }
 }
}

export const unicodeRegExp = /a-zA-Z\u00B7\u00C0-\u00D6\u00D8-\u00F6\u00F8-\u037D\u037F-\u1FFF\u200C-\u200D\u203F-\u2040\u2070-\u218F\u2C00-\u2FEF\u3001-\uD7FF\uF900-\uFDCF\uFDF0-\uFFFD/
const bailRE = new RegExp(`[^${unicodeRegExp.source}.$_\\d]`)
// path为keypath(属性路径) 处理'a.b.c'(即vm.a.b.c) => a[b[c]]
export function parsePath (path: string): any {
 if (bailRE.test(path)) {
  return
 }
 const segments = path.split('.')
 return function (obj) {
  for (let i = 0; i < segments.length; i++) {
   if (!obj) return
   obj = obj[segments[i]]
  }
  return obj
 }
}