Vue源码分析之依赖收集（九）

依赖收集就是订阅数据变化watcher的收集，依赖收集的目的是当响应式数据发生变化时，能够通知相应的订阅者去处理相关的逻辑。

在上一章，介绍了Vue将普通对象变成响应式对象是利用defineReactive()（定义在'core/observer/index.js'中）函数，defineReactive()函数中主要关注的是新建一个dep = new Dep()，以及在设置属性的getter时，对其做依赖收集：dep.depend()

export function defineReactive (
  obj: Object,
  key: string,
  val: any,
  customSetter?: ?Function,
  shallow?: boolean
) {
  const dep = new Dep()

  const property = Object.getOwnPropertyDescriptor(obj, key)
  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]
  }

  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) {
      const value = getter ? getter.call(obj) : val
      /* 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()
      }
      if (setter) {
        setter.call(obj, newVal)
      } else {
        val = newVal
      }
      childOb = !shallow && observe(newVal)
      dep.notify()
    }
  })
}

定义在'core/obersver/dep.js'中的Dep类，主要功能是建立了observer与watcher之间的桥梁。depend()函数的作用是在调用Dep.target的addDep（）函数。

这里的Dep.target是一个静态属性，在全局中唯一保存着一个Watcher，因为在同一时间应该只有一个Watcher被计算。

export default class Dep {
  static target: ?Watcher;
  id: number;
  subs: Array<Watcher>;

  constructor () {
    this.id = uid++
    this.subs = []
  }

  addSub (sub: Watcher) {
    this.subs.push(sub)
  }

  removeSub (sub: Watcher) {
    remove(this.subs, sub)
  }

  depend () {
    if (Dep.target) {
      Dep.target.addDep(this)
    }
  }

  notify () {
    // stabilize the subscriber list first
    const subs = this.subs.slice()
    for (let i = 0, l = subs.length; i < l; i++) {
      subs[i].update()
    }
  }
}

在watcher.js中定义了Watcher类，其中的deps和newDeps数组中保存了用来保存watcher的依赖类。

其中的addDep()函数主要是先通过id判断在新收集的依赖中是否有相同的依赖，没有的话，将其保存在newDeps数组中，然后再判断在以前保存的依赖中是否有相同的，没有则调用dep的addSub函数，将依赖的Watcher收集起来。这个过程主要是为了去重。

export default class Watcher {
  vm: Component;
  expression: string;
  cb: Function;
  id: number;
  deep: boolean;
  user: boolean;
  computed: boolean;
  sync: boolean;
  dirty: boolean;
  active: boolean;
  dep: Dep;
  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.computed = !!options.computed
      this.sync = !!options.sync
      this.before = options.before
    } else {
      this.deep = this.user = this.computed = this.sync = false
    }
    this.cb = cb
    this.id = ++uid // uid for batching
    this.active = true
    this.dirty = this.computed // for computed 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 = 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
        )
      }
    }
    if (this.computed) {
      this.value = undefined
      this.dep = new Dep()
    } else {
      this.value = 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)
      }
    }
  }

以渲染watcher为例说明一下过程。在执行$mount函数的过程中，最后会调用mountComponent函数，其中有一段这样的代码。

在实例化Watcher是，将updateComponent函数传入作为getter。

let updateComponent
/* istanbul ignore if */
if (process.env.NODE_ENV !== 'production' && config.performance && mark) {
  updateComponent = () => {
    const name = vm._name
    const id = vm._uid
    const startTag = `vue-perf-start:${id}`
    const endTag = `vue-perf-end:${id}`

    mark(startTag)
    const vnode = vm._render()
    mark(endTag)
    measure(`vue ${name} render`, startTag, endTag)

    mark(startTag)
    vm._update(vnode, hydrating)
    mark(endTag)
    measure(`vue ${name} patch`, startTag, endTag)
  }
} else {
  updateComponent = () => {
    vm._update(vm._render(), hydrating)
  }
}

// we set this to vm._watcher inside the watcher's constructor
// since the watcher's initial patch may call $forceUpdate (e.g. inside child
// component's mounted hook), which relies on vm._watcher being already defined
new Watcher(vm, updateComponent, noop, {
  before () {
    if (vm._isMounted) {
      callHook(vm, 'beforeUpdate')
    }
  }
}, true /* isRenderWatcher */)
hydrating = false

// manually mounted instance, call mounted on self
// mounted is called for render-created child components in its inserted hook
if (vm.$vnode == null) {
  vm._isMounted = true
  callHook(vm, 'mounted')
}

在watcher中，会调用get函数

} else {
  this.value = this.get()
}

可以看到，在get函数中，首先会pushTarget()，这个函数的功能是在将当前执行的Dep.target保存起来。以便在组件嵌套时能够恢复上一轮运行的Dep.target。

然后调用this.getter.call(vm,vm)，这里的getter就是updateComponent()函数，于是它会执行render函数。render函数在执行过程中会存在访问数据的情况，于是就触发getter。

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
}

export function pushTarget (_target: ?Watcher) {
  if (Dep.target) targetStack.push(Dep.target)
  Dep.target = _target
}

在每个对象的属性中都持有一个dep，在触发getter之后，就会调用dep.depend( )函数，然后调用Dep.target.addDep( )函数，该函数中经过去重判断之后，调用dep.addSub将渲染watcher保存在subs数组中。

回到watcher中的get( )函数，在执行完getter之后，还会执行traverse( )函数，这是为了将value中存在的属性递归执行。

if (this.deep) {
  traverse(value)
}

以及popTarget( )函数，该函数的作用是恢复上一轮执行的Dep.target。

popTarget()

然后执行cleanupDeps( ) 函数。其功能是判断在新收集的依赖中是否出现在之前的依赖收集中，此前在addDep( ) 函数中也判断过一次，但那个判断的目的是为了在deps数组和newDeps数组中去重。这次判断是为了丢掉不需要再监听的属性，避免重复触发渲染。然后再将

this.cleanupDeps()

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
}

总结

在触发数据的getter之后，会收集当前运行的watcher保存在dep的subs数组中。同时，在watcher类中，保存了相关的deps和newDeps，用来到达去重的同时，还可以清除不再需要观察的依赖。