vue源码解析六 -- 依赖收集，派发更新和nextTick

依赖收集

上篇文章中我们讲了响应对象，这次看看他是如何来做依赖收集的，首先我们看看一下依赖收集的源码

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()
      }
      // #7981: for accessor properties without setter
      if (getter && !setter) return
      if (setter) {
        setter.call(obj, newVal)
      } else {
        val = newVal
      }
      childOb = !shallow && observe(newVal)
      dep.notify()
    }
  })

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()
    if (process.env.NODE_ENV !== 'production' && !config.async) {
      // subs aren't sorted in scheduler if not running async
      // we need to sort them now to make sure they fire in correct
      // order
      subs.sort((a, b) => a.id - b.id)
    }
    for (let i = 0, l = subs.length; i < l; i++) {
      subs[i].update()
    }
  }
}

// The current target watcher being evaluated.
// This is globally unique because only one watcher
// can be evaluated at a time.
Dep.target = null
const targetStack = []

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

export function popTarget () {
  targetStack.pop()
  Dep.target = targetStack[targetStack.length - 1]
}

  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
    }
  }
}

解释1

• vue中做依赖收集主要是通过Dep来做的，首先判断有没有dep.target，这个target就是一个watcher实例，
  如果有那么调用dep.depend();这个函数在dep的源码里看我们知道其实是调用了watcher实例的addDep方法，那么addDep的方法主要做的事情是把dep实例的id和实例给到newDepIds,newDeps做一份备份，但是最重要的还是调用dep.addSub(this)；

• dep.addSub是dep实例上的一个方法，我们去他的源码里面看到这个方法的作用是把watcher实例push到this.subs数组，此时我们可以看到这是一个典型的观察者模式，把所有的数据对应的watcher都放到this.subs里面为了就是notify来更新数据，接下来我们就来讲如何派发更新的

解释2

上面我们看到是如何做依赖收集，那么现在我们来看vue中是如何派发更新的，显然派发更新的代码在setter里面，首先比较的是旧值和新设置的值是不是一样的，是一样的直接return。然后调用自定义的setter,然后执行了!shallow && observe(newVal)，目的是防止新设置的值是对象，如果是对象的把对象变成响应式的。最后调用notify非常重要

• 我们看一个notify做的事情，他主要是遍历了this.subs让其中的watcher对象都执行update方法，我们在进入到watcher源码里面看看update这个函数做了什么事情。这个函数进行了两次判断，判断有没有开启lazy,判断是不是同步watcher最后执行queueWatcher(this),为什么要执行这个了，因为vue的更新是合并的，同一个数据对象的多次改变指挥触发一次watcher,并且这么做是要把vue数据的更改到dom的渲染变成异步。要在下一次的nextTick时候触发，所以下面我们来看看queueWatcher做的事情

queueWatcher源码

const queue: Array<Watcher> = []
const activatedChildren: Array<Component> = []
let has: { [key: number]: ?true } = {}
let circular: { [key: number]: number } = {}
let waiting = false
let flushing = false
let index = 0

/**
 * Reset the scheduler's state.
 */
function resetSchedulerState () {
  index = queue.length = activatedChildren.length = 0
  has = {}
  if (process.env.NODE_ENV !== 'production') {
    circular = {}
  }
  waiting = flushing = false
}

// Async edge case #6566 requires saving the timestamp when event listeners are
// attached. However, calling performance.now() has a perf overhead especially
// if the page has thousands of event listeners. Instead, we take a timestamp
// every time the scheduler flushes and use that for all event listeners
// attached during that flush.
export let currentFlushTimestamp = 0

// Async edge case fix requires storing an event listener's attach timestamp.
let getNow: () => number = Date.now

// Determine what event timestamp the browser is using. Annoyingly, the
// timestamp can either be hi-res (relative to page load) or low-res
// (relative to UNIX epoch), so in order to compare time we have to use the
// same timestamp type when saving the flush timestamp.
// All IE versions use low-res event timestamps, and have problematic clock
// implementations (#9632)
if (inBrowser && !isIE) {
  const performance = window.performance
  if (
    performance &&
    typeof performance.now === 'function' &&
    getNow() > document.createEvent('Event').timeStamp
  ) {
    // if the event timestamp, although evaluated AFTER the Date.now(), is
    // smaller than it, it means the event is using a hi-res timestamp,
    // and we need to use the hi-res version for event listener timestamps as
    // well.
    getNow = () => performance.now()
  }
}

/**
 * Flush both queues and run the watchers.
 */
function flushSchedulerQueue () {
  currentFlushTimestamp = getNow()
  flushing = true
  let watcher, id

  // Sort queue before flush.
  // This ensures that:
  // 1. Components are updated from parent to child. (because parent is always
  //    created before the child)
  // 2. A component's user watchers are run before its render watcher (because
  //    user watchers are created before the render watcher)
  // 3. If a component is destroyed during a parent component's watcher run,
  //    its watchers can be skipped.
  queue.sort((a, b) => a.id - b.id)

  // do not cache length because more watchers might be pushed
  // as we run existing watchers
  for (index = 0; index < queue.length; index++) {
    watcher = queue[index]
    if (watcher.before) {
      watcher.before()
    }
    id = watcher.id
    has[id] = null
    watcher.run()
    // in dev build, check and stop circular updates.
    if (process.env.NODE_ENV !== 'production' && has[id] != null) {
      circular[id] = (circular[id] || 0) + 1
      if (circular[id] > MAX_UPDATE_COUNT) {
        warn(
          'You may have an infinite update loop ' + (
            watcher.user
              ? `in watcher with expression "${watcher.expression}"`
              : `in a component render function.`
          ),
          watcher.vm
        )
        break
      }
    }
  }

  // keep copies of post queues before resetting state
  const activatedQueue = activatedChildren.slice()
  const updatedQueue = queue.slice()

  resetSchedulerState()

  // call component updated and activated hooks
  callActivatedHooks(activatedQueue)
  callUpdatedHooks(updatedQueue)

  // devtool hook
  /* istanbul ignore if */
  if (devtools && config.devtools) {
    devtools.emit('flush')
  }
}

function callUpdatedHooks (queue) {
  let i = queue.length
  while (i--) {
    const watcher = queue[i]
    const vm = watcher.vm
    if (vm._watcher === watcher && vm._isMounted && !vm._isDestroyed) {
      callHook(vm, 'updated')
    }
  }
}

/**
 * Queue a kept-alive component that was activated during patch.
 * The queue will be processed after the entire tree has been patched.
 */
export function queueActivatedComponent (vm: Component) {
  // setting _inactive to false here so that a render function can
  // rely on checking whether it's in an inactive tree (e.g. router-view)
  vm._inactive = false
  activatedChildren.push(vm)
}

function callActivatedHooks (queue) {
  for (let i = 0; i < queue.length; i++) {
    queue[i]._inactive = true
    activateChildComponent(queue[i], true /* true */)
  }
}

/**
 * Push a watcher into the watcher queue.
 * Jobs with duplicate IDs will be skipped unless it's
 * pushed when the queue is being flushed.
 */
export function queueWatcher (watcher: Watcher) {
  const id = watcher.id
  if (has[id] == null) {
    has[id] = true
    if (!flushing) {
      queue.push(watcher)
    } else {
      // if already flushing, splice the watcher based on its id
      // if already past its id, it will be run next immediately.
      let i = queue.length - 1
      while (i > index && queue[i].id > watcher.id) {
        i--
      }
      queue.splice(i + 1, 0, watcher)
    }
    // queue the flush
    if (!waiting) {
      waiting = true

      if (process.env.NODE_ENV !== 'production' && !config.async) {
        flushSchedulerQueue()
        return
      }
      nextTick(flushSchedulerQueue)
    }
  }
}

/* @flow */
/* globals MutationObserver */

import { noop } from 'shared/util'
import { handleError } from './error'
import { isIE, isIOS, isNative } from './env'

export let isUsingMicroTask = false

const callbacks = []
let pending = false

function flushCallbacks () {
  pending = false
  const copies = callbacks.slice(0)
  callbacks.length = 0
  for (let i = 0; i < copies.length; i++) {
    copies[i]()
  }
}

// Here we have async deferring wrappers using microtasks.
// In 2.5 we used (macro) tasks (in combination with microtasks).
// However, it has subtle problems when state is changed right before repaint
// (e.g. #6813, out-in transitions).
// Also, using (macro) tasks in event handler would cause some weird behaviors
// that cannot be circumvented (e.g. #7109, #7153, #7546, #7834, #8109).
// So we now use microtasks everywhere, again.
// A major drawback of this tradeoff is that there are some scenarios
// where microtasks have too high a priority and fire in between supposedly
// sequential events (e.g. #4521, #6690, which have workarounds)
// or even between bubbling of the same event (#6566).
let timerFunc

// The nextTick behavior leverages the microtask queue, which can be accessed
// via either native Promise.then or MutationObserver.
// MutationObserver has wider support, however it is seriously bugged in
// UIWebView in iOS >= 9.3.3 when triggered in touch event handlers. It
// completely stops working after triggering a few times... so, if native
// Promise is available, we will use it:
/* istanbul ignore next, $flow-disable-line */
if (typeof Promise !== 'undefined' && isNative(Promise)) {
  const p = Promise.resolve()
  timerFunc = () => {
    p.then(flushCallbacks)
    // In problematic UIWebViews, Promise.then doesn't completely break, but
    // it can get stuck in a weird state where callbacks are pushed into the
    // microtask queue but the queue isn't being flushed, until the browser
    // needs to do some other work, e.g. handle a timer. Therefore we can
    // "force" the microtask queue to be flushed by adding an empty timer.
    if (isIOS) setTimeout(noop)
  }
  isUsingMicroTask = true
} else if (!isIE && typeof MutationObserver !== 'undefined' && (
  isNative(MutationObserver) ||
  // PhantomJS and iOS 7.x
  MutationObserver.toString() === '[object MutationObserverConstructor]'
)) {
  // Use MutationObserver where native Promise is not available,
  // e.g. PhantomJS, iOS7, Android 4.4
  // (#6466 MutationObserver is unreliable in IE11)
  let counter = 1
  const observer = new MutationObserver(flushCallbacks)
  const textNode = document.createTextNode(String(counter))
  observer.observe(textNode, {
    characterData: true
  })
  timerFunc = () => {
    counter = (counter + 1) % 2
    textNode.data = String(counter)
  }
  isUsingMicroTask = true
} else if (typeof setImmediate !== 'undefined' && isNative(setImmediate)) {
  // Fallback to setImmediate.
  // Techinically it leverages the (macro) task queue,
  // but it is still a better choice than setTimeout.
  timerFunc = () => {
    setImmediate(flushCallbacks)
  }
} else {
  // Fallback to setTimeout.
  timerFunc = () => {
    setTimeout(flushCallbacks, 0)
  }
}

export function nextTick (cb?: Function, ctx?: Object) {
  let _resolve
  callbacks.push(() => {
    if (cb) {
      try {
        cb.call(ctx)
      } catch (e) {
        handleError(e, ctx, 'nextTick')
      }
    } else if (_resolve) {
      _resolve(ctx)
    }
  })
  if (!pending) {
    pending = true
    timerFunc()
  }
  // $flow-disable-line
  if (!cb && typeof Promise !== 'undefined') {
    return new Promise(resolve => {
      _resolve = resolve
    })
  }
}

解释

• 这个函数首先判断has对象有没有watcher.id.最开始肯定是没有直接进来，然后判断flushing从源码可以看到是false然后把watcher实例push到queue队列中，然后判断waiting最开始是false直接进来判断config.async根据代码查看是true,所以最后执行的是 nextTick(flushSchedulerQueue)，flushSchedulerQueue是一个回调函数等会在说
• nextTick从源码可以看到每次调用他的时候，他都会把传入的回调此时也就是flushSchedulerQueue，push到callbacks数组里面然后调用timerFunc，这个函数是根据浏览器的支持状况二不一样，从代码里面看到如果支持promise那么就用promise来模拟异步，如果不支持的话判断是否支持MutationObserver ，在判断是否支持setImmediate，都不支持的话降级为setTimeout，但是无论哪种都是异步执行的。
• 不管此时有多少个数组被改变，他都会收集flushSchedulerQueue到callback里面在下一个nextTick时候在执行
• flushSchedulerQueue做的事情是首先把queue队列排序从小到大，目的注释已经说了，下面是翻译的
  1.组件的更新由父到子；因为父组件的创建过程是先于子的，所以watcher的创建也是先父后子，执行顺序也应该保持先父后子。
  2.用户的自定义watcher要优先于渲染watcher执行；因为用户自定义watcher是在渲染watcher之前创建的。
  3.如果一个组件在父组件的watcher执行期间被销毁，那么它对应的watcher执行都可以被跳过，所以父组件的watcher应该先执行。
  然后has[id]设置为null,为下次做准备。关键的就是执行watcher.run()，这个是触发更新的主要函数，run函数先通过this.get()得到它当前的值，然后做判断，如果满足新旧值不等、新值是对象类型、deep模式任何一个条件，则执行watcher的回调，注意回调函数执行的时候会把第一个和第二个参数传入新值value和旧值oldValue，这就是当我们添加自定义watcher的时候能在回调函数的参数中拿到新旧值的原因。渲染watcher在执行this.get()时候就执行了updateComponent = () => {vm._update(vm._render(), hydrating)}