学习vue源码—mvvm稍微有去了解过vue是如何双向绑定的我们都很容易知道vue是通过Object.definePro

这一篇主要是讲解一下vue里mvvm的原理，以及如何理解vue实现mvvm。

稍微有去了解过vue是如何双向绑定的我们都很容易知道vue是通过Object.defineProperty劫持data属性的setter和getter，但是这仅仅只是实现的一部分，在这个实现里我们还要理解dep（订阅中心）和watcher（订阅者）的概念。

`dep`—订阅中心

dep代码在./src/core/observer/dep.js文件里，下面简单讲解一下：

dep的定义参考了观察者设计模式，每一个dep有自己的唯一标识id和订阅者列表subs。
addSub和removeSub用来管理订阅者列表subs。
depend用来收集watcher（订阅者）。
notify用来通知watcher（订阅者）执行更新。
Dep.target刚开始看是比较难理解的一个概念，Dep.target其实是调用当前dep对应属性的watcher。举个例子：假如data有个属性name，那么当data.name的getter被触发时，我们需要知道是谁在调用这个data.name的getter，这就是Dep.target。

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]
}

`watcher`—订阅者

watcher代码在./src/core/observer/watcher.js文件里，关于watcher的选项配置就不细说了，在这里我们只需要重点关注其中的get 、update、run、evaluate这几个方法。

这几个方法的作用稍后解释，现在我们要先理解怎样才会产生一个watcher。在vue里面，有三种类型的watcher：

每一个组件的实例都是一个watcher
在组件的watch选项中声明的watcher
计算属性所使用的依赖值会给对应的依赖值添加一个watcher

讲完watcher的来源后我们再来看这几个方法的讲解：

先从update讲起，当某个响应属性发生变化时触发setter后，执行dep.notify通知每个watcher执行update，代码比较简单，三个逻辑分支，判断this.lazy，这是应用于计算属性时会触发的逻辑分支，this.sync则用于判断同步执行watcher的回调，否则推入queueWatcher后续执行。
run和evaluate都是会调用get方法，只是run方法是用于组件实例的watcher和watch选项中声明的watcher，watch选项中声明的watcher的this.user为true，在run方法中的this.cb.call(this.vm, value, oldValue)这段代码则是我们watch选项中触发的回调。至于evaluate方法则更加简单了，调用get方法然后设置this.dirty为false则是为了后续其他地方使用这个计算属性的时候不需要重新计算，这也是计算属性缓存的一部分逻辑。
接下来讲讲get方法，pushTarget(this)这段则是设置Dep.target为当前watcher实例，其实就是告诉dep是谁在获取属性。value = this.getter.call(vm, vm)则是获取当前值，在这里三种类型的watcher的getter是不一样的。
最后提一下，计算属性的值一般是在组件实例的watcher执行getter的过程中执行计算的。

watcher类型	getter
组件实例	render函数
watch	执行parsePath方法生成的函数
计算属性	执行createComputedGetter方法生成的函数

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

其实学习vue的mvvm，重点在于dep和watcher的理解，要明白这两个类的实例在双向绑定的过程中扮演的是一个什么样角色，单纯从代码上可能不太容易理解这样设计的意图，但是如果能有一个比较具象化的东西来对应，相信对你的理解会有非常大的帮助。

学习vue源码—mvvm

dep—订阅中心

watcher—订阅者

总结

`dep`—订阅中心

`watcher`—订阅者