Vue源码阅读连载之响应式设计

Vue源码阅读连载之响应式设计

书接上回，谈到Vue是在Vue.prototype._init里调用了initState初始化了响应式的组成，看一下initState的源码

export function initState (vm: Component) {
  vm._watchers = []
  const opts = vm.$options
  if (opts.props) initProps(vm, opts.props)
  if (opts.methods) initMethods(vm, opts.methods)
  if (opts.data) {
    initData(vm)
  } else {
    observe(vm._data = {}, true /* asRootData */)
  }
  if (opts.computed) initComputed(vm, opts.computed)
  if (opts.watch && opts.watch !== nativeWatch) {
    initWatch(vm, opts.watch)
  }
}

这里面除了methods，其它像props、data之类的都是响应式的，如果在模板里面引用到了其中某个变量，在变量发生变化的时候会主动通知进而更新DOM。

Object.defineProperty

只要读过官网文档就都知道Vue里响应式设计的基本原理就是这个方法，这个方法在es5里被引入，属于元编程，就是说在之前版本的JavaScript里无法使用既有的语法来对它进行模拟，这也就是Vue无法支持IE9以下的浏览器的原因。

但是Object.defineProperty也有它的缺陷，一些变化还是追踪不到的，比如在对象上新增一个值，delete一个值，以及数组上针对索引直接赋值等，Vue对此提供了$set和$delete方法来补充，后面会讲到。

要真正弥补缺陷，还是要依靠es6的Proxy，但它也是属于元编程。即便是Babel目前也对它无能为力。

所以要等到完美实现自动挡的响应式，还是要让es6得到浏览器的全面支持才行。

看一下Object.defineProperty的用法

Object.defineProperties(obj, props)

其中props可以配置get和set，这样就有了一个切入点能让我来干点什么。

var obj = {};
Object.defineProperty(obj, "data", {
    get: function() {
        //把依赖收集起来
    },
    set: function() {
        //通知依赖要更新了
    }
})

所以可以抽象出一个依赖的对象来，叫Dep，此时可以把Object.defineProperty重新封装一下，在/src/core/observer/index.js里

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
      }
      // #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()
    }
  })
}

这个方法里有一个Dep对象，在get的时候就更新这个Dep对象实例维护的依赖数组，在set的时候就通知Dep对象里的依赖数组去执行更新。

这里看不懂没关系，一直往下走，走到框图那里就好了，到时候记得再回来看代码。

Dep和Watcher

Dep对象的定义在/src/core/observer/dep.js

/* @flow */

import type Watcher from './watcher'
import { remove } from '../util/index'
import config from '../config'

let uid = 0

/**
 * A dep is an observable that can have multiple
 * directives subscribing to it.
 */
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]
}

这里先记住结论，Dep就是一个中间商。Dep实例里面还有一个叫Watcher的对象。这个Watcher对象的定义在/src/core/observer/watcher.js里

let uid = 0

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
    }
  }
}

代码贴的有点敷衍[流汗]

其实Watcher对象就代表了一个响应式的内容，比如模板的{{ }}标记，在{{ }}标记里引用到的变量发生变化时更新DOM，或者某一个computed变量，在它引用到的变量发生变化时进行重新计算，如果它也是被{{ }}标记绑在模板上的话进而触发DOM的更新，还有就是在创建Vue时传入显式定义的watch了，它跟computed也是类似的。

更正：其实Vue2.0后用了虚拟DOM，所以不会为每一个模板中使用到的变量创建一个Watcher而是整个组件创建一个Watcher，剩下比较的事情就交给虚拟DOM了。

其实可以在源码里全局搜索一下"new Watcher"关键词，总共也就是四处

Watcher对象和Dep对象里都含有一个属性互相指向了对方，Dep里是subs，Watcher里是deps。

流程

重新再捋一下流程，以模板上用到props里的message为例

1. 新建Vue时，传入配置，其中props里包含这个message
2. 在调用initProps时，在Vue实例上新建一个内部的空对象_props
3. 经过defineReactive之后，这个props里的message转为_props其中的一个对象，这个对象还有get和set方法
4. 为虚拟DOM的组件新建一个Watcher对象
5. 在新建Watcher对象的构造函数里，会去取一下message的值，这样就触发了get
6. 在get里把Watcher对象跟Dep对象关联了起来
7. 下一次更新props里message的时候就触发了set，从而更新了DOM

再仔细地走一下，

new Vue( ) → Vue.prototype._init( ) → initState( ) 其中

if (opts.props) initProps(vm, opts.props) 

就去初始化props，先建出内部_props空对象，然后循环把props里面内容都通过defineReactive包装为 _props的一个属性，这个属性包含了get和set。

function initProps (vm: Component, propsOptions: Object) {
  const propsData = vm.$options.propsData || {}
  const props = vm._props = {}
  // cache prop keys so that future props updates can iterate using Array
  // instead of dynamic object key enumeration.
  const keys = vm.$options._propKeys = []
  const isRoot = !vm.$parent
  // root instance props should be converted
  if (!isRoot) {
    toggleObserving(false)
  }
  for (const key in propsOptions) {
    keys.push(key)
    const value = validateProp(key, propsOptions, propsData, vm)
    defineReactive(props, key, value)

    if (!(key in vm)) {
      proxy(vm, `_props`, key)
    }
  }
  toggleObserving(true)
}

等到模板解析的时候发现用到了这个message，于是去新建一个Watcher对象，在它的构造函数最后有调用

this.value = this.lazy
      ? undefined
      : this.get()

而get( )里就去获得这个值

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
  }

注意一下pushTarget和popTarget两个方法，它是把当前的Watcher对象放入Dep.target上，注意这个是Dep，也就是类，不是Dep的实例，这样保证了唯一性，反正JavaScript里是单线程的。this.getter在构造函数里被声明为

this.getter = parsePath(expOrFn)

也就是针对表达式的解析，比如"data.x.y.z"这种样子的，

value = this.getter.call(vm, vm)

这一句就能获得这个表达式的值，也就相当于访问了data.x.y.z。

既然访问了这个属性，那么就触发了这个属性上的get，从而把Dep和Watcher建立起连接来，等到下一次触发set的时候自然也就能找到这个属性上挂载的所有的Watcher，从而得到更新。