vue2响应式原理（6）-- 计算属性computed

安汉生
309 阅读3分钟

计算属性computed

<body>
    <div id="app">
      <div ref="name">{{getTotal}}</div>
      <button @click="change">change</button>
    </div>
    <script src="./vue.js"></script>
    <script>
      let vm = new Vue({
        el: '#app',
        data() {
          return {
            a: 1,
          }
        },
        computed: {
          getTotal() {
            return this.a + 7
          }
        },
        methods: {
          change() {
            this.a = 2
          }
        }
      })
    </script>
  </body>

接下来以上面这个例子举例分析

为每个计算属性创建watcher

src/core/instance/state.ts

export function initState(vm: Component) {
  const opts = vm.$options
  if (opts.computed) initComputed(vm, opts.computed)
}


const computedWatcherOptions = { lazy: true }

function initComputed(vm: Component, computed: Object) {
  // $flow-disable-line
  const watchers = (vm._computedWatchers = Object.create(null))
  // computed properties are just getters during SSR
  const isSSR = isServerRendering()

  for (const key in computed) {
    const userDef = computed[key]
    const getter = isFunction(userDef) ? userDef : userDef.get
    if (__DEV__ && getter == null) {
      warn(`Getter is missing for computed property "${key}".`, vm)
    }

    if (!isSSR) {
      // create internal watcher for the computed property.
      watchers[key] = new Watcher(
        vm,
        getter || noop,
        noop,
        computedWatcherOptions
      )
    }

    // component-defined computed properties are already defined on the
    // component prototype. We only need to define computed properties defined
    // at instantiation here.
    if (!(key in vm)) {
      defineComputed(vm, key, userDef)
    } else if (__DEV__) {
      if (key in vm.$data) {
        warn(`The computed property "${key}" is already defined in data.`, vm)
      } else if (vm.$options.props && key in vm.$options.props) {
        warn(`The computed property "${key}" is already defined as a prop.`, vm)
      } else if (vm.$options.methods && key in vm.$options.methods) {
        warn(
          `The computed property "${key}" is already defined as a method.`,
          vm
        )
      }
    }
  }
}
  1. 首先定义watchers和vm._computedWatchers： const watchers = (vm._computedWatchers = Object.create(null))
  2. 遍历computed选项，const userDef = computed[key]，获取每一个计算属性的getter，用户写的计算属性，要么是一个函数，要么就自己写一个get函数，否则在开发环境下，会抛出警告；
  3. 在浏览器环境下，为每一个计算属性创建一个watcher，存到watchers和vm._computedWatchers中，参数getter就是 getTotalCount，
  4. 如果key不存在当前实例vm，就调用defineComputed(vm, key, userDef),同时计算属性不能与data，props，methods中的属性重名，否则会抛出警告

计算属性的watcher

src/core/observer/watcher.ts

export default class Watcher implements DepTarget {
  vm?: Component | null
  expression: string
  cb: Function
  lazy: boolean
  dirty: boolean
  getter: Function
  value: any

  constructor(
    vm: Component | null,
    expOrFn: string | (() => any),
    cb: Function,
    options?: WatcherOptions | null,
    isRenderWatcher?: boolean
  ) {
   
    if ((this.vm = vm) && isRenderWatcher) {
      vm._watcher = this
    }
    if (options) {
      this.lazy = !!options.lazy
    } else {
     
    }
    this.cb = cb  
    this.dirty = this.lazy 
    if (isFunction(expOrFn)) {
      this.getter = expOrFn
    } else {
      this.getter = parsePath(expOrFn)
    }
    this.value = this.lazy ? undefined : this.get()
  }
 
}

computedWatcherOptions = {lazy:true}，故而，在getTotal计算watcher的构造函数中：this.lazy = true，this.dirty = this.lazy = true，this.value = this.lazy ? undefined : this.get(),计算属性没有立即求值，this.value = undefined

读取计算属性

src/core/instance/state.ts

export function defineComputed(
  target: any,
  key: string,
  userDef: Record<string, any> | (() => any)
) {
  const shouldCache = !isServerRendering()
  if (isFunction(userDef)) {
    sharedPropertyDefinition.get = shouldCache
      ? createComputedGetter(key)
      : createGetterInvoker(userDef)
    sharedPropertyDefinition.set = noop
  } else {
    sharedPropertyDefinition.get = userDef.get
      ? shouldCache && userDef.cache !== false
        ? createComputedGetter(key)
        : createGetterInvoker(userDef.get)
      : noop
    sharedPropertyDefinition.set = userDef.set || noop
  }
  if (__DEV__ && sharedPropertyDefinition.set === noop) {
    sharedPropertyDefinition.set = function () {
      warn(
        `Computed property "${key}" was assigned to but it has no setter.`,
        this
      )
    }
  }
  Object.defineProperty(target, key, sharedPropertyDefinition)
}
  1. 通过Object.defineProperty给每个计算属性设置getter和setter,我们平时比较少给计算属性写set，这里我们主要关注getter，
  2. 通过isFunction(userDef)判断用户写的计算属性是函数还是一个对象形式，如果是函数形式，并且是在浏览器环境下，shuoldCache为true，计算属性的getter = createComputedGetter(key)，如果是对象形式，根据shouldCache && userDef.cache !== false判断，也会得到getter = createComputedGetter(key)，
  3. 所以当我们在模板中读取getTotalCount时，得到的是createComputedGetter(key)

接下来看一下createComputedGetter(key)

src/core/instance/state.ts

function createComputedGetter(key) {
  return function computedGetter() {
    const watcher = this._computedWatchers && this._computedWatchers[key]
    if (watcher) {
      if (watcher.dirty) {
        watcher.evaluate()
      }
      if (Dep.target) {
        watcher.depend()
      }
      return watcher.value
    }
  }
}

watcher = this._computedWatchers && this._computedWatchers[key] ，取出计算属性getTotal对应的watcher，如果dirty为true，才会更新，调用watcher.evaluate()到底做了什么，

export default class Watcher implements DepTarget {
   /**
   * Evaluate the value of the watcher.
   * This only gets called for lazy watchers.
   */
  evaluate() {
    this.value = this.get()
    this.dirty = false
  }
}

模板第一次读取getTotal，dirty为true，因为computedWatcherOptions = { lazy: true }作为watcher的option参数传入，接着执行this.get后,把dirty置为false

重要的逻辑来了：

export default class Watcher implements DepTarget {
  get() {
    pushTarget(this)
    let value
    const vm = this.vm
    try {
      value = this.getter.call(vm, vm)
    } catch (e: any) {
    
    } finally {
      popTarget()
    }
    return value
  }
}

Dep.target = null
const targetStack: Array<DepTarget | null | undefined> = []

export function pushTarget(target?: DepTarget | null) {
  targetStack.push(target)
  Dep.target = target
}

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

通过专栏前面分析，模板读取了getTotal，targetStack=[ 渲染watcher ]，

执行pushTarget(this)：

targetStack=[ 渲染watcher，getTotal计算watcher ]，

Dep.target=getTotal计算watcher，

然后执行value = this.getter.call(vm, vm)又读取了响应式数据a，触发a的getter，a的dep实例执行dep.depend(),接着 Dep.target.addDep(this)，

export default class Dep {
  depend(info?: DebuggerEventExtraInfo) {
     if (Dep.target) {
       Dep.target.addDep(this)
       }
     }
   }
  }

export default class Watcher implements DepTarget {
  addDep(dep: Dep) {
     //省略去重优化
     dep.addSub(this)    
    }
  }  
 }

getTotal计算watcher收集了a的dep，同时a的dep也收集了getTotal计算watcher

接着执行popTarget()：

targetStack=[ 渲染watcher ]

Dep.target=渲染watcherr，

在get中返回value值8，然后把this.dirty置为false

接着执行 if (Dep.target) { watcher.depend() }，此时Dep.target=渲染watcherr，这个watcher是getTotal计算watcher,

export default class Watcher implements DepTarget {
  
  depend() {
    let i = this.deps.length
    while (i--) {
      this.deps[i].depend()
    }
  }

}

通过这个方法，调用了a的dep.depend，然后又是Dep.target.addDep(this)，dep.addSub(this)，此时:

a的subs = [ getTotal计算watcher，渲染watcher]

计算属性中依赖的响应式数据更新

export function defineReactive(
  obj: object,
  key: string,
  val?: any,
  customSetter?: Function | null,
  shallow?: boolean,
  mock?: boolean
) {
 const dep = new Dep() 
 set: function reactiveSetter(newVal) {
      // ...
      dep.notify()   
      // ...
  })  
 }
 
 
 export default class Dep {
   notify(info?: DebuggerEventExtraInfo) {
     const subs = this.subs.slice()
     for (let i = 0, l = subs.length; i < l; i++) {
      subs[i].update()
     }
  }
}

export default class Watcher implements DepTarget {
  update() {
    if (this.lazy) {
      this.dirty = true
    } else if (this.sync) {
      this.run()
    } else {
      queueWatcher(this)
    }
  }
}

当a更新,调用a的dep.notify,变量a的subs，按照顺序:

先调用getTotal计算watcher的update和渲染watcher的update，因为getTotal计算watcher的lazy是true，所以这里只是把this.dirty = true，

接着调用渲染watcher，渲染watcher在模板中会读取计算属性getTotal，继续走到total的getter：

function createComputedGetter(key) {
  return function computedGetter() {
    const watcher = this._computedWatchers && this._computedWatchers[key]
    if (watcher) {
      if (watcher.dirty) {
        watcher.evaluate()
      }
      if (Dep.target) {
        watcher.depend()
      }
      return watcher.value
    }
  }
}

dirty为true了，就更新watcher.evaluate()，通过watcher.value拿到最新值渲染到页面

结论:

1. 计算属性watcher中lazy为true,dirty也是true，第一次读取计算属性，在它的getter中,计算最新值，同时dirty改为false

2. 此后无论怎么读取计算属性，由于dirty为false，都不会重新计算执行watcher.evaluate()

3. 只有当计算属性依赖的数据变化时，依赖数据会触发set,会调用它收集的计算属性watcher的update方法，把dirty改回true，读取计算属性就会重新计算

4. 但是，如果计算属性中依赖的状态变化，但是计算属性的返回值没有变化，组件渲染watcher也会执行，只是页面上没有UI变化看不出来

avatar
文章
阅读
粉丝