Vue中computed分析#

// dev/src/core/instance/state.js line 47
export function initState (vm: Component) {
  vm._watchers = []
  const opts = vm.$options // 获取组件定义的选项
  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) // 定义computed属性则进行初始化
  if (opts.watch && opts.watch !== nativeWatch) {
    initWatch(vm, opts.watch)
  }
}

// dev/src/core/instance/state.js line 169
function initComputed (vm: Component, computed: Object) {
  // $flow-disable-line
  const watchers = vm._computedWatchers = Object.create(null) // 创建一个没有原型链指向的对象
  // computed properties are just getters during SSR
  const isSSR = isServerRendering()

  for (const key in computed) {
    const userDef = computed[key] // 获取计算属性的key值定义
    const getter = typeof userDef === 'function' ? userDef : userDef.get // 由于计算属性接受两种类型的参数 此处判断用以获取getter
    if (process.env.NODE_ENV !== 'production' && getter == null) {
      warn(
        `Getter is missing for computed property "${key}".`,
        vm
      )
    }

    if (!isSSR) {
      // create internal watcher for the computed property.
      // 生成computed watcher(vm, getter, noop, { lazy: true })
      watchers[key] = new Watcher( // 计算属性创建观察者watcher和消息订阅器dep
        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)) { // 检查重名属性
      defineComputed(vm, key, userDef) // 定义属性
    } else if (process.env.NODE_ENV !== 'production') {
      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)
      }
    }
  }
}

// dev/src/core/instance/state.js line 31
const sharedPropertyDefinition = {
  enumerable: true,
  configurable: true,
  get: noop,
  set: noop
}

// dev/src/core/instance/state.js line 210
export function defineComputed (
  target: any,
  key: string,
  userDef: Object | Function
) {
  const shouldCache = !isServerRendering()
  if (typeof userDef === 'function') {
    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 (process.env.NODE_ENV !== 'production' &&
      sharedPropertyDefinition.set === noop) {
    sharedPropertyDefinition.set = function () {
      warn(
        `Computed property "${key}" was assigned to but it has no setter.`,
        this
      )
    }
  }
  Object.defineProperty(target, key, sharedPropertyDefinition)
}

/**
 经过重写之后的属性描述符在某条件分支大致呈现如下
 sharedPropertyDefinition = {
    enumerable: true,
    configurable: true,
    get: function computedGetter () {
      const watcher = this._computedWatchers && this._computedWatchers[key]
      if (watcher) {
        if (watcher.dirty) {
          watcher.evaluate()
        }
        if (Dep.target) {
          watcher.depend()
        }
        return watcher.value
      }
    },
    set: userDef.set || noop
 } 
 当计算属性被调用时便会执行 get 访问函数，从而关联上观察者对象 watcher 然后执行 wather.depend() 收集依赖和 watcher.evaluate() 计算求值。
*/