vue源码 （2.响应式原理）

1.props[keyname]优先级为什么高于methods[keyname]

function initMethods (vm: Component, methods: Object) {
  const props = vm.$options.props
  //判重处理 props上key 优先 methods
  for (const key in methods) {
    if (process.env.NODE_ENV !== 'production') {
      if (typeof methods[key] !== 'function') {
        warn(
          `Method "${key}" has type "${typeof methods[key]}" in the component definition. ` +
          `Did you reference the function correctly?`,
          vm
        )
      }
      //**********************************主要是在此处**********************************
      if (props && hasOwn(props, key)) {
        warn(
          `Method "${key}" has already been defined as a prop.`,
          vm
        )
      }
      if ((key in vm) && isReserved(key)) {
        warn(
          `Method "${key}" conflicts with an existing Vue instance method. ` +
          `Avoid defining component methods that start with _ or $.`
        )
      }
    }
    vm[key] = typeof methods[key] !== 'function' ? noop : bind(methods[key], vm)
  }
}

2.vue中是如何实现props的代理的或者说为什么实用this[propsKey]可以访问到this._props.propsKey

 //initProps方法中调用
if (!(key in vm)) {
    proxy(vm, `_props`, key)
}

const sharedPropertyDefinition = {
  enumerable: true,
  configurable: true,
  get: noop,
  set: noop
}

export function proxy (target: Object, sourceKey: string, key: string) {
  sharedPropertyDefinition.get = function proxyGetter () {
    return this[sourceKey][key]
  }
  sharedPropertyDefinition.set = function proxySetter (val) {
    this[sourceKey][key] = val
  }
  Object.defineProperty(target, key, sharedPropertyDefinition)
}

3.computed实现缓存的原理

  //computed中 initComputed中，本质是通过watcher实现的，每一个computed对象访问的时候相当于是watcher[key]。
 // const computedWatcherOptions = { lazy: true }
function initComputed (vm: Component, computed: Object) {
  // $flow-disable-line
  const watchers = vm._computedWatchers = Object.create(null)
  // computed properties are just getters during SSR
  const isSSR = isServerRendering()
  //便利computed中的对象 
  for (const key in computed) {
    const userDef = computed[key]
    const getter = typeof userDef === 'function' ? userDef : userDef.get
    if (process.env.NODE_ENV !== 'production' && getter == null) {
      warn(
        `Getter is missing for computed property "${key}".`,
        vm
      )
    }

    if (!isSSR) {
      // create internal watcher for the computed property.
      
       // ****************//实例化一个watch 所以 computed 时通过watch实现的*****************
      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 (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)
      }
    }
  }
}

function createComputedGetter (key) {
  return function computedGetter () {
    const watcher = this._computedWatchers && this._computedWatchers[key]
    if (watcher) {
      //**************************执行watcher.evaluate 执行computed.key 得倒执行结果赋值给watch,value**************************
      //一次渲染中指执行一次，在下一次更新时才会执行 参考watcher.update()
      if (watcher.dirty) {
        watcher.evaluate()
        //里边执行的是watch.get() watch.dirty=false ==>watch.getter()==>computed.key==>func
      }
      if (Dep.target) {
        watcher.depend()
      }
      return watcher.value
    }
  }
}

3.computed和watch有什么区别

  computed默认懒执行，不可更改，watch可配置
  使用场景不同（watch常用来异步处理）