Reasons for using asynchronous components: 1. Asynchronous components can reduce the packaging results, package asynchronous components separately, and load components asynchronously, which can effectively solve the problem of a component that is too large. 2. The core of the asynchronous component can be defined as a function, and the import syntax can be used in the function to realize split loading of files.

The operating environment of this tutorial: windows7 system, vue3 version, DELL G3 computer.

Reasons for using asynchronous components

1. Asynchronous components can reduce packaging results. Asynchronous components will be packaged separately and components will be loaded asynchronously, which can effectively solve the problem of a component that is too large. Without using asynchronous components, if the component has more functions, the packaged result will become larger.

2. The core of the asynchronous component can be defined as a function. The import syntax can be used in the function to realize split loading of files. The import syntax is provided by webpack and uses jsonp. (Learning video sharing: vuejs introductory tutorial, Basic programming video)

components:{
  VideoPlay:(resolve)=>import("../components/VideoPlay")
}
 
components:{
  VideoPlay(resolve) {
    require(["../components/VideoPlay"], resolve)
  }
}
 
或者使用回調(diào)函數(shù)

Principle

In the createComponent method, there will be corresponding asynchronous components To process, first define an asyncFactory variable, and then make a judgment. If the component is a function, then the resolveAsyncComponent method will be called, and then the function assigned to the asyncFactory will be passed in, which will cause the asyncFactory to execute immediately, and will not return immediately when executed. As a result, because it is asynchronous, it returns a promise. At this time, the value is undefined, and then an asynchronous component's placeholder, empty virtual node, will be rendered first. If after loading, the factory function will be called to pass in the two parameters resolve and reject. After execution, a successful callback and a failed callback will be returned. If the promise is successful, resolve will be called. The forceRender method will be called in resolve to force the view to be updated and re-rendered. , what is called in forceRender is $forceUpdate, and the result is put on factory.resolved. If the refresh is forced, the resolveAsyncComponent method will be used again. At this time, there is a judgment. If there is a successful result, the result will be put back directly. At this time, the return value of resolveAsyncComponent is not undefined, and the component will be created, initialized, and rendered.

Source code

src/core/vdom/create-component.js

1.createComponent method

export function createComponent (
  Ctor: Class<Component> | Function | Object | void,
  data: ?VNodeData,
  context: Component,
  children: ?Array<VNode>,
  tag?: string
): VNode | Array<VNode> | void {
  let asyncFactory
  if (isUndef(Ctor.cid)) { // 看組件是否是一個函數(shù)
    asyncFactory = Ctor // 異步組件一定是一個函數(shù) 新版本提供了對象的寫法
    Ctor = resolveAsyncComponent(asyncFactory, baseCtor) //默認調(diào)用此函數(shù)時返回undefiend
    // 第二次渲染時Ctor不為undefined
    if (Ctor === undefined) {
      //返回async組件的占位符節(jié)點
      //作為注釋節(jié)點，但保留該節(jié)點的所有原始信息
      //該信息將用于異步服務(wù)器渲染和水合。
      return createAsyncPlaceholder(
        asyncFactory,
        data,
        context,
        children,
        tag
      )
    }
  }
}

2.resolveAsyncComponent method

export function resolveAsyncComponent (
  factory: Function,
  baseCtor: Class<Component>
): Class<Component> | void {
  // 如果有錯誤就返回錯誤結(jié)果
  if (isTrue(factory.error) && isDef(factory.errorComp)) {
    return factory.errorComp
  }
  // 再次渲染時可以拿到獲取的最新組件
  // 如果有成功的結(jié)果，就直接返回去
  if (isDef(factory.resolved)) {
    return factory.resolved
  }

  if (owner && !isDef(factory.owners)) {
    // forceRender 強制刷新渲染
    const forceRender = (renderCompleted: boolean) => {
      for (let i = 0, l = owners.length; i < l; i++) {
        (owners[i]: any).$forceUpdate() // 執(zhí)行$forceUpdate
      }
    }
    // 成功
    const resolve = once((res: Object | Class<Component>) => {
      factory.resolved = ensureCtor(res, baseCtor)
      if (!sync) {
        forceRender(true) // 執(zhí)行強制更新視圖重新渲染方法
      } else {
        owners.length = 0
      }
    })
    // 失敗
    const reject = once(reason => {
      if (isDef(factory.errorComp)) {
        factory.error = true
        forceRender(true)
      }
    })

    // 執(zhí)行factory 將resolve方法和reject方法傳入
    const res = factory(resolve, reject)

    sync = false
    return factory.loading ? factory.loadingComp : factory.resolved // 返回結(jié)果
  }
}

3.createAsyncPlaceholder method

// 創(chuàng)建一個異步組件的占位，空虛擬節(jié)點   也就是一個注釋<!-->
export function createAsyncPlaceholder (
  factory: Function,
  data: ?VNodeData,
  context: Component,
  children: ?Array<VNode>,
  tag: ?string
): VNode {
  const node = createEmptyVNode()
  node.asyncFactory = factory
  node.asyncMeta = { data, context, children, tag }
  return node
}

(Learning video sharing: vuejs introductory tutorial, Basic programming video)

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