What is the use of vue3 asynchronous components?

In vue3, asynchronous components can reduce the results of packaging. 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; if asynchronous components are not used, if If a component has more functions, the packaged result will become larger.

The operating environment of this tutorial: windows7 system, vue2.9.6 version, DELL G3 computer.

Asynchronous components are a new optimization method of vue. For example, they can be used in first-screen loading and other scenarios.

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.

components:{
  VideoPlay:(resolve)=>import("../components/VideoPlay")
}
 
components:{
  VideoPlay(resolve) {
    require(["../components/VideoPlay"], resolve)
  }
}
 
或者使用回调函数

Principle

In the createComponent method, there will be corresponding asynchronous component processing. First define an asyncFactory variable, and then judge. If the component is a function, then the resolveAsyncComponent method will be called. Then pass in the function assigned to asyncFactory, which will cause asyncFactory to execute immediately. The result will not be returned immediately during execution, because it is asynchronous and returns a promise. At this time, the value is undefined, and then it will first Renders a placeholder for an asynchronous component, an empty virtual node. 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)) { // 看组件是否是一个函数
    asyncFactory = Ctor // 异步组件一定是一个函数 新版本提供了对象的写法
    Ctor = resolveAsyncComponent(asyncFactory, baseCtor) //默认调用此函数时返回undefiend
    // 第二次渲染时Ctor不为undefined
    if (Ctor === undefined) {
      //返回async组件的占位符节点
      //作为注释节点，但保留该节点的所有原始信息
      //该信息将用于异步服务器渲染和水合。
      return createAsyncPlaceholder(
        asyncFactory,
        data,
        context,
        children,
        tag
      )
    }
  }
}

2. resolveAsyncComponent method

export function resolveAsyncComponent (
  factory: Function,
  baseCtor: Class<Component>
): Class<Component> | void {
  // 如果有错误就返回错误结果
  if (isTrue(factory.error) && isDef(factory.errorComp)) {
    return factory.errorComp
  }
  // 再次渲染时可以拿到获取的最新组件
  // 如果有成功的结果，就直接返回去
  if (isDef(factory.resolved)) {
    return factory.resolved
  }
 
  if (owner && !isDef(factory.owners)) {
    // forceRender 强制刷新渲染
    const forceRender = (renderCompleted: boolean) => {
      for (let i = 0, l = owners.length; i < l; i++) {
        (owners[i]: any).$forceUpdate() // 执行$forceUpdate
      }
    }
    // 成功
    const resolve = once((res: Object | Class<Component>) => {
      factory.resolved = ensureCtor(res, baseCtor)
      if (!sync) {
        forceRender(true) // 执行强制更新视图重新渲染方法
      } else {
        owners.length = 0
      }
    })
    // 失败
    const reject = once(reason => {
      if (isDef(factory.errorComp)) {
        factory.error = true
        forceRender(true)
      }
    })
 
    // 执行factory 将resolve方法和reject方法传入
    const res = factory(resolve, reject)
 
    sync = false
    return factory.loading ? factory.loadingComp : factory.resolved // 返回结果
  }
}

3. createAsyncPlaceholder method

// 创建一个异步组件的占位，空虚拟节点   也就是一个注释<!-->
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
}

[Related recommendation: "vue.js tutorial"]

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