vue为啥要使用异步组件

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

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
      )
    }
  }
}

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
  }
}

// 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
}