defineComponent

// packages/runtime-core/src/apiDefineComponent.ts
// see https://vuejs.org/api/general.html#definecomponent
function defineComponent(options: unknown) {
  return isFunction(options) ? { setup: options, name: options.name } : options
}

SFC 的本质是 object。

defineAsyncComponent

// see https://vuejs.org/api/general.html#defineasynccomponent
export function defineAsyncComponent<
  T extends Component = { new (): ComponentPublicInstance }
>(source: AsyncComponentLoader<T> | AsyncComponentOptions<T>): T {
  if (isFunction(source)) {
    source = { loader: source }
  }

  const {
    loader,
    loadingComponent,
    errorComponent,
    delay = 200,
    timeout, // undefined = never times out
    suspensible = true,
    onError: userOnError
  } = source

  let pendingRequest: Promise<ConcreteComponent> | null = null
  let resolvedComp: ConcreteComponent | undefined

  // ......
  const load = (): Promise<ConcreteComponent> => {
    let thisRequest: Promise<ConcreteComponent>
    return (
      pendingRequest ||
      (thisRequest = pendingRequest =
        loader()
          .catch(err => {
            // ......
          })
          .then((comp: any) => {
            // ......
            // interop module default
            if (
              comp &&
              (comp.__esModule || comp[Symbol.toStringTag] === 'Module')
            ) {
              comp = comp.default
            }
            //  ......
            resolvedComp = comp
            return comp
          }))
    )
  }

  return defineComponent({
    name: 'AsyncComponentWrapper',

    __asyncLoader: load,

    get __asyncResolved() {
      return resolvedComp
    },

    setup() {
      const instance = currentInstance!

      // already resolved
      if (resolvedComp) {
        return () => createInnerComp(resolvedComp!, instance)
      }

      const onError = (err: Error) => {
        pendingRequest = null
        handleError(
          err,
          instance,
          ErrorCodes.ASYNC_COMPONENT_LOADER,
          !errorComponent /* do not throw in dev if user provided error component */
        )
      }

      // suspense-controlled or SSR.
      if (
        (__FEATURE_SUSPENSE__ && suspensible && instance.suspense) ||
        (__SSR__ && isInSSRComponentSetup)
      ) {
        return load()
          .then(comp => {
            return () => createInnerComp(comp, instance)
          })
          .catch(err => {
            onError(err)
            return () =>
              errorComponent
                ? createVNode(errorComponent as ConcreteComponent, {
                    error: err
                  })
                : null
          })
      }

      const loaded = ref(false)
      const error = ref()
      const delayed = ref(!!delay)

      if (delay) {
        setTimeout(() => {
          delayed.value = false
        }, delay)
      }

      if (timeout != null) {
        setTimeout(() => {
          if (!loaded.value && !error.value) {
            const err = new Error(
              `Async component timed out after ${timeout}ms.`
            )
            onError(err)
            error.value = err
          }
        }, timeout)
      }

      load()
        .then(() => {
          loaded.value = true
          if (instance.parent && isKeepAlive(instance.parent.vnode)) {
            // parent is keep-alive, force update so the loaded component's
            // name is taken into account
            queueJob(instance.parent.update)
          }
        })
        .catch(err => {
          onError(err)
          error.value = err
        })

      return () => {
        if (loaded.value && resolvedComp) {
          return createInnerComp(resolvedComp, instance)
        } else if (error.value && errorComponent) {
          return createVNode(errorComponent as ConcreteComponent, {
            error: error.value
          })
        } else if (loadingComponent && !delayed.value) {
          return createVNode(loadingComponent as ConcreteComponent)
        }
      }
    }
  }) as T
}

function createInnerComp(
  comp: ConcreteComponent,
  {
    vnode: { ref, props, children, shapeFlag },
    parent
  }: ComponentInternalInstance
) {
  const vnode = createVNode(comp, props, children)
  // ensure inner component inherits the async wrapper's ref owner
  vnode.ref = ref
  return vnode
}