// 组件 Object
{
 template: '<div>I am async!</div>'
}
// 经过 compileToFunctions 得到对应的 render function 
with(this) {
 return _c('div', [_v("I am async!")])
}
// 在经过 render 得到 Vnode 再 update 成为真实DOM

// 组件
Vue.component('example', {
 template: '<div>I am async!</div>'
})

// 异步组件
Vue.component('async-example', function (resolve, reject) {
 // 利用 setTimeout 模拟请求
 setTimeout(function () {
  // 向 `resolve` 回调传递组件定义
  resolve({
   template: '<div>I am async!</div>'
  })
 }, 1000)
})

// async component
let asyncFactory
if (isUndef(Ctor.cid)) {
 asyncFactory = Ctor
 Ctor = resolveAsyncComponent(asyncFactory, baseCtor, context)
 if (Ctor === undefined) {
  // return a placeholder node for async component, which is rendered
  // as a comment node but preserves all the raw information for the node.
  // the information will be used for async server-rendering and hydration.
  return createAsyncPlaceholder(
   asyncFactory,
   data,
   context,
   children,
   tag
  )
 }
}

function (resolve, reject) {
 // 利用 setTimeout 模拟请求
 setTimeout(function () {
  // 向 `resolve` 回调传递组件定义
  resolve({
   template: '<div>I am async!</div>'
  })
 }, 1000)
}

// factory 为 Ctor
factory(resolve, reject)

const resolve = once((res: Object | Class<Component>) => {
 // 缓存 resolved
 factory.resolved = ensureCtor(res, baseCtor)
 // 强制渲染
 if (!sync) {
 	forceRender(true)
 }
})

if (isDef(factory.resolved)) {
 return factory.resolved
}