进行动作时扩展ActionBuilder

Question

我正在传播此博客以了解操作：

http://www.tzavellas.com/techblog/2015/02/10/action-composition-in-play-framework/

为什么需要扩展ActionBuilder？仅扩展ActionRefiner还不够吗？这里的ActionBuilder的目的是什么？

object AuthenticatedAction
  extends ActionBuilder[AuthenticatedRequest]
  with ActionRefiner[Request, AuthenticatedRequest] {

  def refine[T](request: Request[T]): Future[Either[Result, AuthenticatedRequest[T]]] = {
    val result = request.session.get("username")
      .flatMap(User.findByUsername(_))
      .map(user => new AuthenticatedRequest[T](user, request))
      .toRight(left = Results.Forbidden)
    Future.successful(result)
  }
}