code: public function responder(ResponderInterface $responder = null) : ResponderInterface
      {
          if ($responder){
              $this->responder = $responder;
          }
          if (!$this->responder){
              $this->responder = new NullResponder();
          }
          return $this->responder;
      }
docs: Set/get responder
  
  @param ResponderInterface $responder
  
  @return ResponderInterface