关于中间件：slim3 php框架中response->getBody()为空

        public function createErrorResponse ( $code , $msg , $response )
        {
                echo $response ;
                echo $response -> getBody ( ) -> getSize ( ) ;
                $response = $response -> getBody ( ) -> write ( json_encode ( 'holla' ) ) ;
                $response = $response -> withHeader ( 'Content-Type' , 'application/json; charset=utf-8' ) ;
                return $response ;
        }

        public function __invoke ( $request , $response , $next )
        {
                $userid = $request -> getHeaderLine ( 'userid' ) ;
                $authorization = $request -> getHeaderLine ( 'Authorization' ) ;
                if ( $this -> isAuthenticated ( $userid , $authorization ) )
                {
                        $response = $next ( $request , $response ) ;
                }
                else
                {
                        $msg = 'You are unauthenticated. Please login again' ;
                        $code = 400 ;
                        $response = $this -> createErrorResponse ( $code , $msg , $response ) ;
                }
                return $response ;
        }
}

use Slim\\Http\
equest ;
use Slim\\Http\
esponse ;

class AuthenticationMiddleware {

    public function isAuthenticated ( $userid , $authorization ) {
        //do data validation here
        return false ;
    }

    public function createErrorResponse ( $code , $msg , Response $response ) {
        return $response -> withStatus ( $code )
            -> withHeader ( 'Content-Type' , 'application/json;charset=utf-8' )
            -> withJson ( $msg ) ;
    }

    public function __invoke (Request $request , Response $response , $next ) {
        $userid = $request -> getHeaderLine ( 'userid' ) ;
        $authorization = $request -> getHeaderLine ( 'Authorization' ) ;
        if ( ! $this -> isAuthenticated ( $userid , $authorization ) ) {
            $msg = 'You are unauthenticated. Please login again' ;
            $code = 400 ;
            $this -> createErrorResponse ( $code , $msg , $response ) ;
        } else {
            $response = $next ( $request , $response ) ;
        }
        return $response ;
    }
}