code: public function handle(ServerRequestInterface $request)
      {
          $callback = new CallbackHandler(self::$container);
  
          if (static::$container->has(self::MIDDLEWARE)) {
              $middleware = static::$container->get(self::MIDDLEWARE);
  
              $delegate = new Delegate($callback);
  
              $result = $middleware->process($request, $delegate);
          }
  
          return isset($result) ? $result : $callback($request);
      }
docs: Handles the ServerRequestInterface to convert it to a ResponseInterface.
  
  @param  \Psr\Http\Message\ServerRequestInterface $request
  @return \Psr\Http\Message\ResponseInterface