public function respondToAccessTokenRequest(ServerRequestInterface $request, ResponseInterface $response)
    {
        foreach ($this->enabledGrantTypes as $grantType) {
            if (!$grantType->canRespondToAccessTokenRequest($request)) {
                continue;
            }
            $tokenResponse = $grantType->respondToAccessTokenRequest(
                $request,
                $this->getResponseType(),
                $this->grantTypeAccessTokenTTL[$grantType->getIdentifier()]
            );

            if ($tokenResponse instanceof ResponseTypeInterface) {
                return $tokenResponse->generateHttpResponse($response);
            }
        }

        throw OAuthServerException::unsupportedGrantType();
    }