class ResourceResponseSubscriber

class ResourceResponseSubscriber implements EventSubscriberInterface {
    
    /**
     * The serializer.
     *
     * @var \Symfony\Component\Serializer\SerializerInterface
     */
    protected $serializer;
    
    /**
     * The renderer.
     *
     * @var \Drupal\Core\Render\RendererInterface
     */
    protected $renderer;
    
    /**
     * The current route match.
     *
     * @var \Drupal\Core\Routing\RouteMatchInterface
     */
    protected $routeMatch;
    
    /**
     * Constructs a ResourceResponseSubscriber object.
     *
     * @param \Symfony\Component\Serializer\SerializerInterface $serializer
     *   The serializer.
     * @param \Drupal\Core\Render\RendererInterface $renderer
     *   The renderer.
     * @param \Drupal\Core\Routing\RouteMatchInterface $route_match
     *   The current route match.
     */
    public function __construct(SerializerInterface $serializer, RendererInterface $renderer, RouteMatchInterface $route_match) {
        $this->serializer = $serializer;
        $this->renderer = $renderer;
        $this->routeMatch = $route_match;
    }
    
    /**
     * Serializes ResourceResponse responses' data, and removes that data.
     *
     * @param \Symfony\Component\HttpKernel\Event\ResponseEvent $event
     *   The event to process.
     */
    public function onResponse(ResponseEvent $event) {
        $response = $event->getResponse();
        if (!$response instanceof ResourceResponseInterface) {
            return;
        }
        $request = $event->getRequest();
        $format = $this->getResponseFormat($this->routeMatch, $request);
        $this->renderResponseBody($request, $response, $this->serializer, $format);
        $event->setResponse($this->flattenResponse($response));
    }
    
    /**
     * Determines the format to respond in.
     *
     * Respects the requested format if one is specified. However, it is common to
     * forget to specify a response format in case of a POST or PATCH. Rather than
     * simply throwing an error, we apply the robustness principle: when POSTing
     * or PATCHing using a certain format, you probably expect a response in that
     * same format.
     *
     * @param \Drupal\Core\Routing\RouteMatchInterface $route_match
     *   The current route match.
     * @param \Symfony\Component\HttpFoundation\Request $request
     *   The current request.
     *
     * @return string
     *   The response format.
     */
    public function getResponseFormat(RouteMatchInterface $route_match, Request $request) {
        $route = $route_match->getRouteObject();
        $acceptable_response_formats = $route->hasRequirement('_format') ? explode('|', $route->getRequirement('_format')) : [];
        $acceptable_request_formats = $route->hasRequirement('_content_type_format') ? explode('|', $route->getRequirement('_content_type_format')) : [];
        $acceptable_formats = $request->isMethodCacheable() ? $acceptable_response_formats : $acceptable_request_formats;
        $requested_format = $request->getRequestFormat();
        $content_type_format = $request->getContentTypeFormat();
        // If an acceptable response format is requested, then use that. Otherwise,
        // including and particularly when the client forgot to specify a response
        // format, then use heuristics to select the format that is most likely
        // expected.
        if (in_array($requested_format, $acceptable_response_formats, TRUE)) {
            return $requested_format;
        }
        // If a request body is present, then use the format corresponding to the
        // request body's Content-Type for the response, if it's an acceptable
        // format for the request.
        if (!empty($request->getContent()) && in_array($content_type_format, $acceptable_request_formats, TRUE)) {
            return $content_type_format;
        }
        // Otherwise, use the first acceptable format.
        if (!empty($acceptable_formats)) {
            return $acceptable_formats[0];
        }
        // Sometimes, there are no acceptable formats.
        return NULL;
    }
    
    /**
     * Renders a resource response body.
     *
     * During serialization, encoders and normalizers are able to explicitly
     * bubble cacheability metadata via the 'cacheability' key-value pair in the
     * received context. This bubbled cacheability metadata will be applied to the
     * the response.
     *
     * @param \Symfony\Component\HttpFoundation\Request $request
     *   The request object.
     * @param \Drupal\rest\ResourceResponseInterface $response
     *   The response from the REST resource.
     * @param \Symfony\Component\Serializer\SerializerInterface $serializer
     *   The serializer to use.
     * @param string|null $format
     *   The response format, or NULL in case the response does not need a format.
     *
     * @todo Add test coverage for language negotiation contexts in
     *   https://www.drupal.org/node/2135829.
     */
    protected function renderResponseBody(Request $request, ResourceResponseInterface $response, SerializerInterface $serializer, $format) {
        $data = $response->getResponseData();
        // If there is data to send, serialize and set it as the response body.
        if ($data !== NULL) {
            $serialization_context = [
                'request' => $request,
                CacheableNormalizerInterface::SERIALIZATION_CONTEXT_CACHEABILITY => new CacheableMetadata(),
            ];
            $output = $serializer->serialize($data, $format, $serialization_context);
            if ($response instanceof CacheableResponseInterface) {
                $response->addCacheableDependency($serialization_context[CacheableNormalizerInterface::SERIALIZATION_CONTEXT_CACHEABILITY]);
            }
            $response->setContent($output);
            $response->headers
                ->set('Content-Type', $request->getMimeType($format));
        }
    }
    
    /**
     * Flattens a fully rendered resource response.
     *
     * Ensures that complex data structures in ResourceResponse::getResponseData()
     * are not serialized. Not doing this means that caching this response object
     * requires unserializing the PHP data when reading this response object from
     * cache, which can be very costly, and is unnecessary.
     *
     * @param \Drupal\rest\ResourceResponseInterface $response
     *   A fully rendered resource response.
     *
     * @return \Drupal\Core\Cache\CacheableResponse|\Symfony\Component\HttpFoundation\Response
     *   The flattened response.
     */
    protected function flattenResponse(ResourceResponseInterface $response) {
        $final_response = $response instanceof CacheableResponseInterface ? new CacheableResponse() : new Response();
        $final_response->setContent($response->getContent());
        $final_response->setStatusCode($response->getStatusCode());
        $final_response->setProtocolVersion($response->getProtocolVersion());
        if ($response->getCharset()) {
            $final_response->setCharset($response->getCharset());
        }
        $final_response->headers = clone $response->headers;
        if ($final_response instanceof CacheableResponseInterface) {
            $final_response->addCacheableDependency($response->getCacheableMetadata());
        }
        return $final_response;
    }
    
    /**
     * {@inheritdoc}
     */
    public static function getSubscribedEvents() : array {
        // Run before \Drupal\dynamic_page_cache\EventSubscriber\DynamicPageCacheSubscriber
        // (priority 100), so that Dynamic Page Cache can cache flattened responses.
        $events[KernelEvents::RESPONSE][] = [
            'onResponse',
            128,
        ];
        return $events;
    }

}