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\FilterResponseEvent $event
   *   The event to process.
   */
  public function onResponse(FilterResponseEvent $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
      ->getContentType();

    // 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, e.g. DELETE routes.
    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.
   *
   * In versions of Drupal prior to 8.5, implicit bubbling of cacheability
   * metadata was allowed because there was no explicit cacheability metadata
   * bubbling API. To maintain backwards compatibility, we continue to support
   * this, but support for this will be dropped in Drupal 9.0.0. This is
   * especially useful when interacting with APIs that implicitly invoke
   * rendering (for example: generating URLs): this allows those to "leak", and
   * we collect their bubbled cacheability metadata automatically in a render
   * context.
   *
   * @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,
   *   for example for the response to a DELETE request.
   *
   * @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(),
      ];

      // @deprecated In Drupal 8.5.0, will be removed before Drupal 9.0.0. Use
      // explicit cacheability metadata bubbling instead. (The wrapping call to
      // executeInRenderContext() will be removed before Drupal 9.0.0.)
      $context = new RenderContext();
      $output = $this->renderer
        ->executeInRenderContext($context, function () use ($serializer, $data, $format, $serialization_context) {
        return $serializer
          ->serialize($data, $format, $serialization_context);
      });
      if ($response instanceof CacheableResponseInterface) {
        if (!$context
          ->isEmpty()) {
          @trigger_error('Implicit cacheability metadata bubbling (onto the global render context) in normalizers is deprecated since Drupal 8.5.0 and will be removed in Drupal 9.0.0. Use the "cacheability" serialization context instead, for explicit cacheability metadata bubbling. See https://www.drupal.org/node/2918937', E_USER_DEPRECATED);
          $response
            ->addCacheableDependency($context
            ->pop());
        }
        $response
          ->addCacheableDependency($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() {

    // 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;
  }

}