class ResourceResponseSubscriber

class ResourceResponseSubscriber implements EventSubscriberInterface {

  /**
   * The serializer.
   *
   * @var \Symfony\Component\Serializer\SerializerInterface
   */
  protected $serializer;

  /**
   * Constructs a ResourceResponseSubscriber object.
   *
   * @param \Symfony\Component\Serializer\SerializerInterface $serializer
   *   The serializer.
   */
  public function __construct(SerializerInterface $serializer) {
    $this->serializer = $serializer;
  }

  /**
   * {@inheritdoc}
   *
   * @see \Drupal\rest\EventSubscriber\ResourceResponseSubscriber::getSubscribedEvents()
   * @see \Drupal\dynamic_page_cache\EventSubscriber\DynamicPageCacheSubscriber
   */
  public static function getSubscribedEvents() : array {

    // Run before the dynamic page cache subscriber (priority 100), so that
    // Dynamic Page Cache can cache flattened responses.
    $events[KernelEvents::RESPONSE][] = [
      'onResponse',
      128,
    ];
    return $events;
  }

  /**
   * 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 ResourceResponse) {
      return;
    }
    $request = $event
      ->getRequest();
    $format = 'api_json';
    $this
      ->renderResponseBody($request, $response, $this->serializer, $format);
    $event
      ->setResponse($this
      ->flattenResponse($response, $request));
  }

  /**
   * Renders a resource response body.
   *
   * Serialization can invoke rendering (e.g., generating URLs), but the
   * serialization API does not provide a mechanism to collect the
   * bubbleable metadata associated with that (e.g., language and other
   * contexts), so instead, allow those to "leak" and collect them here in
   * a render context.
   *
   * @param \Symfony\Component\HttpFoundation\Request $request
   *   The request object.
   * @param \Drupal\jsonapi\ResourceResponse $response
   *   The response from the JSON:API 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, ResourceResponse $response, SerializerInterface $serializer, $format) {
    $data = $response
      ->getResponseData();

    // If there is data to send, serialize and set it as the response body.
    if ($data !== NULL) {

      // First normalize the data. Note that error responses do not need a
      // normalization context, since there are no entities to normalize.
      // @see \Drupal\jsonapi\EventSubscriber\DefaultExceptionSubscriber::isJsonApiExceptionEvent()
      $context = !$response
        ->isSuccessful() ? [] : static::generateContext($request);
      $jsonapi_doc_object = $serializer
        ->normalize($data, $format, $context);

      // Having just normalized the data, we can associate its cacheability with
      // the response object.
      if ($response instanceof CacheableResponseInterface) {
        assert($jsonapi_doc_object instanceof CacheableNormalization);
        $response
          ->addCacheableDependency($jsonapi_doc_object);
      }

      // Finally, encode the normalized data (JSON:API's encoder rasterizes it
      // automatically).
      $response
        ->setContent($serializer
        ->encode($jsonapi_doc_object
        ->getNormalization(), $format));
      $response->headers
        ->set('Content-Type', $request
        ->getMimeType($format));
    }
  }

  /**
   * Generates a top-level JSON:API normalization context.
   *
   * @param \Symfony\Component\HttpFoundation\Request $request
   *   The request from which the context can be derived.
   *
   * @return array
   *   The generated context.
   */
  protected static function generateContext(Request $request) {

    // Build the expanded context.
    $context = [
      'account' => NULL,
      'sparse_fieldset' => NULL,
    ];
    if ($request->query
      ->has('fields')) {
      $context['sparse_fieldset'] = array_map(function ($item) {
        return explode(',', $item);
      }, $request->query
        ->all('fields'));
    }
    return $context;
  }

  /**
   * 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 deserializing the PHP data when reading this response object from
   * cache, which can be very costly, and is unnecessary.
   *
   * @param \Drupal\jsonapi\ResourceResponse $response
   *   A fully rendered resource response.
   * @param \Symfony\Component\HttpFoundation\Request $request
   *   The request for which this response is generated.
   *
   * @return \Drupal\Core\Cache\CacheableResponse|\Symfony\Component\HttpFoundation\Response
   *   The flattened response.
   */
  protected static function flattenResponse(ResourceResponse $response, Request $request) {
    $final_response = $response instanceof CacheableResponseInterface && $request
      ->isMethodCacheable() ? new CacheableResponse() : new Response();
    $final_response
      ->setContent($response
      ->getContent());
    $final_response
      ->setStatusCode($response
      ->getStatusCode());
    $final_response
      ->setProtocolVersion($response
      ->getProtocolVersion());
    if ($charset = $response
      ->getCharset()) {
      $final_response
        ->setCharset($charset);
    }
    $final_response->headers = clone $response->headers;
    if ($final_response instanceof CacheableResponseInterface) {
      $final_response
        ->addCacheableDependency($response
        ->getCacheableMetadata());
    }
    return $final_response;
  }

}