class MediaFilterController

class MediaFilterController implements ContainerInjectionInterface {
    
    /**
     * The renderer service.
     *
     * @var \Drupal\Core\Render\RendererInterface
     */
    protected $renderer;
    
    /**
     * The media storage.
     *
     * @var \Drupal\Core\Entity\ContentEntityStorageInterface
     */
    protected $mediaStorage;
    
    /**
     * The entity repository.
     *
     * @var \Drupal\Core\Entity\EntityRepositoryInterface
     */
    protected $entityRepository;
    
    /**
     * Constructs an MediaFilterController instance.
     *
     * @param \Drupal\Core\Render\RendererInterface $renderer
     *   The renderer service.
     * @param \Drupal\Core\Entity\ContentEntityStorageInterface $media_storage
     *   The media storage.
     * @param \Drupal\Core\Entity\EntityRepositoryInterface $entity_repository
     *   The entity repository.
     */
    public function __construct(RendererInterface $renderer, ContentEntityStorageInterface $media_storage, EntityRepositoryInterface $entity_repository) {
        $this->renderer = $renderer;
        $this->mediaStorage = $media_storage;
        $this->entityRepository = $entity_repository;
    }
    
    /**
     * {@inheritdoc}
     */
    public static function create(ContainerInterface $container) {
        return new static($container->get('renderer'), $container->get('entity_type.manager')
            ->getStorage('media'), $container->get('entity.repository'));
    }
    
    /**
     * Returns a HTML response containing a preview of the text after filtering.
     *
     * Applies all of the given text format's filters, not just the `media_embed`
     * filter, because for example `filter_align` and `filter_caption` may apply
     * to it as well.
     *
     * @param \Symfony\Component\HttpFoundation\Request $request
     *   The request object.
     * @param \Drupal\filter\FilterFormatInterface $filter_format
     *   The text format.
     *
     * @return \Symfony\Component\HttpFoundation\Response
     *   The filtered text.
     *
     * @throws \Symfony\Component\HttpKernel\Exception\NotFoundHttpException
     *   Throws an exception if 'text' parameter is not found in the query
     *   string.
     *
     * @see \Drupal\editor\EditorController::getUntransformedText
     */
    public function preview(Request $request, FilterFormatInterface $filter_format) {
        self::checkCsrf($request, \Drupal::currentUser());
        $text = $request->query
            ->get('text');
        $uuid = $request->query
            ->get('uuid');
        if ($text == '' || $uuid == '') {
            throw new NotFoundHttpException();
        }
        $build = [
            '#type' => 'processed_text',
            '#text' => $text,
            '#format' => $filter_format->id(),
        ];
        $html = $this->renderer
            ->renderInIsolation($build);
        // Load the media item so we can embed the label in the response, for use
        // in an ARIA label.
        $headers = [];
        if ($media = $this->entityRepository
            ->loadEntityByUuid('media', $uuid)) {
            $headers['Drupal-Media-Label'] = $this->entityRepository
                ->getTranslationFromContext($media)
                ->label();
        }
        // Note that we intentionally do not use:
        // - \Drupal\Core\Cache\CacheableResponse because caching it on the server
        //   side is wasteful, hence there is no need for cacheability metadata.
        // - \Drupal\Core\Render\HtmlResponse because there is no need for
        //   attachments nor cacheability metadata.
        return (new Response($html, 200, $headers))->setPrivate()
            ->setMaxAge(300);
    }
    
    /**
     * Checks access based on media_embed filter status on the text format.
     *
     * @param \Drupal\filter\FilterFormatInterface $filter_format
     *   The text format for which to check access.
     *
     * @return \Drupal\Core\Access\AccessResultInterface
     *   The access result.
     */
    public static function formatUsesMediaEmbedFilter(FilterFormatInterface $filter_format) {
        $filters = $filter_format->filters();
        return AccessResult::allowedIf($filters->has('media_embed') && $filters->get('media_embed')->status)
            ->addCacheableDependency($filter_format);
    }
    
    /**
     * Throws an AccessDeniedHttpException if the request fails CSRF validation.
     *
     * This is used instead of \Drupal\Core\Access\CsrfAccessCheck, in order to
     * allow access for anonymous users.
     *
     * @todo Refactor this to an access checker.
     */
    private static function checkCsrf(Request $request, AccountInterface $account) {
        $header = 'X-Drupal-MediaPreview-CSRF-Token';
        if (!$request->headers
            ->has($header)) {
            throw new AccessDeniedHttpException();
        }
        if ($account->isAnonymous()) {
            // For anonymous users, just the presence of the custom header is
            // sufficient protection.
            return;
        }
        // For authenticated users, validate the token value.
        $token = $request->headers
            ->get($header);
        if (!\Drupal::csrfToken()->validate($token, $header)) {
            throw new AccessDeniedHttpException();
        }
    }

}