class RssResponseRelativeUrlFilter

class RssResponseRelativeUrlFilter implements EventSubscriberInterface {

  /**
   * Converts relative URLs to absolute URLs.
   *
   * @param \Symfony\Component\HttpKernel\Event\ResponseEvent $event
   *   The response event.
   */
  public function onResponse(ResponseEvent $event) {

    // Only care about RSS responses.
    if (stripos($event
      ->getResponse()->headers
      ->get('Content-Type', ''), 'application/rss+xml') === FALSE) {
      return;
    }
    $response = $event
      ->getResponse();
    $response
      ->setContent($this
      ->transformRootRelativeUrlsToAbsolute($response
      ->getContent(), $event
      ->getRequest()));
  }

  /**
   * Converts all root-relative URLs to absolute URLs in RSS markup.
   *
   * Does not change any existing protocol-relative or absolute URLs.
   *
   * @param string $rss_markup
   *   The RSS markup to update.
   * @param \Symfony\Component\HttpFoundation\Request $request
   *   The current request.
   *
   * @return string
   *   The updated RSS markup.
   */
  protected function transformRootRelativeUrlsToAbsolute($rss_markup, Request $request) {
    $rss_dom = new \DOMDocument();

    // Load the RSS, if there are parsing errors, abort and return the unchanged
    // markup.
    $previous_value = libxml_use_internal_errors(TRUE);
    $rss_dom
      ->loadXML($rss_markup);
    $errors = libxml_get_errors();
    libxml_use_internal_errors($previous_value);
    if ($errors) {
      return $rss_markup;
    }

    // Invoke Html::transformRootRelativeUrlsToAbsolute() on all HTML content
    // embedded in this RSS feed.
    foreach ($rss_dom
      ->getElementsByTagName('item') as $item) {
      foreach ($item
        ->getElementsByTagName('description') as $node) {
        $html_markup = $node->nodeValue;
        if (!empty($html_markup)) {
          $node
            ->replaceChild($rss_dom
            ->createTextNode(Html::transformRootRelativeUrlsToAbsolute($html_markup, $request
            ->getSchemeAndHttpHost())), $node->firstChild);
        }
      }
    }
    return $rss_dom
      ->saveXML();
  }

  /**
   * {@inheritdoc}
   */
  public static function getSubscribedEvents() : array {

    // Should run after any other response subscriber that modifies the markup.
    // @see \Drupal\Core\EventSubscriber\ActiveLinkResponseFilter
    $events[KernelEvents::RESPONSE][] = [
      'onResponse',
      -512,
    ];
    return $events;
  }

}