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;
    }

}