processFeed example

$attached['drupalSettings'] = $assets->getSettings();

      // Since we can only replace content in the HTML head section if there's a       // placeholder for it, we can safely avoid processing the render array if       // it's not present.       if (!empty($attachment_placeholders['head'])) {
        // 'feed' is a special case of 'html_head_link'. We process them into         // 'html_head_link' entries and merge them.         if (!empty($attached['feed'])) {
          $attached = BubbleableMetadata::mergeAttachments(
            $attached,
            $this->processFeed($attached['feed'])
          );
          unset($attached['feed']);
        }
        // 'html_head_link' is a special case of 'html_head' which can be present         // as a head element, but also as a Link: HTTP header depending on         // settings in the render array. Processing it can add to both the         // 'html_head' and 'http_header' keys of '#attached', so we must address         // it before 'html_head'.         if (!empty($attached['html_head_link'])) {
          // Merge the processed 'html_head_link' into $attached so that its           // 'html_head' and 'http_header' values are present for further