processAssetLibraries example

    if (!empty($attached['html_response_attachment_placeholders'])) {
      // Get the placeholders from attached and then remove them.       $attachment_placeholders = $attached['html_response_attachment_placeholders'];
      unset($attached['html_response_attachment_placeholders']);

      $assets = AttachedAssets::createFromRenderArray(['#attached' => $attached]);
      // Take Ajax page state into account, to allow for something like       // Turbolinks to be implemented without altering core.       // @see https://github.com/rails/turbolinks/       $ajax_page_state = $this->requestStack->getCurrentRequest()->get('ajax_page_state');
      $assets->setAlreadyLoadedLibraries(isset($ajax_page_state) ? explode(',', $ajax_page_state['libraries']) : []);
      $variables = $this->processAssetLibraries($assets, $attachment_placeholders);
      // $variables now contains the markup to load the asset libraries. Update       // $attached with the final list of libraries and JavaScript settings, so       // that $response can be updated with those. Then the response object will       // list the final, processed attachments.       $attached['library'] = $assets->getLibraries();
      $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'])) {