class BigPipeResponseAttachmentsProcessor

class BigPipeResponseAttachmentsProcessor extends HtmlResponseAttachmentsProcessor {
    
    /**
     * The HTML response attachments processor service.
     *
     * @var \Drupal\Core\Render\AttachmentsResponseProcessorInterface
     */
    protected $htmlResponseAttachmentsProcessor;
    
    /**
     * Constructs a BigPipeResponseAttachmentsProcessor object.
     *
     * @param \Drupal\Core\Render\AttachmentsResponseProcessorInterface $html_response_attachments_processor
     *   The HTML response attachments processor service.
     * @param \Drupal\Core\Asset\AssetResolverInterface $asset_resolver
     *   An asset resolver.
     * @param \Drupal\Core\Config\ConfigFactoryInterface $config_factory
     *   A config factory for retrieving required config objects.
     * @param \Drupal\Core\Asset\AssetCollectionRendererInterface $css_collection_renderer
     *   The CSS asset collection renderer.
     * @param \Drupal\Core\Asset\AssetCollectionRendererInterface $js_collection_renderer
     *   The JS asset collection renderer.
     * @param \Symfony\Component\HttpFoundation\RequestStack $request_stack
     *   The request stack.
     * @param \Drupal\Core\Render\RendererInterface $renderer
     *   The renderer.
     * @param \Drupal\Core\Extension\ModuleHandlerInterface $module_handler
     *   The module handler service.
     */
    public function __construct(AttachmentsResponseProcessorInterface $html_response_attachments_processor, AssetResolverInterface $asset_resolver, ConfigFactoryInterface $config_factory, AssetCollectionRendererInterface $css_collection_renderer, AssetCollectionRendererInterface $js_collection_renderer, RequestStack $request_stack, RendererInterface $renderer, ModuleHandlerInterface $module_handler) {
        $this->htmlResponseAttachmentsProcessor = $html_response_attachments_processor;
        parent::__construct($asset_resolver, $config_factory, $css_collection_renderer, $js_collection_renderer, $request_stack, $renderer, $module_handler);
    }
    
    /**
     * {@inheritdoc}
     */
    public function processAttachments(AttachmentsInterface $response) {
        assert($response instanceof HtmlResponse);
        // First, render the actual placeholders; this will cause the BigPipe
        // placeholder strategy to generate BigPipe placeholders. We need those to
        // exist already so that we can extract BigPipe placeholders. This is hence
        // a bit of unfortunate but necessary duplication.
        // @see \Drupal\big_pipe\Render\Placeholder\BigPipeStrategy
        // (Note this is copied verbatim from
        // \Drupal\Core\Render\HtmlResponseAttachmentsProcessor::processAttachments)
        try {
            $response = $this->renderPlaceholders($response);
        } catch (EnforcedResponseException $e) {
            return $e->getResponse();
        }
        // Extract BigPipe placeholders; HtmlResponseAttachmentsProcessor does not
        // know (nor need to know) how to process those.
        $attachments = $response->getAttachments();
        $big_pipe_placeholders = [];
        $big_pipe_nojs_placeholders = [];
        if (isset($attachments['big_pipe_placeholders'])) {
            $big_pipe_placeholders = $attachments['big_pipe_placeholders'];
            unset($attachments['big_pipe_placeholders']);
        }
        if (isset($attachments['big_pipe_nojs_placeholders'])) {
            $big_pipe_nojs_placeholders = $attachments['big_pipe_nojs_placeholders'];
            unset($attachments['big_pipe_nojs_placeholders']);
        }
        $html_response = clone $response;
        $html_response->setAttachments($attachments);
        // Call HtmlResponseAttachmentsProcessor to process all other attachments.
        $processed_html_response = $this->htmlResponseAttachmentsProcessor
            ->processAttachments($html_response);
        // Restore BigPipe placeholders.
        $attachments = $processed_html_response->getAttachments();
        $big_pipe_response = clone $processed_html_response;
        if (count($big_pipe_placeholders)) {
            $attachments['big_pipe_placeholders'] = $big_pipe_placeholders;
        }
        if (count($big_pipe_nojs_placeholders)) {
            $attachments['big_pipe_nojs_placeholders'] = $big_pipe_nojs_placeholders;
        }
        $big_pipe_response->setAttachments($attachments);
        return $big_pipe_response;
    }

}