class AjaxResponseAttachmentsProcessor

class AjaxResponseAttachmentsProcessor implements AttachmentsResponseProcessorInterface {
    
    /**
     * The asset resolver service.
     *
     * @var \Drupal\Core\Asset\AssetResolverInterface
     */
    protected $assetResolver;
    
    /**
     * A config object for the system performance configuration.
     *
     * @var \Drupal\Core\Config\Config
     */
    protected $config;
    
    /**
     * The CSS asset collection renderer service.
     *
     * @var \Drupal\Core\Asset\AssetCollectionRendererInterface
     */
    protected $cssCollectionRenderer;
    
    /**
     * The JS asset collection renderer service.
     *
     * @var \Drupal\Core\Asset\AssetCollectionRendererInterface
     */
    protected $jsCollectionRenderer;
    
    /**
     * The request stack.
     *
     * @var \Symfony\Component\HttpFoundation\RequestStack
     */
    protected $requestStack;
    
    /**
     * The renderer.
     *
     * @var \Drupal\Core\Render\RendererInterface
     */
    protected $renderer;
    
    /**
     * The module handler.
     *
     * @var \Drupal\Core\Extension\ModuleHandlerInterface
     */
    protected $moduleHandler;
    
    /**
     * Constructs a AjaxResponseAttachmentsProcessor object.
     *
     * @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.
     */
    public function __construct(AssetResolverInterface $asset_resolver, ConfigFactoryInterface $config_factory, AssetCollectionRendererInterface $css_collection_renderer, AssetCollectionRendererInterface $js_collection_renderer, RequestStack $request_stack, RendererInterface $renderer, ModuleHandlerInterface $module_handler) {
        $this->assetResolver = $asset_resolver;
        $this->config = $config_factory->get('system.performance');
        $this->cssCollectionRenderer = $css_collection_renderer;
        $this->jsCollectionRenderer = $js_collection_renderer;
        $this->requestStack = $request_stack;
        $this->renderer = $renderer;
        $this->moduleHandler = $module_handler;
    }
    
    /**
     * {@inheritdoc}
     */
    public function processAttachments(AttachmentsInterface $response) {
        // @todo Convert to assertion once https://www.drupal.org/node/2408013 lands
        if (!$response instanceof AjaxResponse) {
            throw new \InvalidArgumentException('\\Drupal\\Core\\Ajax\\AjaxResponse instance expected.');
        }
        $request = $this->requestStack
            ->getCurrentRequest();
        if ($response->getContent() == '{}') {
            $response->setData($this->buildAttachmentsCommands($response, $request));
        }
        return $response;
    }
    
    /**
     * Prepares the AJAX commands to attach assets.
     *
     * @param \Drupal\Core\Ajax\AjaxResponse $response
     *   The AJAX response to update.
     * @param \Symfony\Component\HttpFoundation\Request $request
     *   The request object that the AJAX is responding to.
     *
     * @return array
     *   An array of commands ready to be returned as JSON.
     */
    protected function buildAttachmentsCommands(AjaxResponse $response, Request $request) {
        $ajax_page_state = $request->request
            ->get('ajax_page_state');
        // Aggregate CSS/JS if necessary, but only during normal site operation.
        $optimize_css = !defined('MAINTENANCE_MODE') && $this->config
            ->get('css.preprocess');
        $optimize_js = !defined('MAINTENANCE_MODE') && $this->config
            ->get('js.preprocess');
        $attachments = $response->getAttachments();
        // Resolve the attached libraries into asset collections.
        $assets = new AttachedAssets();
        $assets->setLibraries(isset($attachments['library']) ? $attachments['library'] : [])
            ->setAlreadyLoadedLibraries(isset($ajax_page_state['libraries']) ? explode(',', $ajax_page_state['libraries']) : [])
            ->setSettings(isset($attachments['drupalSettings']) ? $attachments['drupalSettings'] : []);
        $css_assets = $this->assetResolver
            ->getCssAssets($assets, $optimize_css);
        list($js_assets_header, $js_assets_footer) = $this->assetResolver
            ->getJsAssets($assets, $optimize_js);
        // First, AttachedAssets::setLibraries() ensures duplicate libraries are
        // removed: it converts it to a set of libraries if necessary. Second,
        // AssetResolver::getJsSettings() ensures $assets contains the final set of
        // JavaScript settings. AttachmentsResponseProcessorInterface also mandates
        // that the response it processes contains the final attachment values, so
        // update both the 'library' and 'drupalSettings' attachments accordingly.
        $attachments['library'] = $assets->getLibraries();
        $attachments['drupalSettings'] = $assets->getSettings();
        $response->setAttachments($attachments);
        // Render the HTML to load these files, and add AJAX commands to insert this
        // HTML in the page. Settings are handled separately, afterwards.
        $settings = [];
        if (isset($js_assets_header['drupalSettings'])) {
            $settings = $js_assets_header['drupalSettings']['data'];
            unset($js_assets_header['drupalSettings']);
        }
        if (isset($js_assets_footer['drupalSettings'])) {
            $settings = $js_assets_footer['drupalSettings']['data'];
            unset($js_assets_footer['drupalSettings']);
        }
        // Prepend commands to add the assets, preserving their relative order.
        $resource_commands = [];
        if ($css_assets) {
            $css_render_array = $this->cssCollectionRenderer
                ->render($css_assets);
            $resource_commands[] = new AddCssCommand($this->renderer
                ->renderPlain($css_render_array));
        }
        if ($js_assets_header) {
            $js_header_render_array = $this->jsCollectionRenderer
                ->render($js_assets_header);
            $resource_commands[] = new PrependCommand('head', $this->renderer
                ->renderPlain($js_header_render_array));
        }
        if ($js_assets_footer) {
            $js_footer_render_array = $this->jsCollectionRenderer
                ->render($js_assets_footer);
            $resource_commands[] = new AppendCommand('body', $this->renderer
                ->renderPlain($js_footer_render_array));
        }
        foreach (array_reverse($resource_commands) as $resource_command) {
            $response->addCommand($resource_command, TRUE);
        }
        // Prepend a command to merge changes and additions to drupalSettings.
        if (!empty($settings)) {
            // During Ajax requests basic path-specific settings are excluded from
            // new drupalSettings values. The original page where this request comes
            // from already has the right values. An Ajax request would update them
            // with values for the Ajax request and incorrectly override the page's
            // values.
            // @see system_js_settings_alter()
            unset($settings['path']);
            $response->addCommand(new SettingsCommand($settings, TRUE), TRUE);
        }
        $commands = $response->getCommands();
        $this->moduleHandler
            ->alter('ajax_render', $commands);
        return $commands;
    }

}