trait CommandWithAttachedAssetsTrait

trait CommandWithAttachedAssetsTrait {
    
    /**
     * The attached assets for this Ajax command.
     *
     * @var \Drupal\Core\Asset\AttachedAssets
     */
    protected $attachedAssets;
    
    /**
     * Processes the content for output.
     *
     * If content is a render array, it may contain attached assets to be
     * processed.
     *
     * @return string|\Drupal\Component\Render\MarkupInterface
     *   HTML rendered content.
     */
    protected function getRenderedContent() {
        $this->attachedAssets = new AttachedAssets();
        if (is_array($this->content)) {
            if (!$this->content) {
                return '';
            }
            $html = \Drupal::service('renderer')->renderRoot($this->content);
            $this->attachedAssets = AttachedAssets::createFromRenderArray($this->content);
            return $html;
        }
        else {
            return $this->content;
        }
    }
    
    /**
     * Gets the attached assets.
     *
     * @return \Drupal\Core\Asset\AttachedAssets|null
     *   The attached assets for this command.
     */
    public function getAttachedAssets() {
        return $this->attachedAssets;
    }

}