Renderer.php

<?php

namespace Drupal\colorbox_load;

use Drupal\Core\Ajax\AjaxResponse;
use Drupal\Core\Render\MainContent\MainContentRendererInterface;
use Drupal\Core\Render\RendererInterface;
use Drupal\Core\Routing\RouteMatchInterface;
use Symfony\Component\HttpFoundation\Request;

/**
 * Render content in a colorbox.
 */
class Renderer implements MainContentRendererInterface {
    
    /**
     * The renderer service.
     *
     * @var \Drupal\Core\Render\RendererInterface
     */
    protected $renderer;
    
    /**
     * Constructs a new HtmlRenderer.
     *
     * @param \Drupal\Core\Render\RendererInterface $renderer
     *   The renderer service.
     */
    public function __construct(RendererInterface $renderer) {
        $this->renderer = $renderer;
    }
    
    /**
     * {@inheritdoc}
     */
    public function renderResponse(array $main_content, Request $request, RouteMatchInterface $route_match) {
        $response = new AjaxResponse();
        $content = $this->renderer
            ->renderPlain($main_content);
        $response->setAttachments($main_content['#attached']);
        $response->addCommand(new OpenCommand($content));
        return $response;
    }

}