class FormAjaxResponseBuilder

class FormAjaxResponseBuilder implements FormAjaxResponseBuilderInterface {
    
    /**
     * The main content to AJAX Response renderer.
     *
     * @var \Drupal\Core\Render\MainContent\MainContentRendererInterface
     */
    protected $ajaxRenderer;
    
    /**
     * The current route match.
     *
     * @var \Drupal\Core\Routing\RouteMatchInterface
     */
    protected $routeMatch;
    
    /**
     * Constructs a new FormAjaxResponseBuilder.
     *
     * @param \Drupal\Core\Render\MainContent\MainContentRendererInterface $ajax_renderer
     *   The ajax renderer.
     * @param \Drupal\Core\Routing\RouteMatchInterface $route_match
     *   The current route match.
     */
    public function __construct(MainContentRendererInterface $ajax_renderer, RouteMatchInterface $route_match) {
        $this->ajaxRenderer = $ajax_renderer;
        $this->routeMatch = $route_match;
    }
    
    /**
     * {@inheritdoc}
     */
    public function buildResponse(Request $request, array $form, FormStateInterface $form_state, array $commands) {
        // If the form build ID has changed, issue an Ajax command to update it.
        if (isset($form['#build_id_old']) && $form['#build_id_old'] !== $form['#build_id']) {
            $commands[] = new UpdateBuildIdCommand($form['#build_id_old'], $form['#build_id']);
        }
        // We need to return the part of the form (or some other content) that needs
        // to be re-rendered so the browser can update the page with changed
        // content. It is up to the #ajax['callback'] function of the element (may
        // or may not be a button) that triggered the Ajax request to determine what
        // needs to be rendered.
        $callback = NULL;
        if (($triggering_element = $form_state->getTriggeringElement()) && isset($triggering_element['#ajax']['callback'])) {
            $callback = $triggering_element['#ajax']['callback'];
        }
        $callback = $form_state->prepareCallback($callback);
        if (empty($callback) || !is_callable($callback)) {
            throw new HttpException(500, 'The specified #ajax callback is empty or not callable.');
        }
        $result = call_user_func_array($callback, [
            &$form,
            &$form_state,
            $request,
        ]);
        // If the callback is an #ajax callback, the result is a render array, and
        // we need to turn it into an AJAX response, so that we can add any commands
        // we got earlier; typically the UpdateBuildIdCommand when handling an AJAX
        // submit from a cached page.
        if ($result instanceof AjaxResponse) {
            $response = $result;
        }
        else {
            // At this point we know callback returned a render element. If the
            // element is part of the group (#group is set on it) it won't be rendered
            // unless we remove #group from it. This is caused by
            // \Drupal\Core\Render\Element\RenderElement::preRenderGroup(), which
            // prevents all members of groups from being rendered directly.
            if (!empty($result['#group'])) {
                unset($result['#group']);
            }
            
            /** @var \Drupal\Core\Ajax\AjaxResponse $response */
            $response = $this->ajaxRenderer
                ->renderResponse($result, $request, $this->routeMatch);
        }
        foreach ($commands as $command) {
            $response->addCommand($command, TRUE);
        }
        return $response;
    }

}