class ConfigHandlerGroup

class ConfigHandlerGroup extends ViewsFormBase {
    
    /**
     * Constructs a new ConfigHandlerGroup object.
     */
    public function __construct($type = NULL, $id = NULL) {
        $this->setType($type);
        $this->setID($id);
    }
    
    /**
     * {@inheritdoc}
     */
    public function getFormKey() {
        return 'handler-group';
    }
    
    /**
     * {@inheritdoc}
     */
    public function getForm(ViewEntityInterface $view, $display_id, $js, $type = NULL, $id = NULL) {
        $this->setType($type);
        $this->setID($id);
        return parent::getForm($view, $display_id, $js);
    }
    
    /**
     * {@inheritdoc}
     */
    public function getFormId() {
        return 'views_ui_config_item_group_form';
    }
    
    /**
     * {@inheritdoc}
     */
    public function buildForm(array $form, FormStateInterface $form_state) {
        $view = $form_state->get('view');
        $display_id = $form_state->get('display_id');
        $type = $form_state->get('type');
        $id = $form_state->get('id');
        $form = [
            'options' => [
                '#tree' => TRUE,
                '#theme_wrappers' => [
                    'container',
                ],
                '#attributes' => [
                    'class' => [
                        'scroll',
                    ],
                    'data-drupal-views-scroll' => TRUE,
                ],
            ],
        ];
        $executable = $view->getExecutable();
        if (!$executable->setDisplay($display_id)) {
            $form['markup'] = [
                '#markup' => $this->t('Invalid display id @display', [
                    '@display' => $display_id,
                ]),
            ];
            return $form;
        }
        $executable->initQuery();
        $item = $executable->getHandler($display_id, $type, $id);
        if ($item) {
            $handler = $executable->display_handler
                ->getHandler($type, $id);
            if (empty($handler)) {
                $form['markup'] = [
                    '#markup' => $this->t("Error: handler for @table > @field doesn't exist!", [
                        '@table' => $item['table'],
                        '@field' => $item['field'],
                    ]),
                ];
            }
            else {
                $handler->init($executable, $executable->display_handler, $item);
                $types = ViewExecutable::getHandlerTypes();
                $form['#title'] = $this->t('Configure aggregation settings for @type %item', [
                    '@type' => $types[$type]['lstitle'],
                    '%item' => $handler->adminLabel(),
                ]);
                $handler->buildGroupByForm($form['options'], $form_state);
                $form_state->set('handler', $handler);
            }
            $view->getStandardButtons($form, $form_state, 'views_ui_config_item_group_form');
        }
        return $form;
    }
    
    /**
     * {@inheritdoc}
     */
    public function submitForm(array &$form, FormStateInterface $form_state) {
        $view = $form_state->get('view');
        $item =& $form_state->get('handler')->options;
        $type = $form_state->get('type');
        $handler = Views::handlerManager($type)->getHandler($item);
        $executable = $view->getExecutable();
        $handler->init($executable, $executable->display_handler, $item);
        $handler->submitGroupByForm($form, $form_state);
        // Store the item back on the view
        $executable->setHandler($form_state->get('display_id'), $form_state->get('type'), $form_state->get('id'), $item);
        // Write to cache
        $view->cacheSet();
    }

}