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();
  }

}