buildExtraOptionsForm example

  protected function defineOptions() {
    $options = parent::defineOptions();

    $options['depth'] = ['default' => 0];

    return $options;
  }

  public function buildExtraOptionsForm(&$form, FormStateInterface $form_state) {
    parent::buildExtraOptionsForm($form, $form_state);

    $form['depth'] = [
      '#type' => 'weight',
      '#title' => $this->t('Depth'),
      '#default_value' => $this->options['depth'],
      '#description' => $this->t('The depth will match nodes tagged with terms in the hierarchy. For example, if you have the term "fruit" and a child term "apple", with a depth of 1 (or higher) then filtering for the term "fruit" will get nodes that are tagged with "apple" as well as "fruit". If negative, the reverse is true; searching for "apple" will also pick up nodes tagged with "fruit" if depth is -1 (or lower).'),
    ];
  }

  public function query() {
    // If no filter values are present, then do nothing.

$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 extra settings for @type %item', ['@type' => $types[$type]['lstitle'], '%item' => $handler->adminLabel()]);

        $form['#section'] = $display_id . '-' . $type . '-' . $id;

        // Get form from the handler.         $handler->buildExtraOptionsForm($form['options'], $form_state);
        $form_state->set('handler', $handler);
      }

      $view->getStandardButtons($form, $form_state, 'views_ui_config_item_extra_form');
    }
    return $form;
  }

  /** * {@inheritdoc} */