class ConfigHandlerExtra

class ConfigHandlerExtra extends ViewsFormBase {

  /**
   * Constructs a new ConfigHandlerExtra object.
   */
  public function __construct($type = NULL, $id = NULL) {
    $this
      ->setType($type);
    $this
      ->setID($id);
  }

  /**
   * {@inheritdoc}
   */
  public function getFormKey() {
    return 'handler-extra';
  }

  /**
   * {@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_extra_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;
    }
    $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 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}
   */
  public function validateForm(array &$form, FormStateInterface $form_state) {
    $form_state
      ->get('handler')
      ->validateExtraOptionsForm($form['options'], $form_state);
  }

  /**
   * {@inheritdoc}
   */
  public function submitForm(array &$form, FormStateInterface $form_state) {
    $view = $form_state
      ->get('view');
    $handler = $form_state
      ->get('handler');

    // Run it through the handler's submit function.
    $handler
      ->submitExtraOptionsForm($form['options'], $form_state);
    $item = $handler->options;

    // Store the data we're given.
    foreach ($form_state
      ->getValue('options') as $key => $value) {
      $item[$key] = $value;
    }

    // Store the item back on the view
    $view
      ->getExecutable()
      ->setHandler($form_state
      ->get('display_id'), $form_state
      ->get('type'), $form_state
      ->get('id'), $item);

    // Write to cache
    $view
      ->cacheSet();
  }

}