class DummyAjaxWidget

class DummyAjaxWidget extends WidgetBase {
  
  /**
   * {@inheritdoc}
   */
  public function formElement(FieldItemListInterface $items, $delta, array $element, array &$form, FormStateInterface $form_state) {
    $element['select_widget'] = [
      '#type' => 'select',
      '#title' => $this->t('Dummy select'),
      '#options' => [
        'pow' => 'Pow!',
        'bam' => 'Bam!',
      ],
      '#required' => TRUE,
      '#ajax' => [
        'callback' => static::class . '::dummyAjaxCallback',
        'effect' => 'fade',
      ],
    ];
    return $element;
  }
  
  /**
   * Ajax callback for Dummy AJAX test.
   *
   * @param array $form
   *   The build form.
   * @param \Drupal\Core\Form\FormStateInterface $form_state
   *   The form state.
   *
   * @return \Drupal\Core\Ajax\AjaxResponse
   *   Ajax response.
   */
  public static function dummyAjaxCallback(array &$form, FormStateInterface $form_state) {
    return new AjaxResponse();
  }

}