class AjaxTestFormPromise

class AjaxTestFormPromise extends FormBase {
  
  /**
   * {@inheritdoc}
   */
  public function getFormId() {
    return 'ajax_test_form_promise';
  }
  
  /**
   * Form for testing the addition of various types of elements via Ajax.
   */
  public function buildForm(array $form, FormStateInterface $form_state) {
    $form['#attached']['library'][] = 'ajax_test/command_promise';
    $form['custom']['#prefix'] = '<div id="ajax_test_form_promise_wrapper">';
    $form['custom']['#suffix'] = '</div>';
    // Button to test for the execution order of Ajax commands.
    $form['test_execution_order_button'] = [
      '#type' => 'submit',
      '#value' => $this->t('Execute commands button'),
      '#button_type' => 'primary',
      '#ajax' => [
        'callback' => [
          static::class,
          'executeCommands',
        ],
        'progress' => [
          'type' => 'throbber',
          'message' => NULL,
        ],
        'wrapper' => 'ajax_test_form_promise_wrapper',
      ],
    ];
    return $form;
  }
  
  /**
   * Ajax callback for the "Execute commands button" button.
   */
  public static function executeCommands(array $form, FormStateInterface $form_state) {
    $selector = '#ajax_test_form_promise_wrapper';
    $response = new AjaxResponse();
    $response->addCommand(new AppendCommand($selector, '1'));
    $response->addCommand(new AjaxTestCommandReturnPromise($selector, '2'));
    $response->addCommand(new AppendCommand($selector, '3'));
    $response->addCommand(new AppendCommand($selector, '4'));
    $response->addCommand(new AjaxTestCommandReturnPromise($selector, '5'));
    return $response;
  }
  
  /**
   * {@inheritdoc}
   */
  public function submitForm(array &$form, FormStateInterface $form_state) {
    // An empty implementation, as we never submit the actual form.
  }

}