class AjaxTestForm

class AjaxTestForm extends FormBase {
  
  /**
   * {@inheritdoc}
   */
  public function getFormId() {
    return 'ajax_test_form';
  }
  
  /**
   * {@inheritdoc}
   */
  public function buildForm(array $form, FormStateInterface $form_state) {
    $form['#action'] = Url::fromRoute('ajax_test.dialog')->toString();
    $form['description'] = [
      '#markup' => '<p>' . $this->t("Ajax Form contents description.") . '</p>',
    ];
    $form['actions'] = [
      '#type' => 'actions',
    ];
    $form['actions']['submit'] = [
      '#type' => 'submit',
      '#value' => $this->t('Do it'),
    ];
    $form['actions']['preview'] = [
      '#type' => 'submit',
      '#value' => $this->t('Preview'),
      // No regular submit-handler. This form only works via JavaScript.
'#submit' => [],
      '#ajax' => [
        // This means the ::preview() method on this class would be invoked in
        // case of a click event. However, since Drupal core's test runner only
        // is able to execute PHP, not JS, there is no point in actually
        // implementing this method, because we can never let it be called from
        // JS; we'd have to manually call it from PHP, at which point we would
        // not actually be testing it.
        // Therefore we consciously choose to not implement this method, because
        // we cannot meaningfully test it anyway.
'callback' => '::preview',
        'event' => 'click',
      ],
    ];
    return $form;
  }
  
  /**
   * {@inheritdoc}
   */
  public function submitForm(array &$form, FormStateInterface $form_state) {
  }
  
  /**
   * {@inheritdoc}
   */
  public function validateForm(array &$form, FormStateInterface $form_state) {
  }

}