class AjaxTestDialogForm

class AjaxTestDialogForm extends FormBase {

  /**
   * {@inheritdoc}
   */
  public function getFormId() {
    return 'ajax_test_dialog_form';
  }

  /**
   * {@inheritdoc}
   */
  public function buildForm(array $form, FormStateInterface $form_state) {
    $form['button1'] = [
      '#type' => 'submit',
      '#name' => 'button1',
      '#value' => 'Button 1 (modal)',
      '#ajax' => [
        'callback' => '::modal',
      ],
    ];
    $form['button2'] = [
      '#type' => 'submit',
      '#name' => 'button2',
      '#value' => 'Button 2 (non-modal)',
      '#ajax' => [
        'callback' => '::nonModal',
      ],
    ];
    $form['button3'] = [
      '#type' => 'submit',
      '#name' => 'button3',
      '#value' => 'Button 3 (modal from url)',
      '#ajax' => [
        'callback' => '::modalFromUrl',
      ],
    ];
    return $form;
  }

  /**
   * {@inheritdoc}
   */
  public function validateForm(array &$form, FormStateInterface $form_state) {
  }

  /**
   * {@inheritdoc}
   */
  public function submitForm(array &$form, FormStateInterface $form_state) {
    $form_state
      ->setRedirect('ajax_test.dialog_contents');
  }

  /**
   * AJAX callback handler for AjaxTestDialogForm.
   */
  public function modal(&$form, FormStateInterface $form_state) {
    return $this
      ->dialog(TRUE);
  }

  /**
   * AJAX callback handler for Url modal, AjaxTestDialogForm.
   */
  public function modalFromUrl(&$form, FormStateInterface $form_state) {
    return $this
      ->dialog(TRUE, TRUE);
  }

  /**
   * AJAX callback handler for AjaxTestDialogForm.
   */
  public function nonModal(&$form, FormStateInterface $form_state) {
    return $this
      ->dialog(FALSE);
  }

  /**
   * Util to render dialog in ajax callback.
   *
   * @param bool $is_modal
   *   (optional) TRUE if modal, FALSE if plain dialog. Defaults to FALSE.
   * @param bool $is_url
   *   (optional) True if modal is from a URL, Defaults to FALSE.
   *
   * @return \Drupal\Core\Ajax\AjaxResponse
   *   An ajax response object.
   */
  protected function dialog(bool $is_modal = FALSE, bool $is_url = FALSE) {
    $content = AjaxTestController::dialogContents();
    $response = new AjaxResponse();
    $title = $this
      ->t('AJAX Dialog & contents');

    // Attach the library necessary for using the Open(Modal)DialogCommand and
    // set the attachments for this Ajax response.
    $content['#attached']['library'][] = 'core/drupal.dialog.ajax';
    if ($is_modal) {
      if ($is_url) {
        $response
          ->addCommand(new OpenModalDialogWithUrl(Url::fromRoute('ajax_test.dialog_form')
          ->toString(), []));
      }
      else {
        $response
          ->addCommand(new OpenModalDialogCommand($title, $content));
      }
    }
    else {
      $selector = '#ajax-test-dialog-wrapper-1';
      $response
        ->addCommand(new OpenDialogCommand($selector, $title, $content));
    }
    return $response;
  }

}