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.
    }

}