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) {
    }

}