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'] = [
            '#title' => 'Preview',
            '#type' => 'link',
            '#url' => Url::fromRoute('ajax_test.dialog_form'),
            '#attributes' => [
                'class' => [
                    'use-ajax',
                    'button',
                ],
                'data-dialog-type' => 'modal',
            ],
        ];
        $form['actions']['hello_world'] = [
            '#type' => 'submit',
            '#value' => $this->t('Hello world'),
            // No regular submit-handler. This form only works via JavaScript.
'#submit' => [],
            '#ajax' => [
                'callback' => '::helloWorld',
                'event' => 'click',
            ],
        ];
        return $form;
    }
    
    /**
     * An AJAX callback that prints "Hello World!" as a message.
     *
     * @param array $form
     *   The form array to remove elements from.
     * @param \Drupal\Core\Form\FormStateInterface $form_state
     *   The current state of the form.
     *
     * @return \Drupal\Core\Ajax\AjaxResponse
     *   An AJAX response.
     */
    public function helloWorld(array &$form, FormStateInterface $form_state) {
        $response = new AjaxResponse();
        $response->addCommand(new MessageCommand('Hello world!'));
        return $response;
    }
    
    /**
     * {@inheritdoc}
     */
    public function submitForm(array &$form, FormStateInterface $form_state) {
    }
    
    /**
     * {@inheritdoc}
     */
    public function validateForm(array &$form, FormStateInterface $form_state) {
    }

}