class AjaxFormsTestLazyLoadForm

class AjaxFormsTestLazyLoadForm extends FormBase {
    
    /**
     * {@inheritdoc}
     */
    public function getFormId() {
        return 'ajax_forms_test_lazy_load_form';
    }
    
    /**
     * {@inheritdoc}
     */
    public function buildForm(array $form, FormStateInterface $form_state) {
        // We attach a JavaScript setting, so that one of the generated AJAX
        // commands will be a settings command. We can then check the settings
        // command to ensure that the 'currentPath' setting is not part
        // of the Ajax response.
        $form['#attached']['drupalSettings']['test'] = 'currentPathUpdate';
        $form['add_files'] = [
            '#title' => $this->t('Add files'),
            '#type' => 'checkbox',
            '#default_value' => FALSE,
        ];
        $form['submit'] = [
            '#type' => 'submit',
            '#value' => $this->t('Submit'),
            '#ajax' => [
                'wrapper' => 'ajax-forms-test-lazy-load-ajax-wrapper',
                'callback' => 'ajax_forms_test_lazy_load_form_ajax',
            ],
            '#prefix' => '<div id="ajax-forms-test-lazy-load-ajax-wrapper"></div>',
        ];
        return $form;
    }
    
    /**
     * {@inheritdoc}
     */
    public function submitForm(array &$form, FormStateInterface $form_state) {
        $form_state->setRebuild();
    }

}