Ajax callback triggered by checkbox.
function checkboxCallback($form, $form_state) {
$response = new AjaxResponse();
$response
->addCommand(new HtmlCommand('#ajax_checkbox_value', (int) $form_state['values']['checkbox']));
$response
->addCommand(new DataCommand('#ajax_checkbox_value', 'form_state_value_select', (int) $form_state['values']['checkbox']));
return $response;
}