protected function callStringCallback($callback, $args = [])
    {
        if (strpos($callback, '@')) {
            list($klass, $method) = explode('@', $callback);
        } else {
            $klass  = $callback;
            $method = 'render';
        }

        $klass = isset($this->instantiatedWidgets[$klass]) ? $this->instantiatedWidgets[$klass] : new $klass;

        return call_user_func_array([$klass, $method], $args);
    }