public function registerScriptFile($url, $position = null)
    {
        if (($cs = $this->getClientScript()) === false) {
            return null;
        }
        if (isset($this->_assetsUrl)) {
            $url = $this->_assetsUrl . '/' . ltrim($url, '/');
        }
        return $cs->registerScriptFile($url, $position);
    }