public function asset($asset, $scheme = null)
    {
        $suffix = false;
        $ext = strtolower(pathinfo($asset, PATHINFO_EXTENSION));
        if (is_array($this->cacheBust) && in_array($ext, $this->cacheBust,
                true) && is_string($this->assetPath)
        ) {
            $path = $this->assetPath.DIRECTORY_SEPARATOR.$this->normalize($asset);
            $suffix = is_readable($path) ? @filemtime($path) : false;
        }
        if ($suffix) {
            $asset = substr($asset, 0, -1 * strlen($ext)).$suffix.'.'.$ext;
        }

        return $this->addHost($this->assetUrl.$asset, $scheme, true);
    }