protected function getNewPrefix()
    {
        // Removing old sprites
        $find = new Finder();
        $fs = new Filesystem();
        $fs->remove(
            $find->in($this->imgDir)->directories(),
            $this->stylePath
        );

        // New prefix
        $prefix = substr(sha1(microtime(true) . 'sprites'), 1, 10);
        $fs->mkdir($this->imgDir . '/' . $prefix);

        return $prefix;
    }