public function setRootDir(/*# string */ $rootDir)
    {
        $dir = realpath($rootDir);

        if (false === $dir) {
            throw new InvalidArgumentException(
                Message::get(Message::CONFIG_ROOT_INVALID, $rootDir),
                Message::CONFIG_ROOT_INVALID
            );
        }

        $this->root_dir = $dir . \DIRECTORY_SEPARATOR;

        return $this;
    }