public function getParams(array $keys = null)
    {
        $params = $this->params;

        if ($keys) {
            $subset = [];
            foreach ($keys as $key) {
                if (array_key_exists($key, $params)) {
                    $subset[$key] = $params[$key];
                }
            }
            return $subset;
        }

        return $params;
    }