public function isValid($group_name, $object = null)
    {
        if (false === $this->isValidGroup($group_name)) {
            throw new InvalidArgumentException("$group_name is not a valid cache validator group.");
        }

        $is_valid = true;
        foreach ($this->validators[$group_name] as $validator) {
            if (false === $validator->isValid($object)) {
                $is_valid = false;
                break;
            }
        }

        return $is_valid;
    }