public function toUserId($userIds)
    {
        $userIds = is_string($userIds) || is_integer($userIds) ? func_get_args() : $userIds;

        foreach ($userIds as $userId) {
            $fds = $this->room->getClients(static::USER_PREFIX . $userId);
            $this->to($fds);
        }

        return $this;
    }