您不需要在线/离线标记，只需要保存最后活动时间。当显示用户状态时，如果最后活动时间小于现在+ 15分钟，则用户在线，否则离线。

You don't need the online/offline flag, you just need to save the last activitity time. When displaying the user status, if last activity time is less than now+15 minutes then user is online, offline otherwise.