I want my app to allow each user to log in with only one mobile phone. If the second mobile phone logs in, the other one automatically logs out. What should I do?
Use clock and store device id
Sorry everyone, I have already done it. I just need to use screen detection and then save the logged in value in firebase. Then change the value to left after leaving. Every time I log in, I only need to check whether the value of firebase is empty or whether the value is left. Otherwise, login is not allowed.
