when I am in the last activity and I press back, my app goes to the background but when I return my app restarts going back to the "login" screen, if I am in "Home" and I go back it goes to the background because my login activity finished, I understand this is part of how the Android system works. Having an app with an sdk that uses bluetooh with asynTask tasks, I would like to be able to force an activity not to close or cancel when it goes to the background, I am looking for alternative solutions such as making the app stay in "widget" mode or being able to launch some kind of "service" that forces my activity as such not to close or restart when it goes to the background, I'm quite new to all this, any suggestion on how to solve this problem will be welcome

CodePudding user response：

Backing out of the last Activity on the stack treats the app as "closed", and there are expected behaviours associated with that:

The user's assumption in these complete dismissal cases is that they have permanently navigated away from the activity, and if they re-open the activity they expect the activity to start from a clean state. The underlying system behavior for these dismissal scenarios matches the user expectation - the activity instance will get destroyed and removed from memory, along with any state stored in it and any saved instance state record associated with the activity.

So while it is possible to restore the previous state (the example they give is a web browser opening at the same page it was on) the system and components like Navigation are built around this idea that state shouldn't be preserved. You'd have to put in effort to work around that.

Running a Service only keeps your app's process alive - it won't do anything for background Activities, which the system will destroy if it needs the memory. And besides, like it says above, if you back out of an Activity it's considered dismissed and its state is wiped, whether it's been kicked out of memory or not.

Honestly it sounds like you just want to avoid redisplaying the login screen if the user's already logged in? That shouldn't be anything to do with Activity state, you should be storing that data somewhere so you can check it when the app starts, so you can send them to the appropriate screen

There's a whole section about this exact scenario in the Navigation docs. They don't show their UserViewModel (which we're supposed to imagine handling logged-in state, and actually doing the user/password auth if necessary) but the idea is pretty simple - you have a component that exposes a value saying whether the user is logged in or not (which could be stored in e.g. SharedPreferences) and it can automatically navigate away from the login screen based on that.