How to call a method when an Android app is closed or loses focus?

Question

Because an app I'm building will handle rather sensitive data I want to sync the SQLite db with the server every time the user logs in, and remove emty the db every time the app loses focus (because the user moves to the home screen or another app).

Seeing the Activity lifecycle, my idea was to do this by emptying the database in the onDestroy of every Activity. To test the described lifecycle I just Override all lifecycle methods (onCreate, onStart, onResume, onPause, onStop, and onDestroy), included some log messages in them, and fired up my app.

The log messages are included in my SettingsActivity. When I enter my app and move to the Settings it runs onCreate, onStart and onResume (as expected). When I then click a setting and move to the next screen it runs onPause and onStop (still as expected). To move back to the settings screen I click the back button and it runs onStart and onResume again (still as expected), when I now click the back button again to move back to the initial screen, it (to my surprise) runs onPause, onStop AND onDestroy.

So my questions;

1. Why does it destroy the Activity when the app is not finished?
2. And more importantly: how can I run my CleanUp method when the app closes or loses focus?

Answer 1

You can have some more information here : http://developer.android.com/training/basics/activity-lifecycle/stopping.html

Even if I think you already read it because you already study the activity lifecycle, you can see in the first figure that the onDestroy() is called after the onStop() and this call is totally managed by the system : you shouldn't expect any behavior. The system will decide itself WHEN to call this method, and sometimes, this method will never be called (see here : http://developer.android.com/reference/android/app/Activity.html). When system needs memory, your activity will pass in onStop() and nothing more.

So, to answer your second question, you shloud read the note in the documentation about the onDestroy() method :

Note: do not count on this method being called as a place for saving data! For example, if an activity is editing data in a content provider, those edits should be committed in either onPause() or onSaveInstanceState(Bundle), not here. This method is usually implemented to free resources like threads that are associated with an activity, so that a destroyed activity does not leave such things around while the rest of its application is still running. There are situations where the system will simply kill the activity's hosting process without calling this method (or any others) in it, so it should not be used to do things that are intended to remain around after the process goes away.

So it's pretty clear that it's a bad place to make your clean-up process. So you shloud use one of onPause() or onStop() method.

onStop() is described like this in the documentation :

Called when you are no longer visible to the user. You will next receive either onRestart(), onDestroy(), or nothing, depending on later user activity.

onPause() is described like this in the documentation :

Called as part of the activity lifecycle when an activity is going into the background, but has not (yet) been killed. [...] When activity B is launched in front of activity A, this callback will be invoked on A. B will not be created until A's onPause() returns, so be sure to not do anything lengthy here. [...] In situations where the system needs more memory it may kill paused processes to reclaim resources.

We now know that onPause() is designed to allow you to save your data, and also that after onPause() was executed, the system could kill your process. So, making the clean-up in onPause() seems to be the safest place as you're pretty sure it will be called everytime.

Also, as you can read, making your clean up here can make your app slow, but anyway cleaning and recreating your database at each gain/loose of focus is a really heavy process...

To resume : make your clean up process in the onPause() method and your init process in the onResume(). Keep it mind that your application can be really slow with this kind of process.

Hope this can help you.