Android-检测应用移至背景和前景

Question

If application is gone to background and come back after 5 minutes, I need to restart the app. We can leave the restart part, how will we detect the app moving to background and to foreground? Please help.

If there is a link on details of its official impossibility or possibility, please share.

Answer 1

when it moves to the background it does onPause() and when it resumes it does onResume() , see the activity lifecycle.

http://developer.android.com/reference/android/app/Activity.html#ActivityLifecycle

Answer 2

I had a similar problem where every time the app returned from background Fragment B kept recalculating results based on the output from Fragment A in the activity (where Fragment A sends data to Fragment B based on user input). The following solution uses a simple Boolean check and should work for most purposes. This allowed me to keep the old FragmentB results even after a long time away from the app (assuming the results in FragmentB aren't so memory-hungry that Android cleans them after some time).

//Code from Fragment B
public class FragmentB extends Fragment {
    Boolean app_was_in_background;
    @Override public void onPause() {
        super.onPause();
        app_was_in_background = true;
    }
    @Override public void onResume() {
        super.onResume();
        if (getArguments() != null) {
            String output_to_FragmentB = getArguments().getString("input_from_FragmentA");

            if (app_was_in_background == null || !app_was_in_background) {
                app_was_in_background = false;

                PerformMyAlgorithm(output_to_FragmentB);
            }
        }
    }
    public void PerformMyAlgorithm(String input) {
        //Code that includes Spinners and calculations, where I want to app
        //to return to the user's chosen spinner location and display
        //results as the user saw them before switching to another app
        //on the device. All this without explicitly "saving" the Spinner 
        //position and data in memory.
    }
}

As a side note, applying the same logic to onStart() instead of onResume() can lead to unwanted results:

PerformMyAlgorithm receives the FragmentA output (ie. String output_to_FragmentB) again automatically at onResume() without the user choosing to to so by clicking a Button in FragmentA (my guess is that this due to the way Android manages Fragment lifecycles).

I don't want this since when returning from background (ie. onResume) the FragmentB Views and calculations are reset and redisplayed/recalculated instead of immediately showing the user the results prior to sending the app to background (ie. onPause).

Answer 3

Override onStop() in each Activity. This method will be called when activity goes in background. Then note the time.

Override onStart() . This will be called when activity moves from background to foreground. Then note the time here.