Problems understanding the life cycle when screen goes off and on

Question

Information: My device is a Nexus One with 2.2 and I have tested two projects, one on 1.5 and one on 2.1.

Problem: I have trouble to understand the life cycle of my application when the screen is turned off and on.

Here is my output

// activity starts
08-04 17:24:17.643: ERROR/PlayActivity(6215): onStart executes ...
08-04 17:24:17.643: ERROR/PlayActivity(6215): onResume executes ...
// screen goes off
08-04 17:24:28.943: ERROR/PlayActivity(6215): onPause executes ...
08-04 17:24:32.113: ERROR/PlayActivity(6215): onStop executes ...
08-04 17:24:32.113: ERROR/PlayActivity(6215): onDestroy executes ...
08-04 17:24:32.983: ERROR/PlayActivity(6215): onStart executes ...
08-04 17:24:32.983: ERROR/PlayActivity(6215): onResume executes ...
08-04 17:24:32.983: ERROR/PlayActivity(6215): onPause executes ...
// screen goes on
08-04 17:24:47.683: ERROR/PlayActivity(6215): onResume executes ...
// lock removed
08-04 17:24:56.943: ERROR/PlayActivity(6215): onPause executes ...
08-04 17:24:59.663: ERROR/PlayActivity(6215): onStop executes ...
08-04 17:24:59.663: ERROR/PlayActivity(6215): onDestroy executes ...
08-04 17:25:00.943: ERROR/PlayActivity(6215): onStart executes ...
08-04 17:25:00.943: ERROR/PlayActivity(6215): onResume executes ...

I am totally confused. Why restarting the activity when the screen goes off? And why stop and restarting it again when the screen was already on and only the lock was removed?

To make sure I haven't done anything wrong, I created a new project with only this activity. The output is identically...

public class LifeCycleTest extends Activity {

    private final static String DEBUG_TAG = "FirstLifeLog";

    public void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        Log.e(DEBUG_TAG, "onCreate executes ...");
        setContentView(R.layout.main);
    }

    protected void onRestart() {
        super.onRestart();
        Log.e(DEBUG_TAG, "onRestart executes ...");
    }

    protected void onStart() {
        super.onStart();
        Log.e(DEBUG_TAG, "onStart executes ...");
    }

    protected void onResume() {
        super.onResume();
        Log.e(DEBUG_TAG, "onResume executes ...");
    }

    protected void onPause() {
        super.onPause();
        Log.e(DEBUG_TAG, "onPause executes ...");
    }

    protected void onStop() {
        super.onStop();
        Log.e(DEBUG_TAG, "onStop executes ...");
    }

    protected void onDestroy() {
        super.onDestroy();
        Log.e(DEBUG_TAG, "onDestroy executes ...");
    }
}

Does someone have an idea?

Update from today (dont understand why it behaves not like last time, maybe more free resources?)

// activity starts
08-09 12:14:03.122: ERROR/FirstLifeLog(15406): onCreate executes ...
08-09 12:14:03.132: ERROR/FirstLifeLog(15406): onStart executes ...
08-09 12:14:03.132: ERROR/FirstLifeLog(15406): onResume executes ...
// screen off
08-09 12:14:07.412: ERROR/FirstLifeLog(15406): onPause executes ...
// screen on
08-09 12:14:11.722: ERROR/FirstLifeLog(15406): onResume executes ...
// no log for removed screen lock

Answer 1

See Activity Lifecycle documentation for a good description of the lifecycle, with diagrams.

Most likely your activity is killed with the screen goes off to save resources (battery power). As the documentation states, you can basically be killed anytime that Android wants to free resources. So, you should always design your activities to be able to be stopped and restarted at any time.

Answer 2

I had the same issue with my own game. My game works in landscape only, and when turning off the screen, the android screensaver takes the control (in portrait mode), thus sending an orientationChange that destroys and recreates the activity.

A simple solution is to declare that you will manage yourself screen orientation changes:

<activity ... android:configChanges="orientation" ... >

This is quite easy if your activity is declared to be landscape only (you have to do nothing), but can get harder if your activity can rotate...

Answer 3

Ruben's answer is completely correct, but only if your application targets the API level 12 or lower.

But since the API level 13 in addition to the orientation option, you have to declare the screenSize option, because it also gets triggered when a device switches between the portrait and the landscape orientations:

<activity ... android:configChanges="orientation|screenSize" ... >

Otherwise, your activity would still be recreated an additional time when screen goes off on the API 13 or a higher platform.

For reference, see API docs, android:configChanges section notes.

Answer 4

Thats the way. If you read the activity life cycle you will see that the steps are pretty much ordered that way. Its not just when your screen goes on and off but also when you chnage the oreintation of the phone. Android recreated the activity following exactly the steps you have mentioned above. Try rotating you screen, you will see then! =)