App Auto-Closing when device is locked.

[Dimitrees]

Hello, don't know if this project is still being maintained, however I noticed an issue in my own fork of the project, but when I tested it on the app which is available in Google Play it still appeared.

After you've selected the "Play" button in the first activity, and chosen a theme, whenever you lock your device manually, the app stops and closes, and when you unlock the device again, you have to restart the app. Should this be happening?

I am also interested in trying to fixing this myself, if one of the creators points me to the right direction.

Thank you.

[sromku]

I am not sure I understood. What do you mean by "app stops and closes" - Is the app crashing? If yes, can you provide the stacktrace.

[Dimitrees]

Exactly, it crashes.

09-01 17:14:33.625 25054-25054/com.snatik.matches E/AndroidRuntime: FATAL EXCEPTION: main Process: com.snatik.matches, PID: 25054 java.lang.RuntimeException: Unable to start activity ComponentInfo{com.snatik.matches/com.snatik.matches.MainActivity}: java.lang.NullPointerException: Attempt to read from field 'int com.snatik.matches.themes.Theme.id' on a null object reference at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2305) at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2365) at android.app.ActivityThread.handleRelaunchActivity(ActivityThread.java:3927) at android.app.ActivityThread.access$900(ActivityThread.java:148) at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1289) at android.os.Handler.dispatchMessage(Handler.java:102) at android.os.Looper.loop(Looper.java:135) at android.app.ActivityThread.main(ActivityThread.java:5272) at java.lang.reflect.Method.invoke(Native Method) at java.lang.reflect.Method.invoke(Method.java:372) at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:909) at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:704) Caused by: java.lang.NullPointerException: Attempt to read from field 'int com.snatik.matches.themes.Theme.id' on a null object reference at com.snatik.matches.fragments.DifficultySelectFragment.onCreateView(DifficultySelectFragment.java:28) at android.support.v4.app.Fragment.performCreateView(Fragment.java:2192) at android.support.v4.app.FragmentManagerImpl.moveToState(FragmentManager.java:1299) at android.support.v4.app.FragmentManagerImpl.moveFragmentToExpectedState(FragmentManager.java:1528) at android.support.v4.app.FragmentManagerImpl.moveToState(FragmentManager.java:1595) at android.support.v4.app.FragmentManagerImpl.dispatchActivityCreated(FragmentManager.java:2900) at android.support.v4.app.FragmentController.dispatchActivityCreated(FragmentController.java:201) at android.support.v4.app.FragmentActivity.onStart(FragmentActivity.java:603) at android.app.Instrumentation.callActivityOnStart(Instrumentation.java:1220) at android.app.Activity.performStart(Activity.java:5993) at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2268) at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2365)  at android.app.ActivityThread.handleRelaunchActivity(ActivityThread.java:3927)  at android.app.ActivityThread.access$900(ActivityThread.java:148)  at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1289)  at android.os.Handler.dispatchMessage(Handler.java:102)  at android.os.Looper.loop(Looper.java:135)  at android.app.ActivityThread.main(ActivityThread.java:5272)  at java.lang.reflect.Method.invoke(Native Method)  at java.lang.reflect.Method.invoke(Method.java:372)  at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:909)  at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:704)  09-01 17:14:33.651 25054-25054/com.snatik.matches I/Process: Sending signal. PID: 25054 SIG: 9

[sromku]

In fact I never experienced this with this app, but looks like a valid exception. It happens because of static fields in Engine class. What happens is that Android kills the game process due to low memory and once you open the app it tries restoring to previous state - to Fragment that was last in the foreground. The problem is that DifficultySelectFragment uses the static field mSelectedTheme from Engine which is initialized in fragment before - ThemeSelectFragment, but this fragment isn't created after restoring the app state.

So, in general:

1. Using static members is a bad idea :) And if it's used the initialization must happen in Application class only.
2. onSaveInstanceState and restore state should be used if we want to retain Activity and Fragments states.

In this case I would remove fragments at all and use only views. Each screen would be a View. Also, I would create some recycler bin for controlling the memory allocation and views for reusage.
This pretty a big change.
For a quick win, I would go back to the home screen if Engine isn't initialized correctly after being restored - this should resolve the crash.