Is there any drawback to implementing member variable free activities?

Issue

I started to learn the android framework and my biggest problem is the hell around the activity life cycle. So when the user rotate the screen my application just crash. As I understand beside the normal activity life cycle Android hacked a force-instance-deleter-and-partially-recovery service for me which is not a bug but a feature.

So I just want my member variable keeps safe, so a I thought a start storing it in the Application class.

So I want to refactor my program in the following way:

I create an own Application

public class MainApp extends Application {
   LoginActivityData loginActivityData; // create data "segment" for every activity
   FirstActivityData firstActivityData;
   ...

   public static MainApp getInstance(final Context context) {
       if (context == null) return null;
       final Context app = context.getApplicationContext();
       return app instanceof MainApp ? (MainApp) app : null;
   }

}

In activities and fragments I stop using member variables except the one from the MainApp class.

public class LoginActivity extends Activity {
   LoginActivityData loginActivityData;

protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    loginActivityData = MainApp.getInstance(this).loginActivityData;
}

Is there any drawback of this design?

Solution

Yes, there is a drawback. Your application object can and will be destroyed by the system when it needs to recover memory, and the application object does not have callback methods that can be used to save state. See here for a fuller explanation.

There are many ways to persist data/state, but if you use the following approach you generally won't go far wrong:

• Use onPause() to save long-term data to a SQLite DB/SharedPreferences/cloud etc. Restore it wherever appropriate (onCreate(), onResume(), ...).
• Use onSaveInstanceState() to save temporary data to a Bundle. Restore it in onCreate()/onRestoreInstanceState(). The bundle is automatically passed around to the appropriate methods by the system. Note that there is no guarantee that onSaveInstanceState() will be called, so don't use it for critical data.

Answered By - Phil Glass