Android - losing variables data when partially closing an app

Question

I am using a class with static values called DB (for Data Base) in my application. When I first run the app, a static byte array from this class is filled and used. Then, when I partially close my app (not closing it definitily just put in background) if a reopen it after 20 seconds more or less, the value of the variable is still here but if I let my app in the background for more than 1 minute the value turns to null.

How can I avoid this to happen?

store your data in a database or shared preferences like you should be — tyczj, Aug 21 '15 at 22:24
there is no way to know if you are closing the app or the OS is killing your app — tyczj, Aug 21 '15 at 22:25
I mean, I want the byte array to be saved between pausing and resuming my app but between starting and killing my app — Roman Panaget, Aug 21 '15 at 22:27
and like I just said `there is no way to know if you are closing the app or the OS is killing your app`. onPause gets called when your app goes into the background or switching an activity so save there but if you app gets killed by the OS you cant erase it — tyczj, Aug 21 '15 at 22:32
Because the byte array is stored in an auxiliary "static" class — Roman Panaget, Aug 21 '15 at 22:34
The value of the byte array may be turned to null because of Garbage Collector ? If yes, how should I avoid it ? — Roman Panaget, Aug 21 '15 at 22:35
@RomanPanaget I dont know how else we can tell you this, there is no way to do what you want! You either need to save it or else you will lose it end of story — tyczj, Aug 21 '15 at 22:37
Ok, suposing that the byte array is stored in my main activity, how can i save it? — Roman Panaget, Aug 21 '15 at 22:39
in `onPause` save it to either a database or shared preferences — tyczj, Aug 21 '15 at 22:40

score 1 · Answer 1 · answered Aug 21 '15 at 22:41

1

store your variable value to shared preferences and load the value from shared preferences in the onResume() Method of activity and store the value in the onPause() Method.

answered Aug 21 '15 at 22:41

Deepak Goyal

4,747
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score 1 · Answer 2 · answered Aug 22 '15 at 04:23

Handling lifestyle events properly is an important aspect of Android development.

I suggest that you read the following to make sure that you understand what happens to your app when you turn off your screen, change to another application or any other action that might change the state of your app:

http://developer.android.com/training/basics/activity-lifecycle/index.html

My suggestion is to store your data by overriding onSaveInstanceState() like so:

@Override
public void onSaveInstanceState(Bundle savedInstanceState) 
{
    // Save the user's current game state
    savedInstanceState.putInt(STATE_SCORE, mCurrentScore);
    savedInstanceState.putInt(STATE_LEVEL, mCurrentLevel);

    // Always call the superclass so it can save the view hierarchy state
    super.onSaveInstanceState(savedInstanceState);
}

Then on your onCreate(), you can reload it like so:

@Override
protected void onCreate(Bundle savedInstanceState) 
{
    super.onCreate(savedInstanceState); // Always call the superclass first

    // Check whether we're recreating a previously destroyed instance
    if (savedInstanceState != null) 
    {
        // Restore value of members from saved state
        mCurrentScore = savedInstanceState.getInt(STATE_SCORE);
        mCurrentLevel = savedInstanceState.getInt(STATE_LEVEL);
    } 
    else 
    {
        // Probably initialize members with default values for a new instance
    }
...
}

I hope that this helps!

Good luck in your future developing!

This is very good but your answer doesn't fit completely to my problem. My byte[] must be null when running the app and must be the same in all the activities, how can I handle this situation with your method? Because you're saying that I must set the value of the byte array in the onCreate method, but in my onCreate method my byte array is null, its value is set a bit later. Once set, it is shared with the other activities and when the app is killed, the byte[] must be save as null. — Roman Panaget, Aug 22 '15 at 08:18

score 0 · Answer 3 · answered Aug 21 '15 at 23:26

I found a solution with the help of the commenters.

For those who had the same problem:

Copy this in all your Activities to ensure that the data is constantly updated in the preferences:

@Override
public void onPause(){
    super.onPause();
    String bytearray = Base64.encodeToString(DB.bytearray, Base64.DEFAULT);
    prefs.edit().putString("BYTEARRAY", bytearray).apply();
}
@Override
public void onResume(){
    super.onResume();
    String bytearray = prefs.getString("BYTEARRAY", Base64.encodeToString(DB.bytearray, Base64.DEFAULT));
    DB.bytearray = Base64.decode(bytearray, Base64.DEFAULT);
}

Then, add this code in all your Activities to ensure that the values are not saved when you close your app.

@Override
public void onDestroy(){
    super.onDestroy();
    String bytearray = "";
    prefs.edit().putString("BYTEARRAY", bytearray).apply();
}

You should have used the bundle onsavedinstance instead of shared preferences in this case. Though your solution is valid. — Lazy Ninja, Aug 21 '15 at 23:32
How? Post your solution it will probably be better than mine ! — Roman Panaget, Aug 21 '15 at 23:35

Android - losing variables data when partially closing an app

3 Answers3