How to save data and re-open the last used Activity

Question

I've made the majority of my game's mechanics, I now need to be able to:

1. Save all the data of the current activity and retrieve it when coming back (I'd appreciate an example of SharedPreferences if that's what I need)

2. Open back the same Activity I left from and in the same time I was when I left it.

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Just to be clearer: I want not to restart from my Main Activity each time the App gets closed or even killed.

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EDIT:

Alright, I've used this Google article in order to save my Activity and recreate it later.

The code in one of my Activities is the following:

onCreate()

@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_player_selection);

    // Check whether we're recreating a previously destroyed instance
    if (savedInstanceState != null) {
        // Restore value of members from saved state
        village = savedInstanceState.getString(VILLAGE);
        seekBarProgress = savedInstanceState.getInt(PROGRESS);
    } else {
        // Probably initialize members with default values for a new instance
        initializeVariables();
        Bundle bundle = getIntent().getExtras();
        village = bundle.getString(VILLAGE);
    }
        [...] // Other code skipped
    }

onSaveInstanceState()

@Override
public void onSaveInstanceState(Bundle savedInstanceState) {
    // Save the user's current game state
    savedInstanceState.putString(VILLAGE, village);
    savedInstanceState.putInt(PROGRESS, seekBarProgress);
    // Always call the superclass so it can save the view hierarchy state
    super.onSaveInstanceState(savedInstanceState);
}

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Thus, I've created a Dispatcher Activity (which is now my Main) that when the App is killed and then launched back, knows which is the last opened Activity to launch. But when I try to do that, and it should open (i.e.) the PlayerSelection Activity, I get a

java.lang.NullPointerException

as it runs this part of code

else { // Probably initialize members with default values for a new instance initializeVariables(); Bundle bundle = getIntent().getExtras(); village = bundle.getString(VILLAGE); }

instead of the previous if statement it runs the else.

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Dispatcher

This is the Dispatcher Activity which filters which Activity should be launched:

public class Dispatcher extends Activity {

@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);

    Class<?> activityClass;

    try {
        SharedPreferences prefs = getSharedPreferences("X", MODE_PRIVATE);
        activityClass = Class.forName(
                prefs.getString("lastActivity", MainActivity.class.getName()));
    } catch(ClassNotFoundException ex) {
        activityClass = MainActivity.class;
    }

    startActivity(new Intent(this, activityClass));
    }
}

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This is what I send to my Dispatcher Activity:

@Override
protected void onPause() {
    super.onPause();

    SharedPreferences prefs = getSharedPreferences("X", MODE_PRIVATE);
    SharedPreferences.Editor editor = prefs.edit();
    editor.putString("lastActivity", getClass().getName());
    editor.commit();
    }
}

---

Question

Why does it happen? What can I do to fix this?

Answer 1

The onSaveInstanceState will not be called when you press back key to finish it. To save your data all the time you have to use onPause as the code shows:

@Override
public void onPause() {
    super.onPause();
    SharedPreferences sp = getSharedPreferences("X", Context.MODE_PRIVATE);
    SharedPreferences.Editor editor = sp.edit();
    editor.putString("lastActivity", HomeActivity.class.getCanonicalName());
}

Remember to replace HomeActivity with your own class name.BTW, I didn't test the function getCanonicalName, but it should work.

In your Dispatch Activity, you read the value back and do some action to it.

Answer 2

Why does it happen?

The Bundle created in onSaveInstanceState is only kept for short-term restarts, e.g. when your activity is recreated due to an orientation change. If your activity stops normally this Bundle is not kept, and thus can't be delivered to onCreate.

What can I do to fix this?

Instead of using the instanceState model, save the activity state in SharedPreferences (similar to the lastActivity which you already save). You should save your state in onStop or onPause and restore it in onStart or onResume respectively. In most cases the former is enough and the latter produces only unnecessary overhead.

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@Override
protected void onStart() {
    super.onStart();
    String saved = sharedPreferences.getString("myKey", null);
    MyPojo pojo;
    if(saved == null){
        pojo = defaultValue;
    }else {
        pojo = new Gson().fromJson(saved, MyPojo.class);
    }
}

@Override
protected void onStop() {
    sharedPreferences.edit().putString("myKey", new Gson().toJson(pojo)).apply();
    super.onStop();
}