Maintain state between activities

Question

My app has two activities: MasterActivity and DetailActivity . MasterActivity has two visualization modes: list mode and map mode . An action bar item toggles between them.

I'd like to mantain the selected visualization mode when user goes into DetailActivity and comes back. In the beginning I used SharedPreferences but then user would get back his previous visualization mode even after a device boot or a long inactivity time, and that's not what I mean.

Then I switched to Bundle and onSaveInstanceState but, when user clicks on back button of DetailActivity , onCreate 's Bundle is always empty so I can't restore the previous visualization mode and it always reverts to the list one.

App uses a Toolbar and AndroidManifest.xml is configured like that:

<activity
    android:name=".ui.MasterActivity"
    android:label="@string/title_activity_master"
    android:theme="@style/AppTheme.NoActionBar">
    <intent-filter>
        <action android:name="android.intent.action.MAIN" />
        <category android:name="android.intent.category.LAUNCHER" />
    </intent-filter>
</activity>
<activity
    android:name=".ui.DetailActivity"
    android:parentActivityName=".ui.MasterActivity"
    android:theme="@style/AppTheme.NoActionBar">
    <meta-data
        android:name="android.support.PARENT_ACTIVITY"
        android:value="it.returntrue.revalue.ui.MasterActivity" />
</activity>

Answer 1

Not sure why onSaveInstanceState doesn't work for you. Your code would be like:

class MasterActivity extends Activity {
    private int mode;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        if (savedInstanceState != null)
            mode = savedInstanceState.getInt("mode");
    }

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

        if(mode == 1) {
            //list mode
        } else {
            // map mode
        }
    }

    @Override
    protected void onSaveInstanceState(Bundle outState) {
        super.onSaveInstanceState(outState);
        outState.putInt("mode", mode);
    }
}

Nevertheless, consider to use Androjeta framework (maintained by me). It comes with a number of features including @Retain which you can use in your case:

class MasterActivity extends BaseActivity {
    @Retain
    int mode;

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

        if(mode == 1) {
            //list mode
        } else {
            // map mode
        }
    }
}

Note that here MasterActivity extends from BaseActivity so you need to create that too. Please, follow the link for details.

Answer 2

I have two approaches. It's basically the same, but different on how you store the data

1. Use a Singleton
2. Keep your sharedprefs model as it is

On both occasions, delete the values on the Activity's onDestroy method.