Handling Configuration Changes

Overview

There are various situations such as when the screen orientation is rotated where the Activity can actually be destroyed and removed from memory and then recreated from scratch again. In these situations, the best practice is to manage for cases where the Activity is re-created by properly saving and restoring the state.

Saving and Restoring State

As your activity begins to stop, the system calls onSaveInstanceState() so your activity can save state information with a collection of key-value pairs. The default implementation of this method automatically saves information about the state of the activity's view hierarchy, such as the text in an EditText widget or the scroll position of a ListView.

To save additional state information for your activity, you must implement onSaveInstanceState() and add key-value pairs to the Bundle object. For example:

public class MainActivity extends Activity {
    static final String SOME_VALUE = "int_value";
    static final String SOME_OTHER_VALUE = "string_value";

    @Override
    public void onSaveInstanceState(Bundle savedInstanceState) {
        // Save custom values into the bundle
        savedInstanceState.putInt(SOME_VALUE, someIntValue);
        savedInstanceState.putString(SOME_OTHER_VALUE, someStringValue);
        // Always call the superclass so it can save the view hierarchy state
        super.onSaveInstanceState(savedInstanceState);
    }
}

The system will call that method before an Activity is destroyed. Then later the system will call onRestoreInstanceState where we can restore state from the bundle:

public void onRestoreInstanceState(Bundle savedInstanceState) {
    // Always call the superclass so it can restore the view hierarchy
    super.onRestoreInstanceState(savedInstanceState);
   
    // Restore state members from saved instance
    someIntValue = savedInstanceState.getInt(SOME_VALUE);
    someStringValue = savedInstanceState.getString(SOME_OTHER_VALUE);
}

Read more on the Recreating an Activity guide.

Locking Screen Orientation

If you want to lock the screen orientation change of any screen (activity) of your android application you just need to set the android:screenOrientation property of an <activity> within the AndroidManifest.xml:

<activity
    android:name="com.techblogon.screenorientationexample.MainActivity"
    android:screenOrientation="portrait"
    android:label="@string/app_name" >
    <!-- ... -->
</activity>

Now that activity is forced to always be displayed in "portrait" mode.

Retaining Fragments

In many cases, we can avoid problems when an Activity is re-created by simply using fragments. If your views and state are within a fragment, we can easily have the fragment be retained when the activity is re-created:

public class RetainedFragment extends Fragment {
    // data object we want to retain
    private MyDataObject data;

    // this method is only called once for this fragment
    @Override
    public void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        // retain this fragment when activity is re-initialized
        setRetainInstance(true);
    }

    public void setData(MyDataObject data) {
        this.data = data;
    }

    public MyDataObject getData() {
        return data;
    }
}

Now you can check to see if the fragment already exists by tag before creating one and the fragment will retain it's state across configuration changes. See the Handling Runtime Changes guide for more details.

References

• http://developer.android.com/guide/topics/resources/runtime-changes.html
• http://developer.android.com/training/basics/activity-lifecycle/recreating.html
• http://www.vogella.com/tutorials/AndroidLifeCycle/article.html#configurationchange
• http://www.androiddesignpatterns.com/2013/04/retaining-objects-across-config-changes.html
• http://www.intertech.com/Blog/saving-and-retrieving-android-instance-state-part-1/
• http://sunil-android.blogspot.com/2013/03/save-and-restore-instance-state.html