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 re-created from scratch again. In these situations, the best practice is to prepare for cases where the Activity is re-created by properly saving and restoring the state.
Saving and Restoring Activity 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
protected 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:@Override
protected 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);
}
Instance state can also be restored in the standard
Activity#onCreate method but it is convenient to do it inonRestoreInstanceState which ensures all of the initialization has been done and allows subclasses to decide whether to use the default implementation. Read this stackoverflow post for details.
Note that
onSaveInstanceState and onRestoreInstanceState are not guaranteed to be called together. Android invokes onSaveInstanceState() when there's a chance the activity might be destroyed. However, there are cases where onSaveInstanceState is called but the activity is not destroyed and as a result onRestoreInstanceState is not invoked.
Read more on the Recreating an Activity guide.
Saving and Restoring Fragment State
Fragments also have a
onSaveInstanceState() method which is called when their state needs to be saved:public class MySimpleFragment extends Fragment {
private int someStateValue;
private final String SOME_VALUE_KEY = "someValueToSave";
// Fires when a configuration change occurs and fragment needs to save state
@Override
protected void onSaveInstanceState(Bundle outState) {
outState.putInt(SOME_VALUE_KEY, someStateValue);
super.onSaveInstanceState(outState);
}
}
Then we can pull data out of this saved state in
onCreateView:public class MySimpleFragment extends Fragment {
// ...
// Inflate the view for the fragment based on layout XML
@Override
public View onCreateView(LayoutInflater inflater, ViewGroup container, Bundle savedInstanceState) {
View view = inflater.inflate(R.layout.my_simple_fragment, container, false);
if (savedInstanceState != null) {
someStateValue = savedInstanceState.getInt(SOME_VALUE_KEY);
// Do something with value if needed
}
return view;
}
}
For the fragment state to be saved properly, we need to be sure that we aren't unnecessarily recreating the fragment on configuration changes. This means being careful not to reinitialize existing fragments when they already exist. Any fragments being initialized in an Activity need to be looked up by tag after a configuration change:
public class ParentActivity extends AppCompatActivity {
private MySimpleFragment fragmentSimple;
private final String SIMPLE_FRAGMENT_TAG = "myfragmenttag";
@Override
protected void onCreate(Bundle savedInstanceState) {
if (savedInstanceState != null) { // saved instance state, fragment may exist
// look up the instance that already exists by tag
fragmentSimple = (MySimpleFragment)
getSupportFragmentManager().findFragmentByTag(SIMPLE_FRAGMENT_TAG);
} else if (fragmentSimple == null) {
// only create fragment if they haven't been instantiated already
fragmentSimple = new MySimpleFragment();
}
}
}
This requires us to be careful to include a tag for lookup whenever putting a fragment into the activity within a transaction:
public class ParentActivity extends AppCompatActivity {
private MySimpleFragment fragmentSimple;
private final String SIMPLE_FRAGMENT_TAG = "myfragmenttag";
@Override
protected void onCreate(Bundle savedInstanceState) {
// ... fragment lookup or instantation from above...
// Always add a tag to a fragment being inserted into container
if (!fragmentSimple.isInLayout()) {
getSupportFragmentManager()
.beginTransaction()
.replace(R.id.container, fragmentSimple, SIMPLE_FRAGMENT_TAG)
.commit();
}
}
}
With this simple pattern, we can properly re-use fragments and restore their state across configuration changes.
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;
}
}
This approach keeps the fragment from being destroyed during the activity lifecycle. They are instead retained inside the Fragment Manager. See the Android official docs for more information.
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.
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.
Manually Managing Configuration Changes
If your application doesn't need to update resources during a specific configuration change and you have a performance limitation that requires you to avoid the activity restart, then you can declare that your activity handles the configuration change itself, which prevents the system from restarting your activity.
However, this technique should be considered a last resort when you must avoid restarts due to a configuration change and is not recommended for most applications. To take this approach, we must add the
android:configChanges node to the activity within the AndroidManifest.xml:<activity android:name=".MyActivity"
android:configChanges="orientation|screenSize|keyboardHidden"
android:label="@string/app_name">
Now, when one of these configurations change, the activity does not restart but instead receives a call to
onConfigurationChanged():// Within the activity which receives these changes
// Checks the current device orientation, and toasts accordingly
@Override
public void onConfigurationChanged(Configuration newConfig) {
super.onConfigurationChanged(newConfig);
// Checks the orientation of the screen
if (newConfig.orientation == Configuration.ORIENTATION_LANDSCAPE) {
Toast.makeText(this, "landscape", Toast.LENGTH_SHORT).show();
} else if (newConfig.orientation == Configuration.ORIENTATION_PORTRAIT){
Toast.makeText(this, "portrait", Toast.LENGTH_SHORT).show();
}
}
See the Handling the Change docs. For more about which configuration changes you can handle in your activity, see the android:configChanges documentation and the Configuration class.
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