Android is changing configurations

Handling Runtime Changes

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Some device configurations can change during runtime (such as screen orientation, keyboard availability, and language). When such a change occurs, Android restarts the running Activity ( onDestroy() is called, followed by onCreate() ). The restart behavior is designed to help your application adapt to new configurations by automatically reloading your application with alternative resources that match the new device configuration.

To properly handle a restart, it is important that your activity restores its previous state through the normal Activity lifecycle, in which Android calls onSaveInstanceState() before it destroys your activity so that you can save data about the application state. You can then restore the state during onCreate() or onRestoreInstanceState() .

To test that your application restarts itself with the application state intact, you should invoke configuration changes (such as changing the screen orientation) while performing various tasks in your application. Your application should be able to restart at any time without loss of user data or state in order to handle events such as configuration changes or when the user receives an incoming phone call and then returns to your application much later after your application process may have been destroyed. To learn how you can restore your activity state, read about the Activity lifecycle.

However, you might encounter a situation in which restarting your application and restoring significant amounts of data can be costly and create a poor user experience. In such a situation, you have two other options:

Allow your activity to restart when a configuration changes, but carry a stateful object to the new instance of your activity.

Prevent the system from restarting your activity during certain configuration changes, but receive a callback when the configurations do change, so that you can manually update your activity as necessary.

Retaining an Object During a Configuration Change

If restarting your activity requires that you recover large sets of data, re-establish a network connection, or perform other intensive operations, then a full restart due to a configuration change might be a slow user experience. Also, it might not be possible for you to completely restore your activity state with the Bundle that the system saves for you with the onSaveInstanceState() callback—it is not designed to carry large objects (such as bitmaps) and the data within it must be serialized then deserialized, which can consume a lot of memory and make the configuration change slow. In such a situation, you can alleviate the burden of reinitializing your activity by retaining a Fragment when your activity is restarted due to a configuration change. This fragment can contain references to stateful objects that you want to retain.

When the Android system shuts down your activity due to a configuration change, the fragments of your activity that you have marked to retain are not destroyed. You can add such fragments to your activity to preserve stateful objects.

To retain stateful objects in a fragment during a runtime configuration change:

1. Extend the Fragment class and declare references to your stateful objects.
2. Call setRetainInstance(boolean) when the fragment is created.
3. Add the fragment to your activity.
4. Use FragmentManager to retrieve the fragment when the activity is restarted.

For example, define your fragment as follows:

Caution: While you can store any object, you should never pass an object that is tied to the Activity , such as a Drawable , an Adapter , a View or any other object that’s associated with a Context . If you do, it will leak all the views and resources of the original activity instance. (Leaking resources means that your application maintains a hold on them and they cannot be garbage-collected, so lots of memory can be lost.)

Then use FragmentManager to add the fragment to the activity. You can obtain the data object from the fragment when the activity starts again during runtime configuration changes. For example, define your activity as follows:

In this example, onCreate() adds a fragment or restores a reference to it. onCreate() also stores the stateful object inside the fragment instance. onDestroy() updates the stateful object inside the retained fragment instance.

Handling the Configuration Change Yourself

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.

Note: Handling the configuration change yourself can make it much more difficult to use alternative resources, because the system does not automatically apply them for you. 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 declare that your activity handles a configuration change, edit the appropriate element in your manifest file to include the android:configChanges attribute with a value that represents the configuration you want to handle. Possible values are listed in the documentation for the android:configChanges attribute (the most commonly used values are «orientation» to prevent restarts when the screen orientation changes and «keyboardHidden» to prevent restarts when the keyboard availability changes). You can declare multiple configuration values in the attribute by separating them with a pipe | character.

For example, the following manifest code declares an activity that handles both the screen orientation change and keyboard availability change:

Now, when one of these configurations change, MyActivity does not restart. Instead, the MyActivity receives a call to onConfigurationChanged() . This method is passed a Configuration object that specifies the new device configuration. By reading fields in the Configuration , you can determine the new configuration and make appropriate changes by updating the resources used in your interface. At the time this method is called, your activity’s Resources object is updated to return resources based on the new configuration, so you can easily reset elements of your UI without the system restarting your activity.

Caution: Beginning with Android 3.2 (API level 13), the «screen size» also changes when the device switches between portrait and landscape orientation. Thus, if you want to prevent runtime restarts due to orientation change when developing for API level 13 or higher (as declared by the minSdkVersion and targetSdkVersion attributes), you must include the «screenSize» value in addition to the «orientation» value. That is, you must decalare android:configChanges=»orientation|screenSize» . However, if your application targets API level 12 or lower, then your activity always handles this configuration change itself (this configuration change does not restart your activity, even when running on an Android 3.2 or higher device).

For example, the following onConfigurationChanged() implementation checks the current device orientation:

The Configuration object represents all of the current configurations, not just the ones that have changed. Most of the time, you won’t care exactly how the configuration has changed and can simply re-assign all your resources that provide alternatives to the configuration that you’re handling. For example, because the Resources object is now updated, you can reset any ImageView s with setImageResource() and the appropriate resource for the new configuration is used (as described in Providing Resources).

Notice that the values from the Configuration fields are integers that are matched to specific constants from the Configuration class. For documentation about which constants to use with each field, refer to the appropriate field in the Configuration reference.

Remember: When you declare your activity to handle a configuration change, you are responsible for resetting any elements for which you provide alternatives. If you declare your activity to handle the orientation change and have images that should change between landscape and portrait, you must re-assign each resource to each element during onConfigurationChanged() .

If you don’t need to update your application based on these configuration changes, you can instead not implement onConfigurationChanged() . In which case, all of the resources used before the configuration change are still used and you’ve only avoided the restart of your activity. However, your application should always be able to shutdown and restart with its previous state intact, so you should not consider this technique an escape from retaining your state during normal activity lifecycle. Not only because there are other configuration changes that you cannot prevent from restarting your application, but also because you should handle events such as when the user leaves your application and it gets destroyed before the user returns to it.

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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What Are Configuration Changes?

Configuration changes can sometimes be hard to understand. This blog post aims to shed some light on them and help you understand them better. One of the most common causes of your app experiencing a configuration change is the user rotating their device. So let’s start by looking at what rotating the device does to your app. Then we’ll continue with an overview of how to handle configuration changes, along with a discussion of their advantages and disadvantages.

How Rotation on Android Works

The way Android handles orientation changes is by destroying and recreating your Activity . This means that your Activity will go through all the lifecycle events associated with being closed. Then it will be restarted and go through its lifecycle events again.

Since your Activity will be recreated, this means any data not persisted will be gone after rotation. This is, of course, far from ideal, so let’s look at one of the simplest solutions for this, which is disabling the behavior entirely.

Handling Configuration Changes

Android provides a way for your application to opt out of being restarted when the orientation changes by defining android:configChanges for your Activity in the manifest. This tells the system that you will be handling the specified configuration changes yourself instead of relying on the system to handle them for you. You can very granularly specify what kind of changes you want to handle, with the most common ones being orientation and screenSize . This will let you handle the user rotating their device, and instead of destroying the Activity and going through the associated lifecycle events, the system will just call onConfigurationChanged() with the new configuration and keep your Activity alive.

Advantages of Handling Configuration Changes

One clear advantage of using android:configChanges , and probably the reason it is used most often, is because it saves you from having to write code to persist your Activity state. Since the Activity isn’t recreated, any data associated with it survives an orientation change. Another advantage is that, especially on slower devices, the experience when rotating the device is far smoother when android:configChanges is used. Now, if all this sounds too good to be true, you’re right, because there are disadvantages as well.

Disadvantages of Handling Configuration Changes

The biggest disadvantage of using android:configChanges is that even when manually handling the configuration changes, you still have to implement onSaveInstanceState() so your app doesn’t lose its state when going into the background. This means that specifying android:configChanges doesn’t actually save you any work, since after correctly implementing onSaveInstanceState() , rotation will also automatically work. In addition, now the automatic resource switching that Android provides no longer works. So if your app requires different resources based on screen size or orientation, it is now on you to update your views in the onConfigurationChanged() method in order to match the new configuration. This can unnecessarily bloat your Activity with code for something Android would have handled for you automatically:

Furthermore, some parts of your application might rely on lifecycle events being triggered when the orientation changes, and as such, don’t work correctly when using android:configChanges . This means that in most cases, using android:configChanges won’t net you any worthwhile benefits, all while causing additional work.

Conclusion

While just specifying android:configChanges in the manifest and forgetting about it can be tempting to quickly get orientation changes to work, it is by no means the correct solution to handle screen rotation. In the end, there is no way of getting around correctly implementing onSaveInstanceState() , since even if you are fine with the drawbacks of android:configChanges , you’ll still need onSaveInstanceState() to correctly work when your app is put into the background.

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Handling Configuration Changes

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.

If a user navigates to a different away from the activity, the onPause() and onResume() methods are called. If you need to retain state information in those cases, it’s best to save state through the use of Shared Preferences:

Upon resume, the onResume() gets called:

Note that onSaveInstanceState() is called right before your activity is about to be killed or restarted because of memory pressure or screen orientation. This is different from onPause() which gets called when your activity loses focus (i.e. you transition to another activity). 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 . For other data to persist, you can put the data in the Bundle provided.

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:

Instance state can also be restored in the standard Activity#onCreate method but it is convenient to do it in onRestoreInstanceState 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.

Fragments also have a onSaveInstanceState() method which is called when their state needs to be saved:

Then we can pull data out of this saved state in onCreateView :

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:

This requires us to be careful to include a tag for lookup whenever putting a fragment into the activity within a transaction:

With this simple pattern, we can properly re-use fragments and restore their state across configuration changes.

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:

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.

Often when you rotate the screen, the app will lose the scroll position and other state of any lists on screen. To properly retain this state for ListView , you can store the instance state onPause and restore onViewCreated as shown below:

Check out this blog post and stackoverflow post for more details.

Note that you must load the items back into the adapter first before calling onRestoreInstanceState . In other words, don’t call onRestoreInstanceState on the ListView until after the items are loaded back in from the network or the database.

Often when you rotate the screen, the app will lose the scroll position and other state of any lists on screen. To properly retain this state for RecyclerView , you can store the instance state onPause and restore onViewCreated as shown below:

Check out this blog post and stackoverflow post for more details.

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 within the AndroidManifest.xml :

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

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 :

Now, when one of these configurations change, the activity does not restart but instead receives a call to onConfigurationChanged() :

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.

Android’s new Architecture Components helps manage configuration states. All state data related to the UI can be moved to a ViewModel, which will survive rotation changes because they are data tied to the application instead of Activity instance. In this way, you do not need to worry about persisting this configuration data between Activity lifecycles.

First, make sure to add the ViewModel library:

Your ViewModel should extend from the ViewModel class. Move for instance the adapter to be encapsulated within the ViewModel:

Next, create a ViewModel class:

Next, change your references to the adapter to refer to the ViewModel:

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