What happens on a configuration change (such as screen rotation) and how do we preserve data?

Short answer

The system recreates the Activity entirely; we preserve non-transient data through a ViewModel, and essential lightweight state (such as identifiers and input text) through SavedStateHandle or onSaveInstanceState so it survives process death.

Explanation

During a language change or screen rotation, the new resources are applied by rebuilding the Activity (onDestroy and then onCreate are called). The ViewModel retains heavy data and screen states as long as the Activity is not deliberately closed, while onSaveInstanceState or SavedStateHandle is used to save text data and lightweight item identifiers sufficient to rebuild the screen after process death.

Code example

class DetailViewModel(
    private val savedStateHandle: SavedStateHandle
) : ViewModel() {
    // Restores the ID even after the system kills the app
    val itemId: String = checkNotNull(savedStateHandle["item_id"])
}

Common mistakes

• Trying to store large objects or a Bitmap in the onSaveInstanceState bundle, causing a TransactionTooLargeException.
• Using android:configChanges in the manifest to escape handling the screen rebuild properly.

Follow-up questions

• What is the difference between full process death and the user closing the Activity?

Sources

• Android Developers — Handle configuration changes