onSaveInstanceState

onSaveInstanceState is a callback in Android that allows an activity or fragment to save transient UI state before it may be destroyed by the system. It is typically invoked when the user rotates the device, switches away from the app, or the system replaces the process to reclaim memory. The method receives a Bundle where you can store small pieces of information that are needed to restore the user interface to its previous state.

In an activity, you override onSaveInstanceState(Bundle outState) and put values into the bundle using methods like putString, putInt, or putParcelable. The saved data is then supplied to the activity if it is recreated, via onCreate(Bundle savedInstanceState) and optionally onRestoreInstanceState(Bundle savedInstanceState). For fragments, a similar pattern applies: you override onSaveInstanceState and place values into the bundle, which can be read back when the fragment is recreated, typically during onCreate or onViewCreated, depending on the lifecycle.

Best practices emphasize saving only lightweight, transient UI state rather than large data or persistent records. Use persistent storage (such as SharedPreferences or a database) for long-term data, and consider architecture components like ViewModel and SavedStateHandle for managing UI-related state across configuration changes. Also, always call the superclass implementation when overriding onSaveInstanceState to preserve default behavior.

Note that onSaveInstanceState is not guaranteed to be called in all shutdown scenarios, such as a process termination without a configuration change. Rely on it for short-term state restoration, not for permanent data storage.