When a user runs a mobile app and then selects another app to run, the first app is moved to the background, or backgrounded. The operating system (both iOS and Android) often kill the backgrounded app to release memory or improve performance for the app running in the foreground.
You can use the
RestorationManager (and related)
classes to handle state restoration.
An iOS app requires a bit of extra setup in Xcode,
but the restoration classes otherwise work the same on
both iOS and Android.