Read original post by Joost van Schaik at .NET by Example
Some time ago I discovered that you can trap Window.Current.VisibilityChanged in a Windows 8 app to check if the app was running in the background, and that you can use that moment to pause a game and/or save the state to storage. The fun thing is, this now works in Universal apps too, that is – and thus, also on Windows Phone 8.1.Although we can discuss if the app is really running in the background or that is actually being suspended when it runs on Windows Phone – the mechanism is the same and it can be used to the same end.