How can you respond to state transitions on your app ?

On state transitions can be responded to state changes in an appropriate way by calling corresponding methods on app’s delegate object.
applicationDidBecomeActive method can be used to prepare to run as the foreground app.
applicationDidEnterBackground method can be used to execute some code when app is running in the background and may be suspended at any time.

applicationWillEnterForeground method can be used to execute some code when your app is moving out of the background
applicationWillTerminate method is called when your app is being terminated.