On state transitions can be responded to state changes in an appropriate way by calling corresponding methods on app's delegate object.

For example:

application Did Become Active method can be used to prepare to run as the foreground app.

application Did Enter Background method can be used to execute some code when app is running in the background and may be suspended at any time.

application Will Enter Foreground method can be used to execute some code when your app is moving out of the background application Will Terminate method is called when your app is being terminated.