It is possible to get a notification or something when the user opens the application, but the application was in the background (so it wasn't closed)?
Your App Delegate will have its function applicationWillEnterForeground called before coming back.
Also, the OS will have queued a series of potential notifications for significant OS changes if it were freeze dried when those events were fired, ie: UIApplicationSignificantTimeChangeNotification and UIDeviceBatteryLevelDidChangeNotification
来源：https://stackoverflow.com/questions/3531014/ios-4-resume-from-background