问题
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)?
回答1:
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