There are two possibilities for this:
1) When reactivated, the method applicationDidBecomeActive:
in your application delegate will be called. Note that this will also be called when the app is being started for the first time.
2) An UIApplicationDidBecomeActiveNotification
is being posted to the notification center. Objects (views) that need to react to that may register for that notification. I think this would probably the way to go for your case: register for this notification in the view that needs to be reloaded. Registering for notifications is handled in Notification Programming Topics – Registering for Notifications
solved Hide/visible app mode in iphone [closed]