iOS NSNotificationCenter to check whether the app came from background to foreground

Have you tried UIApplicationWillEnterForegroundNotification?

The app also posts a UIApplicationWillEnterForegroundNotification notification shortly before calling applicationWillEnterForeground: to give interested objects a chance to respond to the transition.

Subscribe to notification:

[[NSNotificationCenter defaultCenter] addObserver:self
                                         selector:@selector(yourUpdateMethodGoesHere:)
                                             name:UIApplicationWillEnterForegroundNotification
                                           object:nil];

Implement a code, that need to be called:

- (void) yourUpdateMethodGoesHere:(NSNotification *) note {
// code
}

Don’t forget to unsubscribe:

[[NSNotificationCenter defaultCenter] removeObserver:self];