iOS: How to tell when an app is going to suspend?

You can also add Notification observer

[[NSNotificationCenter defaultCenter] addObserver:self
                                         selector:@selector(receiveSuspendNotification:)
                                             name:UIApplicationWillResignActiveNotification
                                           object:nil];

- (void) receiveSuspendNotification:(NSNotification*)notif
{
}

method will get called and you can perform the required tasks.