system notification

for UIApplication

These notifications are sent out after the equivalent delegate message is called
UIKIT_EXTERN NSString *const UIApplicationDidEnterBackgroundNotification       NS_AVAILABLE_IOS(4_0);

NSNotificationCenter *notificationCenter = [NSNotificationCenter defaultCenter];

[notificationCenter addObserver:self selector:@selector(xx:) name:UIApplicationWillTerminateNotification object:nil];
           
[notificationCenter addObserver:self selector:@selector(xx:) name:UIApplicationDidEnterBackgroundNotification object:nil];