iCloud unique identifier

- (BOOL)application:(UIApplication *)application didFinishLaunchingWithOptions:(NSDictionary *)launchOptions {
    [[NSNotificationCenter defaultCenter] addObserver:self selector:@selector(ubiquitousKeyValueStoreDidChange:) name:NSUbiquitousKeyValueStoreDidChangeExternallyNotification object:[NSUbiquitousKeyValueStore defaultStore]];
    
    [[NSUbiquitousKeyValueStore defaultStore] synchronize];
    
    return YES;
}

#pragma mark - NSNotifications

- (void)ubiquitousKeyValueStoreDidChange:(NSNotification*)notification {
    NSUbiquitousKeyValueStore *ubiquitousKeyValueStore = notification.object;
    
    NSString *UUID;
    if ((UUID = [ubiquitousKeyValueStore stringForKey:@"UUID"])) {
        NSLog(@"Got UUID: %@", UUID);
    } else {
        UUID = [[NSUUID UUID] UUIDString];
        [ubiquitousKeyValueStore setString:UUID forKey:@"UUID"];
        
        NSLog(@"Set UUID: %@", UUID);
    }
    
    [ubiquitousKeyValueStore synchronize];
}