Работа с сокетом и сворачивание приложения

    NSString * address = @"255.255.255.255";
    UInt16 port = 888888;
    NSData * data = [string dataUsingEncoding:NSUTF8StringEncoding];
    [udpSocket sendData:data toHost:address port:port withTimeout:-1 tag:1];

@interface AppDelegate()
 
@property (nonatomic, assign) UIBackgroundTaskIdentifier bgTask;
 
@end
 
 
@implementation AppDelegate
 
@synthesize bgTask;
 
- (void)applicationWillResignActive:(UIApplication *)application
{
    self.bgTask = [application beginBackgroundTaskWithExpirationHandler:^{
        [[UIApplication sharedApplication] endBackgroundTask:self.bgTask];
        self.bgTask = UIBackgroundTaskInvalid;
    }];
    // do something
}
 
- (void)applicationDidBecomeActive:(UIApplication *)application
{
    if (self.bgTask != UIBackgroundTaskInvalid)
    {
        [[UIApplication sharedApplication] endBackgroundTask:self.bgTask];
        self.bgTask = UIBackgroundTaskInvalid;
    }
    // do something
}
 
// ...
 
@end

- (BOOL)application:(UIApplication *)application didFinishLaunchingWithOptions:(NSDictionary *)launchOptions
{
    [[UIApplication sharedApplication] setMinimumBackgroundFetchInterval:UIApplicationBackgroundFetchIntervalMinimum];
    return YES;
}
 
- (void)application:(UIApplication *)application performFetchWithCompletionHandler:(void (^)(UIBackgroundFetchResult))completionHandler
{
    // implementation here
}

if ([UIApplication sharedApplication].applicationState != UIApplicationStateBackground)
        [udpSocket sendData:data toHost:address port:port withTimeout:-1 tag:1];