gistfile1.m

static NSOperationQueue *__updateUIQueue;

- (void)viewDidAppear:(BOOL)animated
{
    [super viewDidAppear:animated];
    if (!__updateUIQueue) {
        __updateUIQueue = [[NSOperationQueue alloc] init];
    }
    [__updateUIQueue addOperationWithBlock:^{
        while (TRUE) {
            dispatch_async(dispatch_get_main_queue(), ^{
                [self updateUI];
            });
            sleep(1);
        }
    }];
}

- (void)viewWillDisappear:(BOOL)animated
{
    [super viewWillDisappear:animated];
    [__updateUIQueue cancelAllOperations];
    __updateUIQueue = nil;
}