dispatch_once.m

dispatch_once_t once;

- (BOOL)application:(UIApplication *)application didFinishLaunchingWithOptions:(NSDictionary *)launchOptions
{
    dispatch_once(&once, ^{
        NSLog(@"one");
        dispatch_async(dispatch_get_main_queue(), ^{
            once = 0;
            dispatch_once(&once, ^{
                NSLog(@"two");
            });
        });
    });