我在这里等你（GCD）

BOOL ifYouBack = false;

- (void)waitForYou{
    dispatch_queue_t ourLifeQueue = dispatch_get_main_queue(); 
//    dispatch_queue_t waitForYouQueue =      dispatch_queue_create("I.AM.ALWAYS.HERE", DISPATCH_QUEUE_CONCURRENT);
    dispatch_queue_t waitForYouQueue = dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_HIGH, 0);
    dispatch_async(waitForYouQueue, ^{
        while (![self ifYouback]) {
            dispatch_sync(ourLifeQueue, ^{
                NSLog(@"let's begin a happy life, 115-110-111-119");
            });
        }
    });
    
    while (![self ifYouback]) {
        sleep(1);
        dispatch_sync(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0), ^{
            while (![self ifYouback]) {
                NSLog(@"do some thing for life");
            }
        });
    } 
}

- (BOOL)ifYouback{
    return ifYouBack;
}