how CFRunLoop Change the order of execution

Question

- (void)viewDidLoad
{
    [super viewDidLoad];
    dispatch_async(dispatch_queue_create("oneQueue", DISPATCH_QUEUE_SERIAL), ^
    {
        [self syncAction];
    });
//    CFRunLoopRun();     //  1
    NSLog(@"mainQueue execut 1");
}

- (void)syncAction
{
    dispatch_sync(dispatch_get_main_queue(), ^
    {
//        CFRunLoopStop(CFRunLoopGetCurrent());
        NSLog(@"mainQueue execut 2");   // 2
    });
}

If I commented CFRunLoopRun(); and CFRunLoopStop(CFRunLoopGetCurrent());

output:

mainQueue execut 1
mainQueue execut 2

but if I not comment CFRunLoopRun(); and CFRunLoopStop(CFRunLoopGetCurrent());

output:

mainQueue execut 2
mainQueue execut 1

I always think,if viewDidLoad not excute complation, the mainQueue will not execute new function block,but why CFRunLoopRun can make mainQueue execute skip current function and execute new block which added later by other asyncQueue. and when CFRunLoopStop(CFRunLoopGetCurren)),viewDidLoad continue execute . what CFRunLoop done?