一个很“傻”的面试题

请写出，下面的打印是？

dispatch_async(dispatch_get_global_queue(0, 0), ^{
        NSLog(@"1");
        [self performSelector:@selector(pintLog) withObject:nil afterDelay:0];
         //[self performSelectorOnMainThread:@selector(pintLog) withObject:nil waitUntilDone:false];//
//        [self performSelectorInBackground:@selector(pintLog) withObject:nil];//
        NSLog(@"3");
    });
- (void)pintLog{
    
     NSLog(@"2");
}

请说为什么