dispatch_queue_t concurrentQueue = dispatch_queue_create("my.concurrent.queue", DISPATCH_QUEUE_CONCURRENT);
dispatch_async(concurrentQueue, ^(){
    NSLog(@"dispatch-1");
});
dispatch_async(concurrentQueue, ^(){
    NSLog(@"dispatch-2");
});
dispatch_barrier_async(concurrentQueue, ^(){
    NSLog(@"dispatch-barrier"); 
});
dispatch_async(concurrentQueue, ^(){
    NSLog(@"dispatch-3");
});
dispatch_async(concurrentQueue, ^(){
    NSLog(@"dispatch-4");
});

dispatch_barrier_async 作用是在并行队列中，等待前面两个操作并行操作完成，这里是并行输出

dispatch-1，dispatch-2

然后执行

dispatch_barrier_async中的操作，(现在就只会执行这一个操作)执行完成后，即输出

dispatch-barrier

最后该并行队列恢复原有执行状态，继续并行执行

dispatch-3,dispatch-4