dispatch_barrier_async

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