GCD之dispatch_barrier_async(栅栏函数)

dispatch_queue_t concurrentQueue = dispatch_queue_create("my.concurrent.queue", DISPATCH_QUEUE_CONCURRENT);
  dispatch_async(concurrentQueue, ^(){
    sleep(3);
    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