栅栏函数

 NSLog(@"start");
    //异步并发队列
    dispatch_queue_t queue = dispatch_queue_create("current", DISPATCH_QUEUE_CONCURRENT);
    dispatch_async(queue, ^{
        NSLog(@"%@--1",[NSThread currentThread]);
    });
    dispatch_async(queue, ^{
        NSLog(@"%@--2",[NSThread currentThread]);
    });
    //栅栏函数
    dispatch_barrier_async(queue, ^{
        NSLog(@"++++barrier+++++");
    });
    NSLog(@"11111111");
    dispatch_async(queue, ^{
        NSLog(@"%@--3",[NSThread currentThread]);
        
    });
    dispatch_async(queue, ^{
        NSLog(@"%@--4",[NSThread currentThread]);
        
    });

需要用自定义并发队列

dispatch_barrier_sync(queue,void(^block)())会将queue中barrier前面添加的任务block全部执行后,再执行barrier任务的block,再执行barrier后面添加的任务block.

dispatch_barrier_async(queue,void(^block)())会将queue中barrier前面添加的任务block只添加不执行,继续添加barrier的block,再添加barrier后面的block,同时不影响主线程(或者操作添加任务的线程)中代码的执行!