iOS-多线程-barrier

dispatch_barrier_(a)sync
注意：如果传入的是一个全局并发队列或串行队列，dispatch_barrier_sync 就相当于 dispatch_async

dispatch_queue_t queue = dispatch_queue_create("thread", DISPATCH_QUEUE_CONCURRENT);
    dispatch_async(queue, ^{
        NSLog(@"test1");
    });
    dispatch_async(queue, ^{
        NSLog(@"test2");
    });
    dispatch_async(queue, ^{
        NSLog(@"test3");
    });
    
    dispatch_barrier_sync(queue, ^{
        for (int i = 0; i < 500000000; i++) {
            if (5000 == i) {
                NSLog(@"point1");
            } else if (500000 == i) {
                NSLog(@"point2");
            } else if (50000000 == i) {
                NSLog(@"point3");
            }
        }
        NSLog(@"barrier");
    });
    NSLog(@"aaa");
    dispatch_async(queue, ^{
        NSLog(@"test4");
    });
    NSLog(@"bbb");
    dispatch_async(queue, ^{
        NSLog(@"test5");
    });
    dispatch_async(queue, ^{
        NSLog(@"test6");
    });

https://blog.csdn.net/willluckysmile/article/details/61195571
dispatch_barrier负责对该队列中的任务进行前后分割，sync/async负责是否阻塞位于其之后的非该队列的任务。一般用sync比较符合直观逻辑。

dispatch_queue_t queue = dispatch_get_global_queue(0, 0);
    dispatch_sync(queue, ^{
        NSLog(@"%zu", 10);
    });
    NSLog(@"done");
// 输出：10 done