dispatch_barrier_sync dispatch_barrier_async 区别

dispatch_barrier_sync

// 测试 dispatch_barrier_sync
    NSLog(@"--- start log");
    dispatch_queue_t queue = dispatch_queue_create("barrier_sync", DISPATCH_QUEUE_CONCURRENT);
    dispatch_async(queue, ^{
        for (int i = 0; i < 10; i++) {
            NSLog(@"--- async 1 - %ud", i);
        }
    });
    dispatch_async(queue, ^{
        NSLog(@"--- async 2");
    });
    
    // 1. 在 queue 这个队列中
    // 2. 在当前线程中执行（主线程）
    // 3. 栅栏作用，需要等队列中之前的block 都执行完，才执行这个 block，后边加入 queue 的 block 等该 block 执行完，才会执行
    // 4. 该方法需要等 block 执行完才会返回 (阻塞线程)
    // 5. 如果 queue 不是通过 dispatch_queue_create 创建的， 效果等同于 dispatch_sync
    dispatch_barrier_sync(queue, ^{
        NSLog(@"--- barrier_sync");
    });
    NSLog(@"--- middle log");
    dispatch_async(queue, ^{
        for (int i = 0; i < 10; i++) {
            NSLog(@"--- async 3 - %d", i);
        }
    });
    dispatch_async(queue, ^{
        NSLog(@"--- async 4");
    });
    NSLog(@"--- end log");

执行结果：

2019-02-26 13:13:32.397122+0800 testRuntime01[27982:984495] --- start log
2019-02-26 13:13:32.397289+0800 testRuntime01[27982:984572] --- async 2
2019-02-26 13:13:32.397291+0800 testRuntime01[27982:984570] --- async 1 - 0d
2019-02-26 13:13:32.397421+0800 testRuntime01[27982:984570] --- async 1 - 1d
2019-02-26 13:13:32.397501+0800 testRuntime01[27982:984570] --- async 1 - 2d
2019-02-26 13:13:32.397570+0800 testRuntime01[27982:984570] --- async 1 - 3d
2019-02-26 13:13:32.397637+0800 testRuntime01[27982:984570] --- async 1 - 4d
2019-02-26 13:13:32.397704+0800 testRuntime01[27982:984570] --- async 1 - 5d
2019-02-26 13:13:32.397768+0800 testRuntime01[27982:984570] --- async 1 - 6d
2019-02-26 13:13:32.397915+0800 testRuntime01[27982:984570] --- async 1 - 7d
2019-02-26 13:13:32.398092+0800 testRuntime01[27982:984570] --- async 1 - 8d
2019-02-26 13:13:32.398249+0800 testRuntime01[27982:984570] --- async 1 - 9d
2019-02-26 13:13:32.398429+0800 testRuntime01[27982:984495] --- barrier_sync
2019-02-26 13:13:32.398600+0800 testRuntime01[27982:984495] --- middle log
2019-02-26 13:13:32.398737+0800 testRuntime01[27982:984495] --- end log
2019-02-26 13:13:32.398752+0800 testRuntime01[27982:984572] --- async 4
2019-02-26 13:13:32.398748+0800 testRuntime01[27982:984570] --- async 3 - 0
2019-02-26 13:13:32.399185+0800 testRuntime01[27982:984570] --- async 3 - 1
2019-02-26 13:13:32.410667+0800 testRuntime01[27982:984570] --- async 3 - 2
2019-02-26 13:13:32.410761+0800 testRuntime01[27982:984570] --- async 3 - 3
2019-02-26 13:13:32.410840+0800 testRuntime01[27982:984570] --- async 3 - 4
2019-02-26 13:13:32.410936+0800 testRuntime01[27982:984570] --- async 3 - 5
2019-02-26 13:13:32.411013+0800 testRuntime01[27982:984570] --- async 3 - 6
2019-02-26 13:13:32.411081+0800 testRuntime01[27982:984570] --- async 3 - 7
2019-02-26 13:13:32.411156+0800 testRuntime01[27982:984570] --- async 3 - 8
2019-02-26 13:13:32.411348+0800 testRuntime01[27982:984570] --- async 3 - 9

dispatch_barrier_async

// 测试 dispatch_barrier_async
    NSLog(@"--- start log");
    dispatch_queue_t queue = dispatch_queue_create("barrier_async", DISPATCH_QUEUE_CONCURRENT);
    dispatch_async(queue, ^{
        for (int i = 0; i < 5; i++) {
            NSLog(@"--- async 1 - %d", i);
        }
    });
    dispatch_async(queue, ^{
        NSLog(@"--- async 2");
    });
    
    // 1. 在 queue 这个队列中
    // 2. 在子线程中执行
    // 3. 栅栏作用，需要等队列中之前的block 都执行完，才执行这个 block，然后在执行 dispatch_barrier_async 之后添加到 queue 中的 block
    // 4. 不阻塞主线程，dispatch_barrier_async 执行完方法返回，异步
    // 5. 如果 queue 不是通过 dispatch_queue_create 创建的， 效果等同于 dispatch_async
    dispatch_barrier_async(queue, ^{
        NSLog(@"--- barrier_async");
    });
    NSLog(@"--- middle log");
    dispatch_async(queue, ^{
        for (int i = 0; i < 5; i++) {
            NSLog(@"--- async 3 - %d", i);
        }
    });
    dispatch_async(queue, ^{
        NSLog(@"--- async 4");
    });
    NSLog(@"--- end log");

执行结果：

2019-02-26 13:25:46.655572+0800 testRuntime01[28076:989404] --- start log
2019-02-26 13:25:46.655902+0800 testRuntime01[28076:989463] --- async 1 - 0
2019-02-26 13:25:46.655909+0800 testRuntime01[28076:989462] --- async 2
2019-02-26 13:25:46.655976+0800 testRuntime01[28076:989463] --- async 1 - 1
2019-02-26 13:25:46.655878+0800 testRuntime01[28076:989404] --- middle log
2019-02-26 13:25:46.656042+0800 testRuntime01[28076:989463] --- async 1 - 2
2019-02-26 13:25:46.656099+0800 testRuntime01[28076:989463] --- async 1 - 3
2019-02-26 13:25:46.656165+0800 testRuntime01[28076:989463] --- async 1 - 4
2019-02-26 13:25:46.656237+0800 testRuntime01[28076:989463] --- barrier_async
2019-02-26 13:25:46.656573+0800 testRuntime01[28076:989463] --- async 3 - 0
2019-02-26 13:25:46.656595+0800 testRuntime01[28076:989462] --- async 4
2019-02-26 13:25:46.656646+0800 testRuntime01[28076:989463] --- async 3 - 1
2019-02-26 13:25:46.656714+0800 testRuntime01[28076:989404] --- end log
2019-02-26 13:25:46.656972+0800 testRuntime01[28076:989463] --- async 3 - 2
2019-02-26 13:25:46.657169+0800 testRuntime01[28076:989463] --- async 3 - 3
2019-02-26 13:25:46.796646+0800 testRuntime01[28076:989463] --- async 3 - 4

理解

1. barrier 栅栏的作用，以及配合同步、异步的特点，以及所在线程