- (void)demo1 {
    //创建组
    dispatch_group_t group = dispatch_group_create();
    //创建队列
    dispatch_queue_t queue = dispatch_queue_create("hm", DISPATCH_QUEUE_CONCURRENT);

    //任务1
    dispatch_group_enter(group);
    dispatch_async(queue, ^{
        NSLog(@"任务1");
        dispatch_group_leave(group);
    });
    
    //任务2
    dispatch_group_enter(group);
    dispatch_async(queue, ^{
        NSLog(@"任务2");
        dispatch_group_leave(group);
    });
    
    //任务3
    dispatch_group_enter(group);
    dispatch_async(queue, ^{
        [NSThread sleepForTimeInterval:2.0];
        NSLog(@"任务3");
        dispatch_group_leave(group);
    });

    //等待组中的任务都执行完毕，才会执行
    dispatch_group_notify(group, dispatch_get_main_queue(), ^{
        NSLog(@"over");
    });
    
    //等待组中的任务都执行完毕，才会继续执行后续的代码
    dispatch_group_wait(group, DISPATCH_TIME_FOREVER);
    NSLog(@"hello ");
}

结果 ：
2016-09-13 13:50:23.992 12-调度组[2598:1484594] 任务2
2016-09-13 13:50:23.992 12-调度组[2598:1484584] 任务1
2016-09-13 13:50:25.993 12-调度组[2598:1484590] 任务3
2016-09-13 13:50:25.993 12-调度组[2598:1484334] hello
2016-09-13 13:50:25.999 12-调度组[2598:1484334] over

- (void)demo2 {
    //创建组
    dispatch_group_t group = dispatch_group_create();
    //队列
    dispatch_queue_t queue = dispatch_get_global_queue(0, 0);

    dispatch_group_async(group, queue, ^{
        NSLog(@"正在下载第一个歌曲");
    });
    
    dispatch_group_async(group, queue, ^{
        NSLog(@"正在下载第二个歌曲");
        [NSThread sleepForTimeInterval:2.0];
    });

    dispatch_group_async(group, queue, ^{
        NSLog(@"正在下载第三个歌曲");
    });
    
    //当三个异步任务都执行完成，才执行
    dispatch_group_notify(group, dispatch_get_main_queue(), ^{
        NSLog(@"over %@",[NSThread currentThread]);
    });    
}

2016-09-13 13:56:18.718 12-调度组[2625:1529382] 正在下载第一个歌曲
2016-09-13 13:56:18.718 12-调度组[2625:1529377] 正在下载第二个歌曲
2016-09-13 13:56:18.718 12-调度组[2625:1529384] 正在下载第三个歌曲
2016-09-13 13:56:20.724 12-调度组[2625:1529302] over <NSThread: 0x7f9d63e03f30>{number = 1, name = main}