- (void)demo1 {
    //創建組
    dispatch_group_t group = dispatch_group_create();
    //創建隊列
    dispatch_queue_t queue = dispatch_queue_create("hm", DISPATCH_QUEUE_CONCURRENT);

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

    //等待組中的任務都執行完畢，才會執行
    dispatch_group_notify(group, dispatch_get_main_queue(), ^{
        NSLog(@"over");
    });
    
    //等待組中的任務都執行完畢，才會繼續執行后續的代碼
    dispatch_group_wait(group, DISPATCH_TIME_FOREVER);
    NSLog(@"hello ");
}

結果 ：
2016-09-13 13:50:23.992 12-調度組[2598:1484594] 任務2
2016-09-13 13:50:23.992 12-調度組[2598:1484584] 任務1
2016-09-13 13:50:25.993 12-調度組[2598:1484590] 任務3
2016-09-13 13:50:25.993 12-調度組[2598:1484334] hello
2016-09-13 13:50:25.999 12-調度組[2598:1484334] over

- (void)demo2 {
    //創建組
    dispatch_group_t group = dispatch_group_create();
    //隊列
    dispatch_queue_t queue = dispatch_get_global_queue(0, 0);

    dispatch_group_async(group, queue, ^{
        NSLog(@"正在下載第一個歌曲");
    });
    
    dispatch_group_async(group, queue, ^{
        NSLog(@"正在下載第二個歌曲");
        [NSThread sleepForTimeInterval:2.0];
    });

    dispatch_group_async(group, queue, ^{
        NSLog(@"正在下載第三個歌曲");
    });
    
    //當三個異步任務都執行完成，才執行
    dispatch_group_notify(group, dispatch_get_main_queue(), ^{
        NSLog(@"over %@",[NSThread currentThread]);
    });    
}

2016-09-13 13:56:18.718 12-調度組[2625:1529382] 正在下載第一個歌曲
2016-09-13 13:56:18.718 12-調度組[2625:1529377] 正在下載第二個歌曲
2016-09-13 13:56:18.718 12-調度組[2625:1529384] 正在下載第三個歌曲
2016-09-13 13:56:20.724 12-調度組[2625:1529302] over <NSThread: 0x7f9d63e03f30>{number = 1, name = main}