让队列并发执行任务1和任务2，当两个任务都执行完毕后回到主线程执行任务3:

    1. // 创建队列组
    2. dispatch_group_t group = dispatch_group_create();
    4. // 创建并发队列
    5. dispatch_queue_t queue = dispatch_queue_create("queue", DISPATCH_QUEUE_CONCURRENT);
    7. // 执行任务1
    8. dispatch_group_async(group, queue, ^{
    9.     for (int i = 0; i < 3; i++) {
    10.         NSLog(@"任务1 - %d - %@", i, [NSThread currentThread]);
    11.     }
    12. });
    14. // 执行任务2
    15. dispatch_group_async(group, queue, ^{
    16.     for (int i = 0; i < 3; i++) {
    17.         NSLog(@"任务2 - %d - %@", i, [NSThread currentThread]);
    18.     }
    19. });
    21. // 等前面的任务执行完毕后，会自动执行这个任务
    22. dispatch_group_notify(group, queue, ^{
    23.     dispatch_async(dispatch_get_main_queue(), ^{
    24.         for (int i = 0; i < 3; i++) {
    25.             NSLog(@"任务3 - %d - %@", i, [NSThread currentThread]);
    26.         }
    27.     });
    28. });

    打印结果：

    1. ～: 任务2 - 0 - <NSThread: 0x600001ea43c0>{number = 6, name = (null)}
    2. ～: 任务1 - 0 - <NSThread: 0x600001ee2d40>{number = 4, name = (null)}
    3. ～: 任务2 - 1 - <NSThread: 0x600001ea43c0>{number = 6, name = (null)}
    4. ～: 任务1 - 1 - <NSThread: 0x600001ee2d40>{number = 4, name = (null)}
    5. ～: 任务2 - 2 - <NSThread: 0x600001ea43c0>{number = 6, name = (null)}
    6. ～: 任务1 - 2 - <NSThread: 0x600001ee2d40>{number = 4, name = (null)}
    7. ～: 任务3 - 0 - <NSThread: 0x600001ef8280>{number = 1, name = main}
    8. ～: 任务3 - 1 - <NSThread: 0x600001ef8280>{number = 1, name = main}
    9. ～: 任务3 - 2 - <NSThread: 0x600001ef8280>{number = 1, name = main}

    dispatch_group_notify方法的作用就是监听group中的任务执行结束。