iOS GCD 多线程等待处理事件

    //创建线程组
    dispatch_group_t group = dispatch_group_create();
    //开启异步线程 1
    dispatch_group_async( group, dispatch_get_global_queue(0, 0), ^{
        sleep(5);
        NSLog(@"111");
    });
    //开启异步线程 2
    dispatch_group_async( group, dispatch_get_global_queue(0, 0), ^{
        sleep(2);
        NSLog(@"222");
    });
    //等待全部异步线程完毕
    dispatch_group_notify(group, dispatch_get_global_queue(0, 0), ^{
        NSLog(@"333");
    });
    ```