今天被一个问题纠结了2个小时，一开始失败的代码大概是这样的：

dispatch_group_t group = dispatch_group_create();
                    
while([rs next]){

    dispatch_group_async(group, dispatch_get_global_queue(0, 0), ^{
                            
        [httpHelper getSecure:url completionHandler:^(NSDictionary* dict){
            // 子任务                               
        }];
    });
}
                    
dispatch_group_notify(group, dispatch_get_global_queue(0, 0), ^{
    // 子任务全部完成后，才执行                        
});

或者换一种写法，把dispatch_group_async写到getSecure的callback里面，也不行

最后终于找到一个救命帖：

gcd dispatch groups with an additional level of inception

应该写成这样：

dispatch_group_t group = dispatch_group_create();
                    
while([rs next]){

    dispatch_group_enter(group);

    dispatch_async(dispatch_get_global_queue(0, 0), ^{
                            
        [httpHelper getSecure:url completionHandler:^(NSDictionary* dict){
            // 子任务
            dispatch_group_leave(group);                     
        }];
    });
}
                    
dispatch_group_notify(group, dispatch_get_global_queue(0, 0), ^{
    // 子任务全部完成后，才执行                        
});