今天被一個問題糾結了2個小時。一開始失敗的代碼大概是這種：

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){
            // 子任務                               
        }];
    });
}
                    
dispatch_group_notify(group, dispatch_get_global_queue(0, 0), ^{
    // 子任務所有完畢後。才運行                        
});

或者換一種寫法。把dispatch_group_async寫到getSecure的callback裏面，也不行

最後最終找到一個救命帖：

gcd dispatch groups with an additional level of inception

應該寫成這樣：

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){
            // 子任務
            dispatch_group_leave(group);                     
        }];
    });
}
                    
dispatch_group_notify(group, dispatch_get_global_queue(0, 0), ^{
    // 子任務所有完畢後。才運行                        
});

嵌套的異步回調