AFNetworking 源码里 dispatch_group 的疑问

Question

这里使用diapatch_group的好处是什么？
没看到group之后的回调通知之类，那加个group是为了什么呢？
源码文件：AFURLSessionManager.h

dispatch_group_async(manager.completionGroup ?: url_session_manager_completion_group(), manager.completionQueue ?: dispatch_get_main_queue(), ^{
            if (self.completionHandler) {
                self.completionHandler(task.response, responseObject, error);
            }

            dispatch_async(dispatch_get_main_queue(), ^{
                [[NSNotificationCenter defaultCenter] postNotificationName:AFNetworkingTaskDidCompleteNotification object:task userInfo:userInfo];
            });
        });

dispatch_group_async(manager.completionGroup ?: url_session_manager_completion_group(), manager.completionQueue ?: dispatch_get_main_queue(), ^{
                if (self.completionHandler) {
                    self.completionHandler(task.response, responseObject, serializationError);
                }

                dispatch_async(dispatch_get_main_queue(), ^{
                    [[NSNotificationCenter defaultCenter] postNotificationName:AFNetworkingTaskDidCompleteNotification object:task userInfo:userInfo];
                });
            });

Answer 1

这里使用dispatch_async的意义在于SessionManager有一个公有的completionGroup属性
@property (nonatomic, strong, nullable) dispatch_group_t completionGroup;
个人感觉内部使用dispatch_group_async用来异步派发执行completionHandler回调是为了留给调用者使用dispatch_group_notify等函数有机会监听completionHandler执行完成后的回调