最近看了下AFNetworking3.0的代碼，發(fā)現(xiàn)大師寫的代碼有很多亮點，其中我覺得最可以借鑒的是模塊的解耦性。

解耦性

應(yīng)用方使用AF一般只會用到AFHTTPSessionManger類，而這個類只不過是封裝了AFURLSessionManger類的方法，所以主要功能的實現(xiàn)都在AFURLSessionManger類。
AFURLSessionManger類里面是如何解耦的呢？其中最主要的是，定義了一個字典

@property (readwrite, nonatomic, strong) NSMutableDictionary *mutableTaskDelegatesKeyedByTaskIdentifier;

該mutableTaskDelegatesKeyedByTaskIdentifier的key值為NSURLSessionTask.taskIdentifier（一個NSURLSessionTask任務(wù)的id標識），value值為AFURLSessionManagerTaskDelegate。

而AFURLSessionManager需要實現(xiàn)的NSURLSessionTaskDelegate, NSURLSessionDataDelegate, NSURLSessionDownloadDelegate代理，主要都是通過mutableTaskDelegatesKeyedByTaskIdentifier字典拋回給AFURLSessionManagerTaskDelegate里去實現(xiàn)，實現(xiàn)完成的值又通過block回調(diào)給AFURLSessionManager里面的block。

具體流程：

1、 AFURLSessionManager新建一個Task時，把Task與AFURLSessionManagerTaskDelegate通過字典mutableTaskDelegatesKeyedByTaskIdentifier聯(lián)系起來

- (NSURLSessionDataTask *)dataTaskWithRequest:(NSURLRequest *)request
                               uploadProgress:(nullable void (^)(NSProgress *uploadProgress)) uploadProgressBlock
                             downloadProgress:(nullable void (^)(NSProgress *downloadProgress)) downloadProgressBlock
                            completionHandler:(nullable void (^)(NSURLResponse *response, id _Nullable responseObject,  NSError * _Nullable error))completionHandler {

    __block NSURLSessionDataTask *dataTask = nil;
    url_session_manager_create_task_safely(^{
        dataTask = [self.session dataTaskWithRequest:request];
    });

    [self addDelegateForDataTask:dataTask uploadProgress:uploadProgressBlock downloadProgress:downloadProgressBlock completionHandler:completionHandler];

    return dataTask;
}

其中關(guān)鍵代碼：

[self addDelegateForDataTask:dataTask uploadProgress:uploadProgressBlock downloadProgress:downloadProgressBlock completionHandler:completionHandler];

- (void)addDelegateForDataTask:(NSURLSessionDataTask *)dataTask
                uploadProgress:(nullable void (^)(NSProgress *uploadProgress)) uploadProgressBlock
              downloadProgress:(nullable void (^)(NSProgress *downloadProgress)) downloadProgressBlock
             completionHandler:(void (^)(NSURLResponse *response, id responseObject, NSError *error))completionHandler
{
    AFURLSessionManagerTaskDelegate *delegate = [[AFURLSessionManagerTaskDelegate alloc] initWithTask:dataTask];
    delegate.manager = self;
    delegate.completionHandler = completionHandler;

    dataTask.taskDescription = self.taskDescriptionForSessionTasks;
    [self setDelegate:delegate forTask:dataTask];

    delegate.uploadProgressBlock = uploadProgressBlock;
    delegate.downloadProgressBlock = downloadProgressBlock;
}

2、在回調(diào)函數(shù)中，把需要實現(xiàn)的委托，拋回給AFURLSessionManagerTaskDelegate去實現(xiàn)，拋回后從字典中移除該Task

- (void)URLSession:(NSURLSession *)session
              task:(NSURLSessionTask *)task
didCompleteWithError:(NSError *)error
{
    AFURLSessionManagerTaskDelegate *delegate = [self delegateForTask:task];

    // delegate may be nil when completing a task in the background
    if (delegate) {
        [delegate URLSession:session task:task didCompleteWithError:error];

        [self removeDelegateForTask:task];
    }

    if (self.taskDidComplete) {
        self.taskDidComplete(session, task, error);
    }
}

其中關(guān)鍵代碼為：

[delegate URLSession:session task:task didCompleteWithError:error];
[self removeDelegateForTask:task];

3、AFURLSessionManagerTaskDelegate去實現(xiàn)該委托，實現(xiàn)完成后把結(jié)果給self.completionHandler 的block值，從而就把該值傳遞給了應(yīng)用層的completionHandler

- (void)URLSession:(__unused NSURLSession *)session
              task:(NSURLSessionTask *)task
didCompleteWithError:(NSError *)error
{
    __strong AFURLSessionManager *manager = self.manager;

    __block id responseObject = nil;

    __block NSMutableDictionary *userInfo = [NSMutableDictionary dictionary];
    userInfo[AFNetworkingTaskDidCompleteResponseSerializerKey] = manager.responseSerializer;

    //Performance Improvement from #2672
    NSData *data = nil;
    if (self.mutableData) {
        data = [self.mutableData copy];
        //We no longer need the reference, so nil it out to gain back some memory.
        self.mutableData = nil;
    }

    if (self.downloadFileURL) {
        userInfo[AFNetworkingTaskDidCompleteAssetPathKey] = self.downloadFileURL;
    } else if (data) {
        userInfo[AFNetworkingTaskDidCompleteResponseDataKey] = data;
    }

    if (error) {
        userInfo[AFNetworkingTaskDidCompleteErrorKey] = error;

        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];
            });
        });
    } else {
        dispatch_async(url_session_manager_processing_queue(), ^{
            NSError *serializationError = nil;
            responseObject = [manager.responseSerializer responseObjectForResponse:task.response data:data error:&serializationError];

            if (self.downloadFileURL) {
                responseObject = self.downloadFileURL;
            }

            if (responseObject) {
                userInfo[AFNetworkingTaskDidCompleteSerializedResponseKey] = responseObject;
            }

            if (serializationError) {
                userInfo[AFNetworkingTaskDidCompleteErrorKey] = serializationError;
            }

            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];
                });
            });
        });
    }
}

其中關(guān)鍵代碼為

if (self.completionHandler) {
     self.completionHandler(task.response, responseObject, error);
}

總結(jié)

其實就是把需要實現(xiàn)的Delegate值交給一個類去實現(xiàn)，好處是避免了隨著業(yè)務(wù)不斷發(fā)展出現(xiàn)回調(diào)函數(shù)里代碼過長，而且直觀性比較強，可以在我們工作中借鑒這種寫法。最后，如果想看關(guān)于AF較為詳細的源碼分析，可以推薦看看這篇文章：
《AFNetworking到底做了什么》