AFNetworking 實現(xiàn)了文件的上傳／下載，以及斷點(diǎn)續(xù)傳。
內(nèi)部封裝了 NSURLSession ,替代了之前的 NSURLConnection。

關(guān)于斷點(diǎn)續(xù)傳：把下載文件放到temp里，每間隔一段時間并用文件的形式存儲下載完的數(shù)據(jù)字節(jié)數(shù)，當(dāng)resume的時候，會查詢到之前下載到進(jìn)度，如1000字節(jié)，并放在HTTP 頭部的 range 字段，如：1000-3000，這樣服務(wù)器就會返回1000字節(jié)后的數(shù)據(jù)；斷點(diǎn)上傳道理類似；

AFURLSessionManager 會創(chuàng)建并管理一個NSURLSession 對象，這是一次會話，而這個會話可以創(chuàng)建多個task任務(wù)，每個任務(wù)可以執(zhí)行上傳下載等操作；

依賴的包有：

• Security.framework
• MobileCoreServices.framework
• SystemConfiguration.framework
• UIKit.framework

對于 NSURLSessionDataTask，官方解釋說，這沒有其他任何的附加功能，只是為了區(qū)別和上傳下載的字面意思。但是這個任務(wù)可以用來請求一些html頁面等等；

// 創(chuàng)建session配置：如 認(rèn)證機(jī)制，緩存，cookies存儲位置，超時時間等...
    NSURLSessionConfiguration *configuration = [NSURLSessionConfiguration defaultSessionConfiguration];
    AFURLSessionManager *manager = [[AFURLSessionManager alloc] initWithSessionConfiguration:configuration];
    
    NSURL *URL = [NSURL URLWithString:@"http://123.125.110.24/imtt.dd.qq.com/16891/97EF88D9A32972CDE911B374A46B774D.apk?mkey=58070789ffa35a8e&f=4e1d&c=0&fsname=com.tencent.WeFire_2.7.0_5477.apk&p=.apk"];
    NSURLRequest *request = [NSURLRequest requestWithURL:URL];
    
    //創(chuàng)建downloadtask
    //progress:進(jìn)度信息，會不斷進(jìn)行回調(diào)
    //destination：指定下載完的文件存儲位置
    //completionHandler：執(zhí)行完畢回調(diào)
    _downloadTask = [manager downloadTaskWithRequest:request
                                                                     progress:^(NSProgress *downloadProgress)
    {
        NSLog(@"downloadProgress:%f",downloadProgress.fractionCompleted);
    }
                                                                  destination:^ NSURL *(NSURL *targetPath, NSURLResponse *response)
    {
        
        NSURL *documentsDirectoryURL = [[NSFileManager defaultManager] URLForDirectory:NSDocumentDirectory
                                                                              inDomain:NSUserDomainMask
                                                                     appropriateForURL:nil
                                                                                create:NO
                                                                                 error:nil];
        
        return [documentsDirectoryURL URLByAppendingPathComponent:[response suggestedFilename]];
        
    } completionHandler:^(NSURLResponse *response, NSURL *filePath, NSError *error) {
        NSLog(@"File downloaded to: %@", filePath);
    }];
    
    // 開啟任務(wù)，必須調(diào)用，否則不執(zhí)行
    [_downloadTask resume];
    
    //掛起任務(wù)
// [_downloadTask suspend];

// 創(chuàng)建 downloadTask ，并給 downloadTask 添加代理
- (NSURLSessionDownloadTask *)downloadTaskWithRequest:(NSURLRequest *)request
                                             progress:(void (^)(NSProgress *downloadProgress)) downloadProgressBlock
                                          destination:(NSURL * (^)(NSURL *targetPath, NSURLResponse *response))destination
                                    completionHandler:(void (^)(NSURLResponse *response, NSURL *filePath, NSError *error))completionHandler
{
    __block NSURLSessionDownloadTask *downloadTask = nil;
    
    //創(chuàng)建下載任務(wù)；
    //在修復(fù)bug之前，異步執(zhí)行任務(wù)，修復(fù)bug后直接執(zhí)行 block
    url_session_manager_create_task_safely(^{
        downloadTask = [self.session downloadTaskWithRequest:request];
    });
    
    //為 task 添加代理，此代理為 task 執(zhí)行以下操作：進(jìn)度處理，目標(biāo)文件處理回調(diào)，處理完成回調(diào)；
    [self addDelegateForDownloadTask:downloadTask progress:downloadProgressBlock destination:destination completionHandler:completionHandler];

    return downloadTask;
}

AFURLSessionManagerTaskDelegate:
這個類的作用就是為 downloadTask 服務(wù)，如進(jìn)度回調(diào)／設(shè)置文件最終存儲位置／執(zhí)行任務(wù)完成回調(diào)（completionHandler）／恢復(fù)下載／掛起任務(wù)

- (void)addDelegateForDownloadTask:(NSURLSessionDownloadTask *)downloadTask
                          progress:(void (^)(NSProgress *downloadProgress)) downloadProgressBlock
                       destination:(NSURL * (^)(NSURL *targetPath, NSURLResponse *response))destination
                 completionHandler:(void (^)(NSURLResponse *response, NSURL *filePath, NSError *error))completionHandler
{
    AFURLSessionManagerTaskDelegate *delegate = [[AFURLSessionManagerTaskDelegate alloc] init];
    //設(shè)置代理需要服務(wù)的對象：SessionManager
    delegate.manager = self;
    //代理為 task 執(zhí)行‘完成回調(diào)’blok
    delegate.completionHandler = completionHandler;
    
    if (destination) {
        // 重新封裝 destination ，根據(jù) destination 返回url再把文件轉(zhuǎn)到這個url（這個session參數(shù)沒用到？？？）；
        delegate.downloadTaskDidFinishDownloading = ^NSURL * (NSURLSession * __unused session,
                                                              NSURLSessionDownloadTask *task,
                                                              NSURL *location)
        {
            return destination(location, task.response);
        };
    }
    
    downloadTask.taskDescription = self.taskDescriptionForSessionTasks;
    
    //添加代理 ，delegate 處理進(jìn)度
    [self setDelegate:delegate forTask:downloadTask];
    
    //設(shè)置進(jìn)度回調(diào)
    delegate.downloadProgressBlock = downloadProgressBlock;
}

//存儲 delegate，并設(shè)置 Progress 屬性 以及相關(guān)回調(diào)
- (void)setDelegate:(AFURLSessionManagerTaskDelegate *)delegate
            forTask:(NSURLSessionTask *)task
{
    NSParameterAssert(task);
    NSParameterAssert(delegate);
    
    /*
     sessionManager 可以創(chuàng)建多個任務(wù)，是1對多點(diǎn)關(guān)系
     所以同時添加多個task的時候，下面操作可能會導(dǎo)致線程不安全，需要加鎖；
     */
    [self.lock lock];
    // sessionManager 把taskDelegate 以字典方式存儲 key：delegate唯一標(biāo)識 value：delegate
    self.mutableTaskDelegatesKeyedByTaskIdentifier[@(task.taskIdentifier)] = delegate;
    
    //設(shè)置 progress 相關(guān)屬性和操作
    [delegate setupProgressForTask:task];
    [self addNotificationObserverForTask:task];
    [self.lock unlock];
}

/**
 uploadProgress downloadProgress 負(fù)責(zé)存儲進(jìn)度屬性以及task相關(guān)操作：
 設(shè)置uploadProgress 和 downloadProgress 取消操作 暫停操作 恢復(fù)下載上傳
 監(jiān)聽 uploadProgress ／ downloadProgress
 */
- (void)setupProgressForTask:(NSURLSessionTask *)task {
    __weak __typeof__(task) weakTask = task;

    //獲取下載或者上傳任務(wù)的總字節(jié)數(shù)大?。挥脕碛嬎惝?dāng)前進(jìn)度（ios7之前步兼容，iOS (7.0 and later)）
    self.uploadProgress.totalUnitCount = task.countOfBytesExpectedToSend;
    self.downloadProgress.totalUnitCount = task.countOfBytesExpectedToReceive;
    
    //允許取消 task
    [self.uploadProgress setCancellable:YES];
    
    //取消回調(diào)，需把task轉(zhuǎn)成strong，以保證再task在取消前不被釋放掉
    [self.uploadProgress setCancellationHandler:^{
        __typeof__(weakTask) strongTask = weakTask;
        [strongTask cancel];
    }];
    
    //允許暫停，以下設(shè)置類似取消操作
    [self.uploadProgress setPausable:YES];
    [self.uploadProgress setPausingHandler:^{
        __typeof__(weakTask) strongTask = weakTask;
        [strongTask suspend];
    }];
    
    //恢復(fù)上傳
    if ([self.uploadProgress respondsToSelector:@selector(setResumingHandler:)]) {
        [self.uploadProgress setResumingHandler:^{
            __typeof__(weakTask) strongTask = weakTask;
            [strongTask resume];
        }];
    }

    [self.downloadProgress setCancellable:YES];
    [self.downloadProgress setCancellationHandler:^{
        __typeof__(weakTask) strongTask = weakTask;
        [strongTask cancel];
    }];
    [self.downloadProgress setPausable:YES];
    [self.downloadProgress setPausingHandler:^{
        __typeof__(weakTask) strongTask = weakTask;
        [strongTask suspend];
    }];

    if ([self.downloadProgress respondsToSelector:@selector(setResumingHandler:)]) {
        [self.downloadProgress setResumingHandler:^{
            __typeof__(weakTask) strongTask = weakTask;
            [strongTask resume];
        }];
    }
    
    /** 監(jiān)聽下載 uploadProgress downloadProgress 的 fractionCompleted（百分比） 屬性
        實際上修改 downloadProgress／uploadProgress 的 totalUnitCount 或者 completedUnitCount 屬性
        就會自動計算 fractionCompleted 的值；
        所以在回調(diào)中直接取 fractionCompleted 即可；
     */
    [self.downloadProgress addObserver:self
                            forKeyPath:NSStringFromSelector(@selector(fractionCompleted))
                               options:NSKeyValueObservingOptionNew
                               context:NULL];
    [self.uploadProgress addObserver:self
                          forKeyPath:NSStringFromSelector(@selector(fractionCompleted))
                             options:NSKeyValueObservingOptionNew
                             context:NULL];
}

//添加監(jiān)聽resume 和suspend，以便接收delegate發(fā)送的通知
- (void)addNotificationObserverForTask:(NSURLSessionTask *)task {
    [[NSNotificationCenter defaultCenter] addObserver:self selector:@selector(taskDidResume:) name:AFNSURLSessionTaskDidResumeNotification object:task];
    [[NSNotificationCenter defaultCenter] addObserver:self selector:@selector(taskDidSuspend:) name:AFNSURLSessionTaskDidSuspendNotification object:task];
}

需要實現(xiàn)幾個代理：

• NSURLSessionDelegate,
• NSURLSessionTaskDelegate,
• NSURLSessionDataDelegate,
• NSURLSessionDownloadDelegate

這個方法很關(guān)鍵，用于累加返回的data數(shù)據(jù)：

/**
 更新下載進(jìn)度：
 下載過程會多次調(diào)用這個方法
 每次調(diào)用會接收一段數(shù)據(jù)
 totalBytesWritten:到目前為止接收到的數(shù)據(jù)總大小（累計接收到的數(shù)據(jù)）；
 totalBytesExpectedToWrite：總的下載數(shù)據(jù)大小；
 totalBytesWritten／totalBytesExpectedToWrite：已完成進(jìn)度百分比；
 */
- (void)URLSession:(NSURLSession *)session
      downloadTask:(NSURLSessionDownloadTask *)downloadTask
      didWriteData:(int64_t)bytesWritten
 totalBytesWritten:(int64_t)totalBytesWritten
totalBytesExpectedToWrite:(int64_t)totalBytesExpectedToWrite
{
    
    AFURLSessionManagerTaskDelegate *delegate = [self delegateForTask:downloadTask];
    
    // 傳遞給代理，記錄下載進(jìn)度
    if (delegate) {
        [delegate URLSession:session downloadTask:downloadTask didWriteData:bytesWritten totalBytesWritten:totalBytesWritten totalBytesExpectedToWrite:totalBytesExpectedToWrite];
    }

    if (self.downloadTaskDidWriteData) {
        self.downloadTaskDidWriteData(session, downloadTask, bytesWritten, totalBytesWritten, totalBytesExpectedToWrite);
    }
}

// 下載完成回調(diào)
- (void)URLSession:(NSURLSession *)session
      downloadTask:(NSURLSessionDownloadTask *)downloadTask
didFinishDownloadingToURL:(NSURL *)location
{
    AFURLSessionManagerTaskDelegate *delegate = [self delegateForTask:downloadTask];
    
    // 調(diào)用setDownloadTaskDidFinishDownloadingBlock方法才會初始化downloadTaskDidFinishDownloading
    // 否則會在delegate里移動下載文件到指定位置
    if (self.downloadTaskDidFinishDownloading) {
        // 獲取用戶指定下載路徑
        NSURL *fileURL = self.downloadTaskDidFinishDownloading(session, downloadTask, location);
        if (fileURL) {
            delegate.downloadFileURL = fileURL;
            NSError *error = nil;
            //移動文件到指定位置
            [[NSFileManager defaultManager] moveItemAtURL:location toURL:fileURL error:&error];
            
            //只看到發(fā)送錯誤信息通知，未找到在哪處理錯誤信息
            if (error) {
                [[NSNotificationCenter defaultCenter] postNotificationName:AFURLSessionDownloadTaskDidFailToMoveFileNotification object:downloadTask userInfo:error.userInfo];
            }

            return;
        }
    }
    
    // 通知 delegate 把下載的臨時文件轉(zhuǎn)移到指定地點(diǎn)
    if (delegate) {
        [delegate URLSession:session downloadTask:downloadTask didFinishDownloadingToURL:location];
    }
}

// sessionTaskDelegate
// 完成傳輸任務(wù)
- (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];

        // 移除代理以及監(jiān)聽
        [self removeDelegateForTask:task];
    }

    // 需要手動調(diào)用 setTaskDidCompleteBlock ，才會執(zhí)行 taskDidComplete，如果沒有手動設(shè)置，則在代理里執(zhí)行commectionHandler
    if (self.taskDidComplete) {
        self.taskDidComplete(session, task, error);
    }
}

相應(yīng)的會調(diào)用delegate中的幾個方法，主要用delegate來輔助執(zhí)行：

- (void)URLSession:(__unused NSURLSession *)session
          dataTask:(__unused NSURLSessionDataTask *)dataTask
    didReceiveData:(NSData *)data
{
    self.downloadProgress.completedUnitCount = dataTask.countOfBytesReceived;
    self.downloadProgress.totalUnitCount = dataTask.countOfBytesExpectedToReceive;

    [self.mutableData appendData:data];
}

- (void)URLSession:(NSURLSession *)session task:(NSURLSessionTask *)task
   didSendBodyData:(int64_t)bytesSent
    totalBytesSent:(int64_t)totalBytesSent
totalBytesExpectedToSend:(int64_t)totalBytesExpectedToSend{
    
    self.uploadProgress.completedUnitCount = task.countOfBytesSent;
    self.uploadProgress.totalUnitCount = task.countOfBytesExpectedToSend;
}

#pragma mark - NSURLSessionDownloadDelegate

//用于記錄下載進(jìn)度：
- (void)URLSession:(NSURLSession *)session downloadTask:(NSURLSessionDownloadTask *)downloadTask
      didWriteData:(int64_t)bytesWritten
 totalBytesWritten:(int64_t)totalBytesWritten
totalBytesExpectedToWrite:(int64_t)totalBytesExpectedToWrite{
    
    self.downloadProgress.totalUnitCount = totalBytesExpectedToWrite;
    self.downloadProgress.completedUnitCount = totalBytesWritten;
}

//用于記錄下載進(jìn)度：
- (void)URLSession:(NSURLSession *)session downloadTask:(NSURLSessionDownloadTask *)downloadTask
 didResumeAtOffset:(int64_t)fileOffset
expectedTotalBytes:(int64_t)expectedTotalBytes{
    self.downloadProgress.totalUnitCount = expectedTotalBytes;
    self.downloadProgress.completedUnitCount = fileOffset;
}

//下載完成回調(diào)
- (void)URLSession:(NSURLSession *)session
      downloadTask:(NSURLSessionDownloadTask *)downloadTask
didFinishDownloadingToURL:(NSURL *)location
{
    NSError *fileManagerError = nil;
    self.downloadFileURL = nil;

    // 把下載到臨時文件移動到指定地點(diǎn)
    if (self.downloadTaskDidFinishDownloading) {
        
        // 獲取用戶指定的目標(biāo) url
        self.downloadFileURL = self.downloadTaskDidFinishDownloading(session, downloadTask, location);
        if (self.downloadFileURL) {
            [[NSFileManager defaultManager] moveItemAtURL:location toURL:self.downloadFileURL error:&fileManagerError];

            //只看到發(fā)送錯誤信息通知，未找到在哪處理錯誤信息
            if (fileManagerError) {
                [[NSNotificationCenter defaultCenter] postNotificationName:AFURLSessionDownloadTaskDidFailToMoveFileNotification object:downloadTask userInfo:fileManagerError.userInfo];
            }
        }
    }
}

最后一步，執(zhí)行completionHandler：
completionHandler 交給 delegate 來處理：

/**
 下載完成回調(diào)
 執(zhí)行 complectionHandler 
 并把返回結(jié)果打包發(fā)送通知
 */
- (void)URLSession:(__unused NSURLSession *)session
              task:(NSURLSessionTask *)task
didCompleteWithError:(NSError *)error
{
    
    // 執(zhí)行重要操作之前需要把 weak 類型轉(zhuǎn)成 strong，防止過程中 manager 被釋放
    __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;
    // 把 mutableData（累計下載完的數(shù)據(jù)）拷貝到data，然后釋放
    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;
    }

    /**在didFinishDownloadingToURL 回調(diào)里已經(jīng)記錄了 downloadFileURL 變量
     優(yōu)先記錄 downloadFileURL 其次記錄data，因為存儲url數(shù)據(jù)量小
     */
    if (self.downloadFileURL) {
        userInfo[AFNetworkingTaskDidCompleteAssetPathKey] = self.downloadFileURL;
    } else if (data) {
        userInfo[AFNetworkingTaskDidCompleteResponseDataKey] = data;
    }

    //如果下載出錯，則 執(zhí)行completionHandler block的時候傳遞錯誤信息
    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;
            
            /** 解密 data 數(shù)據(jù) 
                task.response：存儲:url 類型 長度 文件名 加密信息
             */
            responseObject = [manager.responseSerializer responseObjectForResponse:task.response data:data error:&serializationError];

            // 如果下載數(shù)據(jù)成功，即 downloadFileURL 不為空，優(yōu)先使用 url ，如果沒有 url，再使用 responseObject
            if (self.downloadFileURL) {
                responseObject = self.downloadFileURL;
            }
            
            if (responseObject) {
                userInfo[AFNetworkingTaskDidCompleteSerializedResponseKey] = responseObject;
            }

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

            /**
             把任務(wù)添加到 completionQueue 中；
             并發(fā)執(zhí)行
             */
            dispatch_group_async(manager.completionGroup ?: url_session_manager_completion_group(), manager.completionQueue ?: dispatch_get_main_queue(), ^{
                
                // 執(zhí)行 completionHandler ，傳遞解密后的數(shù)據(jù)，或者url ，以及錯誤信息
                if (self.completionHandler) {
                    self.completionHandler(task.response, responseObject, serializationError);
                }
                
                // 把數(shù)據(jù)打包發(fā)送通知，用于用戶自己擴(kuò)展
                // 例如下載操作和 completionHandler 不在一個類中，可以接收這個通知
                dispatch_async(dispatch_get_main_queue(), ^{
                    [[NSNotificationCenter defaultCenter] postNotificationName:AFNetworkingTaskDidCompleteNotification object:task userInfo:userInfo];
                });
            });
        });
    }
}