// 1 指定下载文件地址 URLString 
 // 2 获取保存的文件路径 filePath 
 // 3 创建 NSURLRequest 
 NSURLRequest *request = [NSURLRequest requestWithURL:[NSURL URLWithString:URLString]]; 
 unsigned long long downloadedBytes = 0; 

 if ([[NSFileManager defaultManager] fileExistsAtPath:filePath]) { 
 // 3.1 若之前下载过 , 则在 HTTP 请求头部加入 Range 
  // 获取已下载文件的 size 
  downloadedBytes = [self fileSizeForPath:filePath]; 

  // 验证是否下载过文件
  if (downloadedBytes > 0) { 
    // 若下载过 , 断点续传的时候修改 HTTP 头部部分的 Range 
    NSMutableURLRequest *mutableURLRequest = [request mutableCopy]; 
    NSString *requestRange = 
    [NSString stringWithFormat:@"bytes=%llu-", downloadedBytes]; 
    [mutableURLRequest setValue:requestRange forHTTPHeaderField:@"Range"]; 
    request = mutableURLRequest; 
  } 
 } 

 // 4 创建 AFHTTPRequestOperation 
 AFHTTPRequestOperation *operation 
 = [[AFHTTPRequestOperation alloc] initWithRequest:request]; 

 // 5 设置操作输出流 , 保存在第 2 步的文件中
 operation.outputStream = [NSOutputStream 
 outputStreamToFileAtPath:filePath append:YES]; 

 // 6 设置下载进度处理 block 
 [operation setDownloadProgressBlock:^(NSUInteger bytesRead, 
 long long totalBytesRead, long long totalBytesExpectedToRead) { 
 // bytesRead 当前读取的字节数
 // totalBytesRead 读取的总字节数 , 包含断点续传之前的
 // totalBytesExpectedToRead 文件总大小
 }]; 

 // 7 设置 success 和 failure 处理 block 
 [operation setCompletionBlockWithSuccess:^(AFHTTPRequestOperation 
 *operation, id responseObject) { 

 } failure:^(AFHTTPRequestOperation *operation, NSError *error) { 

 }]; 

 // 8 启动 operation 
 [operation start];

 // SWIFT 
 // 请求失败处理
 func connection(connection: NSURLConnection, 
 didFailWithError error: NSError) { 
  self.failureHandler(error: error) 
 } 

 // 接收到服务器响应是调用
 func connection(connection: NSURLConnection, 
 didReceiveResponse response: NSURLResponse) { 
  if self.totalLength != 0 { 
    return 
  } 

  self.writeHandle = NSFileHandle(forWritingAtPath: 
  FileManager.instance.cacheFilePath(self.fileName!)) 

  self.totalLength = response.expectedContentLength + self.currentLength 
 } 

 // 当服务器返回实体数据是调用
 func connection(connection: NSURLConnection, didReceiveData data: NSData) { 
  let length = data.length 

  // move to the end of file 
  self.writeHandle.seekToEndOfFile() 

  // write data to sanbox 
  self.writeHandle.writeData(data) 

  // calculate data length 
  self.currentLength = self.currentLength + length 

  print("currentLength\(self.currentLength)-totalLength\(self.totalLength)") 

  if (self.downloadProgressHandler != nil) { 
    self.downloadProgressHandler(bytes: length, totalBytes: 
    self.currentLength, totalBytesExpected: self.totalLength) 
  } 
 } 

 // 下载完毕后调用
 func connectionDidFinishLoading(connection: NSURLConnection) { 
  self.currentLength = 0 
  self.totalLength = 0 

  //close write handle 
  self.writeHandle.closeFile() 
  self.writeHandle = nil 

  let cacheFilePath = FileManager.instance.cacheFilePath(self.fileName!) 
  let documenFilePath = FileManager.instance.documentFilePath(self.fileName!) 

  do { 
    try FileManager.instance.moveItemAtPath(cacheFilePath, toPath: documenFilePath) 
  } catch let e as NSError { 
    print("Error occurred when to move file: \(e)") 
  } 

  self.successHandler(responseObject:fileName!) 
 }

 // SWIFT 
 // 接收数据
 func URLSession(session: NSURLSession, dataTask: NSURLSessionDataTask, 
 idReceiveData data: NSData) { 
  //. . . 
 } 
 // 接收服务器响应
 func URLSession(session: NSURLSession, dataTask: NSURLSessionDataTask, 
 didReceiveResponse response: NSURLResponse, completionHandler: 
 (NSURLSessionResponseDisposition) -> Void) { 
  // . . . 
  completionHandler(.Allow) 
 } 

 // 请求完成
 func URLSession(session: NSURLSession, task: NSURLSessionTask, 
 didCompleteWithError error: NSError?) { 
  if error == nil { 
    // . . . 
    self.successHandler(responseObject:self.fileName!) 
  } else { 
    self.failureHandler(error:error!) 
  } 
 }

 //SWIFT 

 //UI 触发 pause 
 func pause(){ 
  self.downloadTask?.cancelByProducingResumeData({data -> Void in 
    if data != nil { 
 data!.writeToFile(FileManager.instance.cacheFilePath(self.fileName!), 
 atomically: false) 
 } 
    }) 
  self.downloadTask = nil 
 } 

 // MARK: - NSURLSessionDownloadDelegate 
 func URLSession(session: NSURLSession, downloadTask: 
 NSURLSessionDownloadTask, didWriteData bytesWritten: Int64, 
 totalBytesWritten: Int64, totalBytesExpectedToWrite: Int64) { 
  if (self.downloadProgressHandler != nil) { 
    self.downloadProgressHandler(bytes: Int(bytesWritten), 
     totalBytes: totalBytesWritten, totalBytesExpected: totalBytesExpectedToWrite) 
  } 
 } 

 func URLSession(session: NSURLSession, task: NSURLSessionTask, 
 didCompleteWithError error: NSError?) { 
  if error != nil {//real error 
    self.failureHandler(error:error!) 
  } 
 } 

 func URLSession(session: NSURLSession, downloadTask: NSURLSessionDownloadTask, 
 didFinishDownloadingToURL location: NSURL) { 
  let cacheFilePath = FileManager.instance.cacheFilePath(self.fileName!) 
  let documenFilePath = FileManager.instance.documentFilePath(self.fileName!) 
  do { 
    if FileManager.instance.fileExistsAtPath(cacheFilePath){ 
      try FileManager.instance.removeItemAtPath(cacheFilePath) 
    } 
    try FileManager.instance.moveItemAtPath(location.path!, toPath: documenFilePath) 
  } catch let e as NSError { 
    print("Error occurred when to move file: \(e)") 
  } 
  self.successHandler(responseObject:documenFilePath) 
 }