AFNetworking下载进度块未调用

Question

我正在使用HTTP GET方法开发AFNetworking 3.0。我能够获得成功的响应，但不会调用下载进度块。以下是我的代码。

AFHTTPSessionManager *op = [AFHTTPSessionManager manager];


    op.responseSerializer = [AFJSONResponseSerializer serializer];

    NSURLSessionDataTask *dataTask = [op dataTaskWithRequest:request
        uploadProgress:nil downloadProgress:^(NSProgress * _Nonnull uploadProgress) {

        [op setDataTaskDidReceiveDataBlock:^(NSURLSession * _Nonnull session, NSURLSessionDataTask * _Nonnull dataTask, NSData * _Nonnull data) {
            if (dataTask.countOfBytesExpectedToReceive == NSURLSessionTransferSizeUnknown) {
                return;
            }
            NSUInteger code = [(NSHTTPURLResponse *)dataTask.response statusCode];

            if (!(code > 199 && code < 400)) {
                return;
            }

            long long bytesReceived = [dataTask countOfBytesReceived];
            long long bytesTotal = [dataTask countOfBytesExpectedToReceive];

            progression(code, bytesReceived, bytesTotal);
        }];

     }
      completionHandler:^(NSURLResponse * _Nonnull response, id  _Nullable responseObject, NSError * _Nullable error) {

        NSHTTPURLResponse *httpResponse = (NSHTTPURLResponse *)response;
        long statuscode = (long)[httpResponse statusCode];

    }];
    [dataTask resume];