- (void)URLSession:(NSURLSession *)session
      downloadTask:(NSURLSessionDownloadTask *)downloadTask
      didWriteData:(int64_t)bytesWritten
 totalBytesWritten:(int64_t)totalBytesWritten
totalBytesExpectedToWrite:(int64_t)totalBytesExpectedToWrite
{
  float prog = (float)totalBytesWritten / (float)totalBytesExpectedToWrite;
  self.progressBar.progress = prog;
  NSLog(@"%f", [self.progressBar progress]);
}