// 进度发生变化时，更新 TransferModel 的 Progress
private void DownloadProgress(DownloadOperation download)
{
TransferModel transfer = _transfers.First(p => p.DownloadOperation == download);
transfer.Progress = download.Progress.Status.ToString() + ": " + download.Progress.BytesReceived.ToString("#,0") + " / " + download.Progress.TotalBytesToReceive.ToString("#,0");
}