相关疑难解决方法(0)

我有一个文件下载功能:

        HttpClientHandler aHandler = new HttpClientHandler();
        aHandler.ClientCertificateOptions = ClientCertificateOption.Automatic;
        HttpClient aClient = new HttpClient(aHandler);
        aClient.DefaultRequestHeaders.ExpectContinue = false;
        HttpResponseMessage response = await aClient.GetAsync(url);
        InMemoryRandomAccessStream randomAccessStream = new InMemoryRandomAccessStream();

        // To save downloaded image to local storage
        var imageFile = await ApplicationData.Current.LocalFolder.CreateFileAsync(
        filename, CreationCollisionOption.ReplaceExisting);
        var fs = await imageFile.OpenAsync(FileAccessMode.ReadWrite);
        DataWriter writer = new DataWriter(fs.GetOutputStreamAt(0));

        writer.WriteBytes(await response.Content.ReadAsByteArrayAsync());

        await writer.StoreAsync();
        //current.image.SetSource(randomAccessStream);
        writer.DetachStream();
        await fs.FlushAsync();

我怎样才能实现进度条功能？也许到目前为止我可以写出写入的字节数？或者其他的东西？

PS我无法使用DownloadOperation(后台传输),因为来自服务器的数据请求证书 - 并且DownloadOperations中不存在此功能.