Question

在WinRT中。我在后台方法下载，下载进度应在UI部分更新。

我的代码是

             public async static Task DownloadSingleFile(string name, SoundClass sc)
             {
                    var dl = new BackgroundDownloader();
                    dl.CostPolicy = BackgroundTransferCostPolicy.Always;
                    file = await localSoundsFolder.CreateFileAsync(name, CreationCollisionOption.ReplaceExisting);

                    var d = dl.CreateDownload(new Uri(uriToDownloadFrom), file);
                    d.Priority = BackgroundTransferPriority.Default;

                    var progressCallback = new Progress<DownloadOperation>(DownloadProgress);
                    try
                    {
                        await d.StartAsync().AsTask(cancellationToken.Token, progressCallback);
                        CancellationTokenSource token = Utility.cancellationList[sc];
                        if (token != null)
                        {
                            token.Cancel();
                            Utility.cancellationList.Remove(sc);
                            Debug.WriteLine("The sc has been removed from the download list");
                        }
                    }
                    catch
                    {
                        return;
                    }
              }

下载方法如下所示

        private static void DownloadProgress(DownloadOperation download)
        {
            Debug.WriteLine("Callback");
            var value = download.Progress.BytesReceived * 100 / download.Progress.TotalBytesToReceive;
            Debug.WriteLine("The bytesReceived is {0} and total bytes is {1}", download.Progress.BytesReceived.ToString(), download.Progress.TotalBytesToReceive.ToString());
            new System.Threading.ManualResetEvent(false).WaitOne(10);
            //Update the UI here
            if (download.Progress.Status == BackgroundTransferStatus.Completed || value >= 100)
            {
               //Perform opertaion
            }
        }

我面临的问题是因为我发生了多次下载操作，所以无法直接执行更新UI的操作。我想知道如何发送绑定到UI的参数DownloadProgress方法并帮助进行更新操作。

Answer 1

你可以使用lambdas：

int downloadId = ...;
var progressCallback = new Progress<DownloadOperation>(x => DownloadProgress(x, downloadId));

然后您的进度更新程序可以使用它：

private static void DownloadProgress(DownloadOperation download, int downloadId)
{
  ... // use downloadId
}

如何在使用带并行下载的BackgroundDownloader时更新UI？

1 个答案: