使用带有并行下载的 BackgroundDownloader 时如何更新 UI？答案

【问题标题】：How to update the UI when using a BackgroundDownloader with parallel downloads?使用带有并行下载的 BackgroundDownloader 时如何更新 UI？
【发布时间】：2016-01-12 22:12:21
【问题描述】：

在 WinRT 中。我有一个后台下载方法，下载进度应该在 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;
                    }
              }

而下载方式是这样的

        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，由于发生了多个下载操作，我无法直接执行更新 UI 的操作。我想知道如何发送一个绑定到 UI 并有助于更新操作的参数 DownloadProgress 方法。

【问题讨论】：

您能否展示或分享一些资源，这些资源有助于实现具有实时进度和取消选项的多文件下载

标签： c# windows-runtime winrt-xaml uwp

【解决方案1】：

您可以为此使用 lambda：

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

那么你的进度更新器就可以使用它了：

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

【讨论】：

谢谢！正是我想要的！