Question

我想为BackgroundDownloader实现超时功能。当我达到超时时，我无法取消下载操作。所以我这样使用它：

   public async void downloadFile(string fileUrl, string fileName) {
        var myFolder = await StorageFolder.GetFolderFromPathAsync(Package.Current.InstalledLocation.Path);
        var myFile = await myFolder.CreateFileAsync(fileName, CreationCollisionOption.ReplaceExisting);

        var downloader = new BackgroundDownloader();
        var downloadOperation = downloader.CreateDownload(new Uri(fileUrl), myFile);

        var task = Task.Run(async () => await downloadOperation.StartAsync().AsTask());
        if ( task.Wait(TimeSpan.FromMilliseconds(1000)) ) {
            // file is downloaded in time

        } else {
            // timeout is reached - how to cancel downloadOperation ?????

        }
    }

我在尝试：

downloadOperation.StartAsync().Cancel();

我得到了

WinRT信息：此操作已经开始。呼叫 AttachAsync附加到正在运行的下载/上传。

downloadOperation.AttachAsync().Cancel();

我得到了

抛出异常：＆＃39; System.Runtime.InteropServices.COMException＆＃39;在 Project.exe WinRT信息：此操作未启动。呼叫 StartAsync启动操作。附加信息：方法是在意想不到的时间召唤。

任何想法都会被贬低！

Answer 1

当我达到超时时，我无法取消下载操作。我在尝试：downloadOperation.AttachAsync（）。取消（）;

根据我的测试，downloadOperation.AttachAsync().Cancel();在我的网站上正常运行。以下是我用于测试的代码：

public async void downloadFile(string fileUrl, string fileName)
{
    var myFolder = await StorageFolder.GetFolderFromPathAsync(Package.Current.InstalledLocation.Path);
    var myFile = await myFolder.CreateFileAsync(fileName, CreationCollisionOption.ReplaceExisting);

    var downloader = new BackgroundDownloader();
    var downloadOperation = downloader.CreateDownload(new Uri(fileUrl), myFile);

    var task = Task.Run(async () => await downloadOperation.StartAsync().AsTask());
    if (task.Wait(TimeSpan.FromMilliseconds(1000)))
    {
        // file is downloaded in time
    }
    else {
        // timeout is reached - how to cancel downloadOperation ?????
        downloadOperation.AttachAsync().Cancel();
    }
}

通常我们使用CancellationToken来取消下载操作。此外，使用Task.Wait方法将阻止UI线程，这会导致糟糕的用户体验。因此，在您的方案中使用CancellationTokenSource.CancelAfter方法可能是更好的选择。

以下是我验证过的代码：

public async void downloadFile(string fileUrl, string fileName)
{
    var myFolder = await StorageFolder.GetFolderFromPathAsync(Package.Current.InstalledLocation.Path);
    var myFile = await myFolder.CreateFileAsync(fileName, CreationCollisionOption.ReplaceExisting);

    var downloader = new BackgroundDownloader();
    var downloadOperation = downloader.CreateDownload(new Uri(fileUrl), myFile);

    // Define the cancellation token.
    CancellationTokenSource cts = new CancellationTokenSource();
    CancellationToken token = cts.Token;

    cts.CancelAfter(1000);
    try
    {
        // Pass the token to the task that listens for cancellation.
        await downloadOperation.StartAsync().AsTask(token);
        // file is downloaded in time
    }
    catch (TaskCanceledException)
    {
        // timeout is reached, downloadOperation is cancled
    }
    finally
    {
        // Releases all resources of cts
        cts.Dispose();
    }
}

取消Windows 10 BackgroundDownloader

1 个答案: