取消Windows 10 BackgroundDownloader

Question

我想为BackgroundDownloader实现一个超时功能.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 ?????

        }
    }

我在努力:

downloadOperation.StartAsync().Cancel();

我明白了

WinRT信息:此操作已经开始.调用AttachAsync以附加到正在运行的下载/上载.

downloadOperation.AttachAsync().Cancel();

我明白了

抛出异常:Project.exe WinRT信息中的"System.Runtime.InteropServices.COMException":此操作未启动.调用StartAsync以启动操作.附加信息:在意外时间调用方法.

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().Cancel();

根据我的测试,downloadOperation.AttachAsync().Cancel();在我的网站上工作正常.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.所以在你的场景中使用CancellationTokenSource.CancelAfter方法可能是更好的选择.

以下是我验证的代码:

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();
    }
}