WebClient.CancelAsync — 文件仍在下载

Question

我正在尝试使用 Asp.NET 核心创建一个 Web API 公开路由以开始和取消大文件的长时间下载。服务器应该能够同时处理多个下载。

下载是使用 WebClient.DownloadFileAsync 执行的，以便获得较短的响应时间并返回 downloadId 供以后使用。 WebClient 的实例作为值存储在静态字典中，其对应的 key 自然是 downloadId.
应在通过访问对应于 downloadId 键的字典值检索到的客户端实例上使用 WebClient.CancelAsync 取消下载。

当下载完成且未被取消时，以下代码可以完美运行； AsyncCompletedEventHandler（OnDownloadFileCompleted 在这种情况下）被正确调用。

问题：当调用 WebClient.CancelAsync 时，文件继续下载并且 OnDownloadFileCompleted 没有立即被调用。 WebClient 在调用处理程序之前似乎要等到下载完成。然而，在这两种情况下，属性 AsyncCompletedEventArgs.Canceled 都已正确设置（例如 true 如果确实调用了 WebClient.CancelAsync。

我是不是漏掉了什么？
是否有 better/smarter 模式来处理 Web 中的多个下载API？

如有任何帮助，我们将不胜感激！

DownloadController.cs

[Route ("api/download")]
public class DownloadController {

    private readonly DownloadService service;

    public DownloadController (DownloadService service) {
        this.service = service;
    }

    [Route ("start")]
    [HttpPost]
    public string Start ([FromForm] string fileUrl) => this.service.StartDownload (fileUrl);

    [Route ("cancel")]
    [HttpPost]
    public void Cancel ([FromForm] string downloadId) => this.service.CancelDownload (downloadId);
}

DownloadService.cs

public class DownloadService {
    public string DOWNLOAD_FOLDER { get => "C:\tmp"; }
    public static Dictionary<string, WebClient> DownloadClients = new Dictionary<string, WebClient> ();

    public string StartDownload (string fileUrl) {
        var downloadId = Guid.NewGuid ().ToString ("N");
        DownloadClients[downloadId] = new WebClient ();
        DownloadClients[downloadId].DownloadFileCompleted += OnDownloadFileCompleted;
        DownloadClients[downloadId].DownloadFileAsync (new Uri (fileUrl), Path.Combine (DOWNLOAD_FOLDER, downloadId), downloadId);
        return downloadId;
    }

    public void CancelDownload (string downloadId) {
        if (DownloadClients.TryGetValue (downloadId, out WebClient client)) {
            client.CancelAsync ();
        }
    }

    private void OnDownloadFileCompleted (object sender, AsyncCompletedEventArgs e) {
        var downloadId = e.UserState.ToString ();
        if (!e.Cancelled) {
            Debug.WriteLine ("Completed");
        } else {
            Debug.WriteLine ("Cancelled"); //will only be reached when the file finishes downloading
        }

        if (DownloadClients.ContainsKey (downloadId)) {
            DownloadClients[downloadId].Dispose ();
            DownloadClients.Remove (downloadId);
        }
    }
}

Answer 1

我能够复制你所看到的：CancelAsync实际上并没有取消下载。

使用 HttpClient，您可以使用 CopyToAsync 获取流并将其保存到文件中，接受 CancellationToken。取消令牌会立即停止下载。

这里是DownloadServiceclass我修改后使用HttpClient.

public class DownloadService {
    public string DOWNLOAD_FOLDER {
        get => "C:\tmp";
    }

    public static readonly ConcurrentDictionary<string, Download> Downloads = new ConcurrentDictionary<string, Download>();

    public async Task<string> StartDownload(string fileUrl) {
        var downloadId = Guid.NewGuid().ToString("N");
        Downloads[downloadId] = new Download(fileUrl);
        await Downloads[downloadId].Start(Path.Combine(DOWNLOAD_FOLDER, downloadId));

        return downloadId;
    }

    public void CancelDownload(string downloadId) {
        if (Downloads.TryRemove(downloadId, out var download)) {
            download.Cancel();
        }
    }

这使用 Download class 看起来像这样：

public class Download {
    private static readonly HttpClient Client = new HttpClient();
    private readonly string _fileUrl;

    private readonly CancellationTokenSource _tokenSource = new CancellationTokenSource();
    private Task _copyTask;
    private Stream _responseStream;
    private Stream _fileStream;

    public Download(string fileUrl) {
        _fileUrl = fileUrl;
    }

    public async Task Start(string saveTo) {
        var response = await Client.GetAsync(_fileUrl, HttpCompletionOption.ResponseHeadersRead);
        _responseStream = await response.Content.ReadAsStreamAsync();
        _fileStream = File.Create(saveTo);
        _copyTask = _responseStream.CopyToAsync(_fileStream, 81920, _tokenSource.Token).ContinueWith(task => {
            if (task.IsCanceled) return;
            _responseStream.Dispose();
            _fileStream.Dispose();
        });
    }

    public void Cancel() {
        _tokenSource.Cancel();
        _responseStream.Dispose();
        _fileStream.Dispose();
    }
}

要从 Downloads 列表中删除成功完成的下载，您还有一些工作要做，但我会把它留给您。

WebClient.CancelAsync — 文件仍在下载

WebClient.CancelAsync — File still downloading

.net

c#

asp.net

webclient

asp.net-core