WebClient.CancelAsync — 文件仍在下载

Question

我正在尝试使用 Asp.NET Core 创建一个 Web API，该 API 公开路由以启动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。

• 下载执行使用WebClient.DownloadFileAsync为了有一个短的响应时间并返回一个downloadId供以后使用。WebClient 的实例作为值存储在一个静态字典中，其对应key的自然是downloadId.
• 应该在WebClient.CancelAsync通过访问与downloadId键对应的字典的值检索到的客户端实例上取消下载。

当下载完成而不被取消时，以下代码完美运行；AsyncCompletedEventHandler（OnDownloadFileCompleted在这种情况下）被正确调用。

问题：调用 时WebClient.CancelAsync，文件会继续下载并且OnDownloadFileCompleted不会立即调用。WebClient 似乎在调用处理程序之前等待下载完成。然而，在这两种情况下，属性都AsyncCompletedEventArgs.Canceled被正确设置（例如，true如果WebClient.CancelAsync确实被调用了。

• 我错过了什么吗？
• 是否有更好/更智能的模式来处理 WebAPI 中的多次下载？

任何帮助将不胜感激！

下载控制器.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);
}

下载服务.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

我能够复制您看到的内容：CancelAsync实际上并没有取消下载。

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

这是DownloadService我修改后使用的类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();
        }
    }

这使用了一个Download看起来像这样的类：

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();
    }
}

您仍然需要做一些工作才能从Downloads列表中删除成功完成的下载，但我会将其留给您。