在 BackgroundWorker 中等待下载完成

Question

我有一个带有进度条的对话框。显示对话框时，Backgroundworker 应下载两个文件（使用 WebClient）并自动将它们复制到指定位置。 如何在复制新文件之前等待文件下载。

我试图用await 做一些事情，但我无法将Backgroundworker 更改为异步方法。如何在工作器中等待下载完成？

运行worker的代码：

private void fmUpdateingDatabaseDialog_Shown(object sender, EventArgs e)
{
    device.Connect();
    lbInformation.Text = "uploading database to " + device.FriendlyName;
    device.Disconnect();

    worker.WorkerReportsProgress = true;
    worker.WorkerSupportsCancellation = true;
    worker.DoWork += new DoWorkEventHandler(worker_DoWork);
    worker.ProgressChanged += 
        new ProgressChangedEventHandler(worker_ProgressChanged);
    worker.RunWorkerCompleted += 
        new RunWorkerCompletedEventHandler(worker_RunWorkerCompleted);
    worker.RunWorkerAsync();
}

DoWork 处理程序中的代码（实际代码中路径不为空）：

private void worker_DoWork(object sender, DoWorkEventArgs e)
{
    BackgroundWorker worker = sender as BackgroundWorker;

    //download files temporary
    WebClient client = new WebClient();
    client.DownloadProgressChanged += new DownloadProgressChangedEventHandler(client_DownloadProgressChanged);
    client.DownloadFileTaskAsync(new Uri(""), Path.Combine(tempPath + ""));

    WebClient client2 = new WebClient();
    client2.DownloadProgressChanged += new DownloadProgressChangedEventHandler(client2_DownloadProgressChanged);
    client2.DownloadFileTaskAsync(new Uri(""), Path.Combine(tempPath + ""));

    //upload files to phone
    device.Connect();
        device.TransferContentToDevice(Path.Combine(tempPath+""), folder.Id, folder, true);
        device.TransferContentToDevice(Path.Combine(tempPath+""), folder.Id, folder, true);
    device.Disconnect();
}

Answer 1

您可以使用同步的WebClient 方法（例如，DownloadFile 代替DownloadFileTaskAsync），或者您可以直接使用async/await 直接代替 @987654328 @。在这种情况下，您主要进行 I/O，因此 async 比 BackgroundWorker 更适合。

async 解决方案如下所示：

private async void fmUpdateingDatabaseDialog_Shown(object sender, EventArgs e)
{
  device.Connect();
  lbInformation.Text = "uploading database to " + device.FriendlyName;
  device.Disconnect();

  var progress = new Progress<T>(data =>
  {
    // TODO: move worker_ProgressChanged code into here.
  });
  await DownloadAsync(progress);
  // TODO: move worker_RunWorkerCompleted code here.
}

private async Task DownloadAsync(IProgress<T> progress)
{
  //download files temporary
  WebClient client = new WebClient();
  client.DownloadProgressChanged += new DownloadProgressChangedEventHandler(client_DownloadProgressChanged);
  await client.DownloadFileTaskAsync(new Uri(""), Path.Combine(tempPath + ""));

  WebClient client2 = new WebClient();
  client2.DownloadProgressChanged += new DownloadProgressChangedEventHandler(client2_DownloadProgressChanged);
  await client2.DownloadFileTaskAsync(new Uri(""), Path.Combine(tempPath + ""));

  //upload files to phone
  // TODO: Check for Async versions of these methods that you can await.
  //  If there aren't any, consider using Task.Run.
  device.Connect();
  device.TransferContentToDevice(Path.Combine(tempPath+""), folder.Id, folder, true);
  device.TransferContentToDevice(Path.Combine(tempPath+""), folder.Id, folder, true);
  device.Disconnect();
}

Answer 2

您可以使用类似的东西，它使用"wait and pulse" 机制来延迟代码，直到您的下载操作完成：

var locker = new object(); 

Thread t = new Thread(new ThreadStart(() =>
{
    lock (locker)
    {
        //peform your downloading operation, and wait for it to finish.
        client.DownloadFileTaskAsync(new Uri(""), Path.Combine(tempPath + ""));
        while (/* not yet downloaded */) { }; 
        //inform the parent thread that the download has finished.
        Monitor.Pulse(locker);
    }
}));

t.Start();

lock(locker)
{
    Monitor.Wait(locker);
}

但是，如果您有资源，我建议您重构代码以完全使用异步等待方法（从而避免后台工作人员）。后台工作器是传统的异步方法之一，而推荐的方法是 TAP。

有关如何执行此操作的示例，请参见 Stephen Cleary 的回答。