Question

我正在尝试封装WebClient异步下载方法，这样我就可以在等待冗长的文件下载时获得进度条更新。 MSDN文档表明FTPDownloadCompleted和FTPDownloadProgressChanged在不同的线程上，但它们永远不会触发。也许我会以错误的方式解决这个问题，但我需要避免同步事件，回调等等......所以我想我会尝试这条路线。我特别好奇为什么如果他们是不同线程的代表，事件就不会被解雇。

class downloadFTP
{ 
    public delegate void ProgressBarUpdate(Int32 val);
    ProgressBarUpdate progressBarUpdate;

    WebClient webClient;

    Int32 waitTime = 60;

    Double percentComplete = 0;

    Boolean transferComplete = false;
    public Boolean TransferComplete { get { return transferComplete; } }

    Byte[] downloadData = null;

    public Boolean DownLoadFTPFile(String filename, ProgressBarUpdate pbUpdate) 
    {
        Boolean result = false;

        progressBarUpdate = pbUpdate;
        AutoResetEvent waiter = new System.Threading.AutoResetEvent (false);
        webClient = new WebClient();
        webClient.Credentials = new NetworkCredential(user.userId, user.passWd);
        webClient.DownloadDataCompleted += new DownloadDataCompletedEventHandler(FTPDownloadCompleted);
        webClient.DownloadProgressChanged += new DownloadProgressChangedEventHandler(FTPDownloadProgressChanged);
        webClient.DownloadDataAsync(new Uri(ftpURL + "//" + filename),waiter);

        waiter.WaitOne(waitTime*1000);
        return TransferComplete;
    }

    private void FTPDownloadProgressChanged(object sender, DownloadProgressChangedEventArgs e)
    {
        percentComplete = e.ProgressPercentage;
    }

    private void FTPDownloadCompleted(object sender, DownloadDataCompletedEventArgs e)
    {
        downloadData = e.Result;
        transferComplete = true;
    }

Answer 1

WebClient在当前SynchronizationContext上运行回调（请参阅AsyncOperationManager.CreateOperation）。

如果当前的SynchronizationContext是WinForms或WPF事件循环，那么您的DownLoadFTPFile方法会阻止该线程，并且事件不会显示。解决方案：删除AutoResetEvent。

WebClient.DownloadDataAsync可以封装在一个类中吗？

1 个答案: