c# - Windows Phone 8.1 中后台任务显示对话框的问题

Question

我有这段代码用于在后台方法中执行 httpwebrequest 和响应，我只想在下载 zip 崩溃时显示对话框以获取信息并且我的代码进入此捕获...<​​/p>

    private void DoSincroFit()
    {
        HttpWebRequest request = HttpWebRequest.CreateHttp(url);
        request.BeginGetResponse(new AsyncCallback(playResponseAsync), request);
    }

    public async void playResponseAsync(IAsyncResult asyncResult)
    {
        //Declaration of variables
        HttpWebRequest webRequest = (HttpWebRequest)asyncResult.AsyncState;

        try
        {
            string fileName = "sincrofit.rar";

            using (HttpWebResponse webResponse = (HttpWebResponse)webRequest.EndGetResponse(asyncResult))
            {
                byte[] buffer = new byte[1024];


                var newZipFile = await ApplicationData.Current.LocalFolder.CreateFileAsync(fileName, CreationCollisionOption.ReplaceExisting);
                var newZipFile = await KnownFolders.DocumentsLibrary.CreateFileAsync(fileName, CreationCollisionOption.ReplaceExisting);

                using (var writeStream = await newZipFile.OpenAsync(FileAccessMode.ReadWrite))
                {
                    using (var outputStream = writeStream.GetOutputStreamAt(0))
                    {
                        using (var dataWriter = new DataWriter(outputStream))
                        {
                            using (Stream input = webResponse.GetResponseStream())
                            {
                                var totalSize = 0;
                                for (int size = input.Read(buffer, 0, buffer.Length); size > 0; size = input.Read(buffer, 0, buffer.Length))
                                {
                                    dataWriter.WriteBytes(buffer);
                                    totalSize += size;    //get the progress of download
                                }
                                await dataWriter.StoreAsync();
                                await outputStream.FlushAsync();
                                dataWriter.DetachStream();
                            }
                        }
                    }
                }

            }
        }
        catch
        {
           SMethods.Message_Dialog("Download has stopped!","Error");
        }
    }

但是当我的代码从这个类执行这个方法时:

class StandarMethods
{
public async void Message_Dialog(string text, string title)
    {
        //Declaration of variables
        MessageDialog MDialog = new MessageDialog(text, title);

        await MDialog.ShowAsync();
    }
}

最后我的应用程序在尝试执行时崩溃了:

await MDialog.ShowAsync();

这个在后台等待任务...有人可以帮助我吗？是时候使用事件处理程序了吗？为什么？如何？提前致谢!

Answer 1

Merli 你的问题是你试图从后台线程访问 UI 线程以向用户显示对话框所以使用 Dispatcher 这个基本示例是:-

// This is for silverlight part
    Deployment.Current.Dispatcher.BeginInvoke(delegate
    {
      var mbr = MessageBox.Show("Are you sure you want to leave this page?", "Warning",      
      MessageBoxButton.OKCancel);

      if(mbr == MessageBoxResult.OK)
      {   OK pressed  } 
      else
      {   Cancel pressed  }

    });

对于 winrt 部分 -

CoreDispatcher dispatcher = CoreWindow.GetForCurrentThread().Dispatcher;
    dispatcher.RunAsync(CoreDispatcherPriority.Normal, async()=>{
          // UI code goes here
          //Declaration of variables
        MessageDialog MDialog = new MessageDialog(text, title);
        await MDialog.ShowAsync();
    });