动态添加的控件在UWP C＃中不可见

Question

我正在尝试在C＃中为UWP创建一个下载管理器。我需要添加ProgressBar用于下载进度，TextBlock用于显示下载状态，另一个TextBox用于显示下载文件名。当我单独编译下载管理器的代码时（在一个新的uwp应用程序上），我的代码工作正常。它在运行时正确显示进度。 我的代码是 -

private async void Downloadbtn_Click(object sender, Windows.UI.Xaml.RoutedEventArgs e)
    {
        Uri url = new Uri(urlBox.Text);
        string fileName = ResolveName(urlBox.Text);
        FolderPicker folderPicker = new FolderPicker();
        folderPicker.ViewMode = PickerViewMode.List;
        folderPicker.FileTypeFilter.Add("*.*");
        StorageFolder downloadfolder = await folderPicker.PickSingleFolderAsync();

        if (folderPicker != null)
        {
            StorageFile downloadFile = await downloadfolder.CreateFileAsync(fileName, CreationCollisionOption.GenerateUniqueName);
            BackgroundDownloader downloader = new BackgroundDownloader();
            operation = downloader.CreateDownload(url, downloadFile);

            try
            {

                TextBlock fileNameText = new TextBlock();
                fileNameText.Text = fileName;
                ProgressBar ProgressIndicator = new ProgressBar();
                ProgressIndicator.Name = "progress";
                TextBlock progressText = new TextBlock();
                progressText.Name = "progressText";

                StackPanel panel = new StackPanel();
                panel.Children.Add(fileNameText);
                panel.Children.Add(ProgressIndicator);
                panel.Children.Add(progressText);
                MainStack.Children.Add(panel);

                cts = new CancellationTokenSource();
                Progress<DownloadOperation> progressCallBack = new Progress<DownloadOperation>(Progress);
                await operation.StartAsync().AsTask(cts.Token,progressCallBack);
            }
            catch (Exception)
            {
                await operation.ResultFile.DeleteAsync();
                operation = null;
            }


        }
    }

    private string ResolveName(string fileName)
    {
        if (fileName.Contains("/"))
        {
            int last = fileName.LastIndexOf("/");
            fileName = fileName.Substring(last + 1, fileName.Length - last - 1);
        }
        if (fileName.Contains("%20"))
        {
            fileName = fileName.Replace("%20", " ");
        }

        return fileName;
    }

    private void Progress(DownloadOperation operation)
    {
        ProgressBar c = new ProgressBar();
        c = (ProgressBar)(MainStack.FindName("progress" ));
        TextBlock c1 = new TextBlock();
        c1 = (TextBlock)(MainStack.FindName("progressText"));

        double received = operation.Progress.BytesReceived;
        double toReceive = operation.Progress.TotalBytesToReceive;
        double progress = received * 100 / toReceive;
        ((ProgressBar)c).Value = progress;
        c1.Text = received + "KB received of " + toReceive + "KB";

        if (received == toReceive) 
            c1.Text = "Download Completed";
    }

MainStack是添加新下载及其ui元素的stackpanel。

但当我将上述代码放在不同的应用中时，UI元素（进度条，文本块）不可见 -

private async void GetDownloadData(Uri uri)
    {
        string fileName = ResolveName(uri.tostring());
        FolderPicker folderPicker = new FolderPicker();
        folderPicker.ViewMode = PickerViewMode.List;
        folderPicker.FileTypeFilter.Add("*.*");
        StorageFolder downloadfolder = await folderPicker.PickSingleFolderAsync();

        if (folderPicker != null)
        {
            StorageFile downloadFile = await downloadfolder.CreateFileAsync(fileName, CreationCollisionOption.GenerateUniqueName);
            BackgroundDownloader downloader = new BackgroundDownloader();
            operation = downloader.CreateDownload(url, downloadFile);

            try
            {

                TextBlock fileNameText = new TextBlock();
                fileNameText.Text = fileName;
                ProgressBar ProgressIndicator = new ProgressBar();
                ProgressIndicator.Name = "progress" ;
                TextBlock progressText = new TextBlock();
                progressText.Name = "progressText";

                StackPanel panel = new StackPanel();
                panel.Children.Add(fileNameText);
                panel.Children.Add(ProgressIndicator);
                panel.Children.Add(progressText);
                MainStack.Children.Add(panel);

                cts = new CancellationTokenSource();
                Progress<DownloadOperation> progressCallBack = new Progress<DownloadOperation>(Progress);
                await operation.StartAsync().AsTask(cts.Token,progressCallBack);
            }
            catch (Exception)
            {
                await operation.ResultFile.DeleteAsync();
                operation = null;
            }


        }
    }

在第二个代码中，我调用GetDownloadData（uri）方法来启动下载，而不是在按钮单击事件时调用它。此方法也放在不同的XAML页面（主页除外）。

第二个代码的唯一问题是，在MainStack（stackpanel）中运行时没有添加进度条或文本块。我尝试通过用listview和listbox替换stackpanel来解决这个问题，但问题无法解决。 任何想法，我在这里缺少什么。