WinUi3 how to get Elements inside dataTemplate

Question

This is my xaml code, a NavigationView contains a NavigationView.HeaderTemplate.

<NavigationView.HeaderTemplate>
        <DataTemplate
            x:Name="HeaderDataTemplate">
            <TextBlock 
                       Name="HeaderText"
                       Text="I am a TextBlock"
                       Margin="0,0,0,10"/>
        </DataTemplate>
    </NavigationView.HeaderTemplate>

I want to know how to get the Texkblock through c# code.

Here is my c# code bu it doesn't work.

private void nvSample_SelectionChanged(NavigationView sender, NavigationViewSelectionChangedEventArgs args)
    {
        var navView = sender as NavigationView;
        NavigationViewItem item = args.SelectedItemContainer as NavigationViewItem;
        if(item.Tag.ToString() == "SatrtPage")
        {
            DataTemplate dataTemplate = sender.HeaderTemplate; 
            DependencyObject dio = dataTemplate.LoadContent();
            var rectangle= FindVisualChild<TextBlock>(dio);
            if (rectangle != null)
            {
                rectangle.Text = "LLLLLLL";

            }
           
           contentFrame.Navigate(typeof(StartPage));
        }
private childItem FindVisualChild<childItem>(DependencyObject obj)
where childItem : DependencyObject
    {
        for (int i = 0; i < VisualTreeHelper.GetChildrenCount(obj); i++)
        {
            DependencyObject child = VisualTreeHelper.GetChild(obj, i);
            if (child != null && child is childItem)
            {
                return (childItem)child;
            }
            else
            {
                childItem childOfChild = FindVisualChild<childItem>(child);
                if (childOfChild != null)
                    return childOfChild;
            }
        }
        return null;
    }

Answer 1

Try this:

private void nvSample_SelectionChanged(NavigationView sender, NavigationViewSelectionChangedEventArgs args)
{
    var navView = sender as NavigationView;
    if (FindVisualChild<SplitView>(navView) is SplitView splitView
        && splitView.Content is Panel panel
        && panel.Children.OfType<ContentControl>().FirstOrDefault() is ContentControl cc)
    {
        var textBlock = FindVisualChild<TextBlock>(cc);
        if (textBlock != null)
            textBlock.Text = "changed text...";
    }
}