c# - Parallel Generation of UI

Question

We have a WPF application that has a ListBox with a VirtualizingStackPanel with caching. Not because it has massively many elements (typically less than 20 but perhaps up to 100 or more in extreme cases) but because elements take time to generate. The elements are in fact UIElement objects. So the application dynamically needs to generate UIElements.

The problem is that even though the virtualization appears to work, the application is still slow to become responsive, and this is in a proof of concept solution with minimal "noise".

So we figured that since the main problem is that we generate complex UIElement objects dynamically, we need to do that in parallel, i.e. off-thread. But we get an error that the code needs to be run on a STA thread:

The calling thread must be STA, because many UI components require this.

Does this mean that we cannot generate UI (UIElement objects) on thread other than the WPF main UI thread?

Here's a relevant code fragment from our proof of concept solution:

public class Person : ObservableBase
{
    // ...

    UIElement _UI;
    public UIElement UI
    {
        get
        {
            if (_UI == null)
            {
                ParallelGenerateUI();
            }
            return _UI;
        }
    }

    private void ParallelGenerateUI()
    {
        var scheduler = TaskScheduler.FromCurrentSynchronizationContext();

        Task.Factory.StartNew(() => GenerateUI())
        .ContinueWith(t =>
        {
            _UI = t.Result;
            RaisePropertyChanged("UI");
        }, scheduler);
    }

    private UIElement GenerateUI()
    {
        var tb = new TextBlock();
        tb.Width = 800.0;
        tb.TextWrapping = TextWrapping.Wrap;
        var n = rnd.Next(10, 5000);
        for (int i = 0; i < n; i++)
        {
            tb.Inlines.Add(new Run("A line of text. "));
        }
        return tb;
    }

    // ...
}

and here is a relevant piece of XAML:

<DataTemplate x:Key="PersonDataTemplate" DataType="{x:Type local:Person}">
    <Grid>
        <Border Margin="4" BorderBrush="Black" BorderThickness="1" MinHeight="40" CornerRadius="3" Padding="3">

            <Grid>
                <Grid.RowDefinitions>
                    <RowDefinition />
                    <!--<RowDefinition />-->
                </Grid.RowDefinitions>
                <Grid.ColumnDefinitions>
                    <ColumnDefinition Width="Auto" />
                    <ColumnDefinition Width="Auto" />
                    <ColumnDefinition Width="Auto" />
                    <ColumnDefinition Width="Auto" />
                    <ColumnDefinition />
                </Grid.ColumnDefinitions>
                <TextBlock Text="Name : " Grid.Row="0" FontWeight="Bold" HorizontalAlignment="Right" />
                <TextBlock Grid.Column="1" Grid.Row="0" Text="{Binding Name}" />
                <TextBlock Text=" - Age : " Grid.Column="2" Grid.Row="0" FontWeight="Bold"
                        HorizontalAlignment="Right" />
                <TextBlock Grid.Column="3" Grid.Row="0" Text="{Binding Age}" />
                <ContentControl Grid.Column="4" Grid.Row="0" Content="{Binding Path=UI}" />

            </Grid>
        </Border>
    </Grid>
</DataTemplate>

As you can see we databind to a property UI of type UIElement.

<ListBox x:Name="listbox" ItemsSource="{Binding Persons}" Background="LightBlue"
    ItemTemplate="{StaticResource PersonDataTemplate}"
    ItemContainerStyle="{StaticResource ListBoxItemStyle}" 
    VirtualizingPanel.IsVirtualizing="True"
    VirtualizingPanel.IsVirtualizingWhenGrouping="True" 
    VirtualizingStackPanel.ScrollUnit="Pixel"  
    VirtualizingStackPanel.CacheLength="10,10"
    VirtualizingStackPanel.CacheLengthUnit="Item"
>
    <ListBox.GroupStyle>
        <GroupStyle HeaderTemplate="{StaticResource GroupHeaderTemplate}" />
    </ListBox.GroupStyle>

</ListBox>

In closing context, what our application does is create a code view where the list is of procedures which again contain a mix of structured content (for parameters and local variables on one hand and statements and expressions on the other.)

In other words our UIElement objects are too complex to create via databinding alone.

Another thought we had was to use "Async" settings in the XAML as it appears possible to create "non-blocking UI" but we have not been able to implement this because we get the same error as above:

The calling thread must be STA, because many UI components require this.

Stacktrace:

System.InvalidOperationException was unhandled by user code
  HResult=-2146233079
  Message=The calling thread must be STA, because many UI components require this.
  Source=PresentationCore
  StackTrace:
       at System.Windows.Input.InputManager..ctor()
       at System.Windows.Input.InputManager.GetCurrentInputManagerImpl()
       at System.Windows.Input.KeyboardNavigation..ctor()
       at System.Windows.FrameworkElement.FrameworkServices..ctor()
       at System.Windows.FrameworkElement.EnsureFrameworkServices()
       at System.Windows.FrameworkElement..ctor()
       at System.Windows.Controls.TextBlock..ctor()
       at WPF4._5_VirtualizingStackPanelNewFeatures.Person.GenerateUI() in c:UsersChristianDesktopWPF4.5_VirtualizingStackPanelNewFeaturesWPF4.5_VirtualizingStackPanelNewFeaturesPerson.cs:line 84
       at WPF4._5_VirtualizingStackPanelNewFeatures.Person.<ParallelGenerateUI>b__2() in c:UsersChristianDesktopWPF4.5_VirtualizingStackPanelNewFeaturesWPF4.5_VirtualizingStackPanelNewFeaturesPerson.cs:line 68
       at System.Threading.Tasks.Task`1.InnerInvoke()
       at System.Threading.Tasks.Task.Execute()
  InnerException: 

Edits:

1) Added more XAML. 2) Added stacktrace.

Answer 1

I am suffering the same problem in normal c# environment. I also tried lots of things. Do you calculate the size of controls to adjust the size of the parent in advance? I am doing this unfortunately.

You may also create a control nesting your children dynamically. By that you can create kind of an UIElement Adapter. The adapter is created at the start time and has all information to create the UIElements. The adapter could create requested children on STA thread on demand just in time. When scrolling up or down you may create children in advance in the direction you are scrolling. This way you can start with e.g. 5-10 UI elements and then you calculate by scrolling up more.

I know this is not so nice and it would be better, if there is some technology within the framework providing something like this, but I did not found it yet.

You may look also at those two things. One helped me much in control responsive. The other is still open, since you need .NET Framework 4.5:

1. SuspendLayout and ResumeLayout don't operate very nice. You may try this:

   /// <summary>
   /// An application sends the WM_SETREDRAW message to a window to allow changes in that 
   /// window to be redrawn or to prevent changes in that window from being redrawn.
   /// </summary>
   private const int WM_SETREDRAW = 11; 
   
   /// <summary>
   /// Suspends painting for the target control. Do NOT forget to call EndControlUpdate!!!
   /// </summary>
   /// <param name="control">visual control</param>
   public static void BeginControlUpdate(Control control)
   {
       Message msgSuspendUpdate = Message.Create(control.Handle, WM_SETREDRAW, IntPtr.Zero,
             IntPtr.Zero);
   
       NativeWindow window = NativeWindow.FromHandle(control.Handle);
       window.DefWndProc(ref msgSuspendUpdate);
   }
   
   /// <summary>
   /// Resumes painting for the target control. Intended to be called following a call to BeginControlUpdate()
   /// </summary>
   /// <param name="control">visual control</param>
   public static void EndControlUpdate(Control control)
   {
       // Create a C "true" boolean as an IntPtr
       IntPtr wparam = new IntPtr(1);
       Message msgResumeUpdate = Message.Create(control.Handle, WM_SETREDRAW, wparam,
             IntPtr.Zero);
   
       NativeWindow window = NativeWindow.FromHandle(control.Handle);
       window.DefWndProc(ref msgResumeUpdate);
       control.Invalidate();
       control.Refresh();
   }
   

2. Dispatcher.Yield