Adding sample of data virtualization (#3645)

src/MainDemo.Wpf/Domain/TreesViewModel.cs

@@ -1,6 +1,7 @@
 using System.Collections;
 using System.Collections.ObjectModel;
 using System.Windows.Documents;
+using CommunityToolkit.Mvvm.ComponentModel;
 using MaterialDesignDemo.Shared.Domain;
 using MaterialDesignThemes.Wpf;
 
@@ -48,12 +49,24 @@ public class Planet
     public double Velocity { get; set; }
 }
 
-public class TestItem
+public partial class TestItem : ObservableObject
 {
     public TestItem? Parent { get; set; }
     public string Name { get; }
     public ObservableCollection<TestItem> Items { get; }
 
+    // This property is used to determine if the item is expanded or not.
+    // With the TreeListView control, the UI items are virtualized. Without
+    // this property, the IsExpanded state of the TreeListViewItem would be lost
+    // when it is recycled.
+    //
+    // For more information see:
+    // https://github.com/MaterialDesignInXAML/MaterialDesignInXamlToolkit/issues/3640#issuecomment-2274086113
+    //
+    // https://learn.microsoft.com/dotnet/desktop/wpf/advanced/optimizing-performance-controls?view=netframeworkdesktop-4.8&WT.mc_id=DT-MVP-5003472
+    [ObservableProperty]
+    private bool _isExpanded;
+
     public TestItem(string name, IEnumerable<TestItem> items)
     {
         Name = name;

src/MainDemo.Wpf/Trees.xaml

@@ -231,6 +231,19 @@
               </DataTemplate>
             </materialDesign:TreeListView.Resources>
 
+            <!--
+            Because Data Virtualization is enabled on this tree view by default, if you don't bind the IsExpanded property to something in the bound view model,
+            you can loose the expanded state of items when the TreeListViewItem is recycled.
+
+            For more information:
+            https://github.com/MaterialDesignInXAML/MaterialDesignInXamlToolkit/issues/3640#issuecomment-2274086113
+            https://learn.microsoft.com/dotnet/desktop/wpf/advanced/optimizing-performance-controls?view=netframeworkdesktop-4.8&WT.mc_id=DT-MVP-5003472
+            -->
+            <materialDesign:TreeListView.ItemContainerStyle>
+              <Style TargetType="materialDesign:TreeListViewItem" BasedOn="{StaticResource {x:Type materialDesign:TreeListViewItem}}">
+                <Setter Property="IsExpanded" Value="{Binding IsExpanded}" />
+              </Style>
+            </materialDesign:TreeListView.ItemContainerStyle>
           </materialDesign:TreeListView>
           <StackPanel Orientation="Horizontal" VerticalAlignment="Bottom" HorizontalAlignment="Right">
             <Button Command="{Binding AddListTreeItemCommand}"