Allow ICollectionView for nested TreeListViewItems (#3358)

This requires supported non-generic IEnumerable

Author: Keboo

MaterialDesignThemes.UITests/WPF/TreeListViews/TreeItem.cs

@@ -1,4 +1,6 @@
-using System.Diagnostics;
+using System.ComponentModel;
+using System.Diagnostics;
+using System.Windows.Data;
 
 namespace MaterialDesignThemes.UITests.WPF.TreeListViews;
 
@@ -12,9 +14,12 @@ public class TreeItem
     //NB: making the assumption changes occur ont he UI thread
     public TestableCollection<TreeItem> Children { get; } = new();
 
+    public ICollectionView ChildrenView { get; }
+
     public TreeItem(string value, TreeItem? parent)
     {
         Value = value;
         Parent = parent;
+        ChildrenView = CollectionViewSource.GetDefaultView(Children);
     }
 }

MaterialDesignThemes.UITests/WPF/TreeListViews/TreeListViewWithCollectionView.xaml

@@ -11,7 +11,7 @@
              mc:Ignorable="d">
   <materialDesign:TreeListView ItemsSource="{Binding Items}">
     <materialDesign:TreeListView.Resources>
-      <HierarchicalDataTemplate DataType="{x:Type local:TreeItem}" ItemsSource="{Binding Children}">
+      <HierarchicalDataTemplate DataType="{x:Type local:TreeItem}" ItemsSource="{Binding ChildrenView}">
         <TextBlock Text="{Binding Value}" />
       </HierarchicalDataTemplate>
     </materialDesign:TreeListView.Resources>

MaterialDesignThemes.Wpf/TreeListViewItem.cs

@@ -1,4 +1,5 @@
-using System.Collections.Specialized;
+using System.Collections;
+using System.Collections.Specialized;
 using MaterialDesignThemes.Wpf.Internal;
 
 namespace MaterialDesignThemes.Wpf;
@@ -16,7 +17,7 @@ public TreeListViewItem()
     private TreeListViewContentPresenter? ContentPresenter { get; set; }
     private TreeListView? TreeListView { get; set; }
 
-    public IEnumerable<object?> GetChildren() => Children ?? Array.Empty<object?>();
+    public IEnumerable<object?> GetChildren() => Children?.OfType<object?>() ?? Array.Empty<object?>();
 
     public bool IsExpanded
     {
@@ -55,14 +56,14 @@ public int Level
         DependencyProperty.Register(nameof(Level), typeof(int), typeof(TreeListViewItem), new PropertyMetadata(0));
 
 
-    internal IEnumerable<object?>? Children
+    internal IEnumerable? Children
     {
-        get => (IEnumerable<object?>)GetValue(ChildrenProperty);
+        get => (IEnumerable?)GetValue(ChildrenProperty);
         set => SetValue(ChildrenProperty, value);
     }
 
     internal static readonly DependencyProperty ChildrenProperty =
-        DependencyProperty.Register("Children", typeof(IEnumerable<object?>), typeof(TreeListViewItem),
+        DependencyProperty.Register(nameof(Children), typeof(IEnumerable), typeof(TreeListViewItem),
             new PropertyMetadata(null, OnChildrenChanged));
 
     private static void OnChildrenChanged(DependencyObject d, DependencyPropertyChangedEventArgs e)
@@ -140,7 +141,7 @@ internal void ClearTreeListViewItem(object item, TreeListView treeListView)
 
     private void UpdateHasChildren()
     {
-        SetCurrentValue(HasItemsProperty, Children?.Any() == true);
+        SetCurrentValue(HasItemsProperty, GetChildren().Any());
     }
 
     protected override void OnMouseDoubleClick(MouseButtonEventArgs e)