added SelectedItems DP and got it to a working state, even with keepi…

…ng state when collapsing items. Tests are missing

src/MainDemo.Wpf/App.xaml

@@ -22,7 +22,7 @@
         If you would prefer to use your own colors, there is an option for that as well:
         PrimaryColor and SecondaryColor also support the constant string "Inherit" to specify the color should use the system theme accent color
         -->
-        <materialDesign:CustomColorTheme BaseTheme="Light" PrimaryColor="Aqua" SecondaryColor="#FF006400" />
+        <!--<materialDesign:CustomColorTheme BaseTheme="Light" PrimaryColor="Aqua" SecondaryColor="#FF006400" />-->
 
         <!-- You can also use the built-in theme dictionaries: -->
         <!--

src/MainDemo.Wpf/Domain/TreesViewModel.cs

@@ -104,6 +104,10 @@ public sealed class TreesViewModel : ViewModelBase
 
     public AnotherCommandImplementation RemoveListTreeItemCommand { get; }
 
+    public AnotherCommandImplementation RemoveSelectedListTreeItemCommand { get; }
+
+    public ObservableCollection<TestItem?> SelectedTreeItems { get; } = [];
+
     public TestItem? SelectedTreeItem
     {
         get => _selectedTreeItem;
@@ -119,7 +123,7 @@ public object? SelectedItem
     public TreesViewModel()
     {
         Random random = new();
-        for(int i = 0; i < 10; i++)
+        for (int i = 0; i < 10; i++)
         {
             TreeItems.Add(CreateTestItem(random, 1));
         }
@@ -128,8 +132,8 @@ static TestItem CreateTestItem(Random random, int depth)
         {
             int numberOfChildren = depth < 5 ? random.Next(0, 6) : 0;
             var children = Enumerable.Range(0, numberOfChildren).Select(_ => CreateTestItem(random, depth + 1));
-            var rv = new TestItem(GenerateString(random.Next(4, 10)), children); 
-            foreach(var child in rv.Items)
+            var rv = new TestItem(GenerateString(random.Next(4, 10)), children);
+            foreach (var child in rv.Items)
             {
                 child.Parent = rv;
             }
@@ -154,7 +158,7 @@ static TestItem CreateTestItem(Random random, int depth)
         {
             if (items is IEnumerable enumerable)
             {
-                foreach(TestItem testItem in enumerable)
+                foreach (TestItem testItem in enumerable)
                 {
                     if (testItem.Parent is { } parent)
                     {
@@ -226,6 +230,14 @@ static TestItem CreateTestItem(Random random, int depth)
                 }
             },
             _ => SelectedItem != null);
+
+        RemoveSelectedListTreeItemCommand = new(item =>
+        {
+            if (item is TestItem treeItem)
+            {
+                SelectedTreeItems.Remove(treeItem);
+            }
+        });
     }
 
     private static string GenerateString(int length)

src/MainDemo.Wpf/Trees.xaml

@@ -209,53 +209,85 @@
                   Grid.Column="2"
                   VerticalContentAlignment="Top"
                   UniqueKey="trees_3">
-        <Grid>
-          <materialDesign:TreeListView MinWidth="220" MaxHeight="450"
-                                     ItemsSource="{Binding TreeItems}"
-                                     SelectedItem="{Binding SelectedTreeItem}">
-            <materialDesign:TreeListView.Resources>
-              <HierarchicalDataTemplate DataType="{x:Type domain:TestItem}"
-                          ItemsSource="{Binding Items, Mode=OneTime}">
-                <TextBlock Margin="3,2" Text="{Binding Name, Mode=OneTime}" />
-              </HierarchicalDataTemplate>
+        <StackPanel Orientation="Horizontal">
+          <Grid>
+            <materialDesign:TreeListView MinWidth="220"
+                                         MaxHeight="450"
+                                         ItemsSource="{Binding TreeItems}"
+                                         SelectedItem="{Binding SelectedTreeItem}"
+                                         SelectedItems="{Binding SelectedTreeItems}">
+              <materialDesign:TreeListView.Resources>
+                <HierarchicalDataTemplate DataType="{x:Type domain:TestItem}"
+                                          ItemsSource="{Binding Items, Mode=OneTime}">
+                  <TextBlock Margin="3,2" Text="{Binding Name, Mode=OneTime}" />
+                </HierarchicalDataTemplate>
+
+                <HierarchicalDataTemplate DataType="{x:Type domain:MovieCategory}"
+                                          ItemsSource="{Binding Movies, Mode=OneTime}">
+                  <TextBlock Margin="3,2" Text="{Binding Name, Mode=OneTime}" />
+                </HierarchicalDataTemplate>
+
+                <DataTemplate DataType="{x:Type domain:Movie}">
+                  <TextBlock Margin="3,2"
+                             Text="{Binding Name, Mode=OneTime}"
+                             ToolTip="{Binding Director, Mode=OneTime}" />
+                </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}"
+                      ToolTip="Add an item"
+                      Content="{materialDesign:PackIcon Kind=Add}"/>
+
+              <Button Command="{Binding RemoveListTreeItemCommand}"
+                      ToolTip="Remove selected item(s)"
+                      Content="{materialDesign:PackIcon Kind=Remove}"/>
 
-              <HierarchicalDataTemplate DataType="{x:Type domain:MovieCategory}"
-                                      ItemsSource="{Binding Movies, Mode=OneTime}">
-                <TextBlock Margin="3,2" Text="{Binding Name, Mode=OneTime}" />
-              </HierarchicalDataTemplate>
+            </StackPanel>
+          </Grid>
 
-              <DataTemplate DataType="{x:Type domain:Movie}">
-                <TextBlock Margin="3,2"
-                           Text="{Binding Name, Mode=OneTime}"
-                           ToolTip="{Binding Director, Mode=OneTime}" />
-              </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}"
-                    ToolTip="Add an item"
-                    Content="{materialDesign:PackIcon Kind=Add}"/>
-
-            <Button Command="{Binding RemoveListTreeItemCommand}"
-                    ToolTip="Remove selected item(s)"
-                    Content="{materialDesign:PackIcon Kind=Remove}"/>
-
-          </StackPanel>
-        </Grid>
+          <GroupBox Margin="4,0,0,0" Header="TreeListView.SelectedItems">
+            <ListBox ItemsSource="{Binding SelectedTreeItems}" HorizontalContentAlignment="Stretch">
+              <ListBox.ItemTemplate>
+                <DataTemplate>
+                  <Grid>
+                    <Grid.ColumnDefinitions>
+                      <ColumnDefinition Width="*" />
+                      <ColumnDefinition Width="auto" />
+                    </Grid.ColumnDefinitions>
+                    <TextBlock Grid.Column="0"
+                               Text="{Binding Name}"
+                               VerticalAlignment="Center" />
+                    <Button Grid.Column="1"
+                            Padding="4"
+                            Width="{Binding Path=ActualHeight, RelativeSource={RelativeSource Mode=Self}}"
+                            Command="{Binding DataContext.RemoveSelectedListTreeItemCommand, RelativeSource={RelativeSource AncestorType=ListBox}}"
+                            CommandParameter="{Binding .}"
+                            Content="{materialDesign:PackIcon Kind=Bin}"
+                            Foreground="Red"
+                            Style="{StaticResource MaterialDesignFlatButton}" />
+                  </Grid>
+                </DataTemplate>
+              </ListBox.ItemTemplate>
+            </ListBox>
+          </GroupBox>
+
+        </StackPanel>
+
       </smtx:XamlDisplay>
 
       <TextBlock Grid.Row="2"

src/MaterialDesignThemes.Wpf/TreeListView.cs

@@ -1,11 +1,11 @@
+using System.Collections;
 using System.Collections.Specialized;
 using System.Windows.Automation.Peers;
 using MaterialDesignThemes.Wpf.Automation.Peers;
 using MaterialDesignThemes.Wpf.Internal;
 
 namespace MaterialDesignThemes.Wpf;
 
-//TODO: Implement bindable property for getting selected items
 //TODO: Implement GridView support for having columns
 public class TreeListView : ListView
 {
@@ -20,6 +20,41 @@ public double LevelIndentSize
 
     internal TreeListViewItemsCollection? InternalItemsSource { get; set; }
 
+    internal bool IsCollapsingItem { get; set; } = false;
+    public new IList SelectedItems
+    {
+        get => (IList)GetValue(SelectedItemsProperty);
+        set => SetValue(SelectedItemsProperty, value);
+    }
+    public static new readonly DependencyProperty SelectedItemsProperty =
+            DependencyProperty.Register(
+                nameof(SelectedItems),
+                typeof(IList),
+                typeof(TreeListView),
+                new FrameworkPropertyMetadata(default, OnSelectedItemsPropertyChanged));
+
+    private static void OnSelectedItemsPropertyChanged(DependencyObject d, DependencyPropertyChangedEventArgs e)
+    {
+        if (d is TreeListView tree)
+        {
+            // Unsubscribe old collection changed
+            if (e.OldValue is INotifyCollectionChanged oldCollection)
+            {
+                oldCollection.CollectionChanged -= tree.SelectedItems_CollectionChanged;
+            }
+
+            // Subscribe new collection changed
+            if (e.NewValue is INotifyCollectionChanged newCollection)
+            {
+                newCollection.CollectionChanged += tree.SelectedItems_CollectionChanged;
+            }
+
+            tree.SyncUISelectionWithSelectedItems();
+        }
+    }
+
+
+
     static TreeListView()
     {
         ItemsSourceProperty.OverrideMetadata(typeof(TreeListView), new FrameworkPropertyMetadata()
@@ -30,6 +65,85 @@ static TreeListView()
 
     public TreeListView()
     {
+        SelectionChanged += TreeListView_SelectionChanged;
+    }
+
+    private void TreeListView_SelectionChanged(object sender, SelectionChangedEventArgs e)
+    {
+        // Keep the DP collection in sync when UI selection changes
+        if (SelectedItems is null)
+        {
+            return;
+        }
+
+        // Remove unselected
+        foreach (var item in e.RemovedItems)
+        {
+            if (!IsCollapsingItem)
+            {
+                SelectedItems.Remove(item);
+            }
+        }
+
+        // Add newly selected
+        foreach (var item in e.AddedItems)
+        {
+            if (!SelectedItems.Contains(item))
+            {
+                SelectedItems.Add(item);
+            }
+        }
+    }
+
+    private void SelectedItems_CollectionChanged(object? sender, NotifyCollectionChangedEventArgs e)
+    {
+        // NB: Everytime we modify the base.SelectedItems we have to unsubscribe from the SelectionChanged event
+        //     The base.SelectedItems only contains the visually selected items,
+        //     while the SelectedItems contains all selected items, even ones which are not "visible"
+        this.SelectionChanged -= TreeListView_SelectionChanged;
+
+        if (e.Action == NotifyCollectionChangedAction.Reset)
+        {
+            base.SelectedItems.Clear();
+            foreach (var item in SelectedItems)
+            {
+                base.SelectedItems.Add(item);
+            }
+        }
+        else
+        {
+            foreach (var item in e.OldItems ?? Array.Empty<object>())
+            {
+                if (!IsCollapsingItem)
+                {
+                    base.SelectedItems.Remove(item);
+                }
+            }
+
+            foreach (var item in e.NewItems ?? Array.Empty<object>())
+            {
+                if (!base.SelectedItems.Contains(item))
+                {
+                    base.SelectedItems.Add(item);
+                }
+            }
+        }
+
+        this.SelectionChanged += TreeListView_SelectionChanged;
+    }
+
+    /// <summary>
+    /// Updates/syncs the List containing all SelectedItems with the visually selected items (base.SelectedItems)
+    /// </summary>
+    private void SyncUISelectionWithSelectedItems()
+    {
+        this.SelectionChanged -= TreeListView_SelectionChanged;
+        base.SelectedItems.Clear();
+        foreach (var item in SelectedItems ?? Array.Empty<object>())
+        {
+            base.SelectedItems.Add(item);
+        }
+        this.SelectionChanged += TreeListView_SelectionChanged;
     }
 
     protected override AutomationPeer OnCreateAutomationPeer()
@@ -96,10 +210,14 @@ internal void ItemExpandedChanged(TreeListViewItem item)
                 {
                     itemsSource.InsertWithLevel(i + index + 1, children[i], parentLevel + 1);
                 }
+                // if a node, which has a selected childs, is expanded (added) we need to visually select the child (i.e.: update base.SelectedItems)
+                SyncUISelectionWithSelectedItems();
             }
             else
             {
+                IsCollapsingItem = true;
                 itemsSource.RemoveChildrenOfOffsetAdjustedItem(index);
+                IsCollapsingItem = false;
             }
         }
     }