feat: add demo. update overrides with internal implementations.

src/Ursa/Controls/ComboBox/TreeComboBoxItem.cs

@@ -0,0 +1,110 @@
+using Avalonia;
+using Avalonia.Controls;
+using Avalonia.Controls.Metadata;
+using Avalonia.Controls.Primitives;
+using Avalonia.Input;
+using Avalonia.LogicalTree;
+using Avalonia.Media;
+using Avalonia.Media.TextFormatting;
+using Irihi.Avalonia.Shared.Common;
+using Irihi.Avalonia.Shared.Helpers;
+
+namespace Ursa.Controls;
+
+[TemplatePart(PartNames.PART_Header, typeof(Control))]
+public class TreeComboBoxItem: HeaderedItemsControl, ISelectable
+{
+    private Control? _header;
+    private TreeComboBox? _treeComboBox;
+    
+    public static readonly StyledProperty<bool> IsSelectedProperty = TreeViewItem.IsSelectedProperty.AddOwner<TreeComboBoxItem>();
+
+    public bool IsSelected
+    {
+        get => GetValue(IsSelectedProperty);
+        set => SetValue(IsSelectedProperty, value);
+    }
+    
+    public static readonly StyledProperty<bool> IsExpandedProperty = TreeViewItem.IsExpandedProperty.AddOwner<TreeComboBoxItem>();
+    
+    public bool IsExpanded
+    {
+        get => GetValue(IsExpandedProperty);
+        set => SetValue(IsExpandedProperty, value);
+    }
+
+    
+
+    public static readonly DirectProperty<TreeComboBoxItem, int> LevelProperty = AvaloniaProperty.RegisterDirect<TreeComboBoxItem, int>(
+        nameof(Level), o => o.Level, (o, v) => o.Level = v);
+    private int _level;
+    public int Level
+    {
+        get => _level;
+        protected set => SetAndRaise(LevelProperty, ref _level, value);
+    }
+
+    protected override void OnApplyTemplate(TemplateAppliedEventArgs e)
+    {
+        base.OnApplyTemplate(e);
+        DoubleTappedEvent.RemoveHandler(OnDoubleTapped, _header);
+        _header = e.NameScope.Find<Control>(PartNames.PART_Header);
+        DoubleTappedEvent.AddHandler(OnDoubleTapped, _header);
+    }
+
+    protected override void OnAttachedToLogicalTree(LogicalTreeAttachmentEventArgs e)
+    {
+        base.OnAttachedToLogicalTree(e);
+        _treeComboBox = this.FindLogicalAncestorOfType<TreeComboBox>();
+        Level = CalculateDistanceFromLogicalParent<TreeComboBox>(this);
+        if (this.ItemTemplate is null && this._treeComboBox?.ItemTemplate is not null)
+        {
+            SetCurrentValue(ItemTemplateProperty, this._treeComboBox.ItemTemplate);
+        }
+        
+        
+        
+    }
+
+    private void OnDoubleTapped(object sender, TappedEventArgs e)
+    {
+        if (this.ItemCount <= 0) return;
+        this.SetCurrentValue(IsExpandedProperty, !IsExpanded);
+        e.Handled = true;
+    }
+
+    protected override bool NeedsContainerOverride(object? item, int index, out object? recycleKey)
+    {
+        return EnsureParent().NeedsContainerInternal(item, index, out recycleKey);
+    }
+
+    protected override Control CreateContainerForItemOverride(object? item, int index, object? recycleKey)
+    {
+        return EnsureParent().CreateContainerForItemInternal(item, index, recycleKey);
+    }
+
+    protected override void ContainerForItemPreparedOverride(Control container, object? item, int index)
+    {
+        EnsureParent().ContainerForItemPreparedInternal(container, item, index);
+    }
+
+    // TODO replace with helper method from shared library. 
+    private static int CalculateDistanceFromLogicalParent<T>(ILogical? logical, int @default = -1) where T: ILogical
+    {
+        int distance = 0;
+        ILogical? parent = logical;
+        while (parent is not null)
+        {
+            if (parent is T) return distance;
+            parent = parent.LogicalParent;
+            distance++;
+        }
+        return @default;
+    }
+
+    private TreeComboBox EnsureParent()
+    {
+        return this._treeComboBox ??
+               throw new InvalidOperationException("TreeComboBoxItem must be a part of TreeComboBox");
+    }
+}