feat: add hierarchical demo.

demo/Semi.Avalonia.TreeDataGrid.Demo/Converters/FileIconConverter.cs

@@ -0,0 +1,28 @@
+using System;
+using System.Collections.Generic;
+using System.Globalization;
+using Avalonia;
+using Avalonia.Data.Converters;
+using Avalonia.Media;
+using Avalonia.Metadata;
+
+namespace Semi.Avalonia.TreeDataGrid.Demo.Converters;
+
+public class FileIconConverter: IMultiValueConverter
+{
+    [Content] 
+    public Dictionary<string, PathGeometry> Items { get; set; } = new Dictionary<string, PathGeometry>();
+
+    public object? Convert(IList<object?> values, Type targetType, object? parameter, CultureInfo culture)
+    {
+        if (values[0] is bool isDirectory && values[1] is bool isOpen)
+        {
+            if (!isDirectory)
+            {
+                return Items["file"];
+            }
+            return isOpen ? Items["folderOpen"] : Items["folderClosed"];
+        }
+        return AvaloniaProperty.UnsetValue;
+    }
+}