Implement TextSearch attached property

src/Avalonia.Controls/Primitives/SelectingItemsControl.cs

@@ -531,11 +531,28 @@ namespace Avalonia.Controls.Primitives
 
                 _textSearchTerm += e.Text;
 
-                bool match(ItemContainerInfo info) =>
-                    info.ContainerControl is IContentControl control &&
-                    control.Content?.ToString()?.StartsWith(_textSearchTerm, StringComparison.OrdinalIgnoreCase) == true;
+                bool Match(ItemContainerInfo info)
+                {
+                    foreach (var child in info.ContainerControl.GetVisualDescendants().OfType<Control>())
+                    {
+                        if (!child.IsSet(TextSearch.TextProperty))
+                        {
+                            continue;
+                        }
+
+                        var searchText = child.GetValue(TextSearch.TextProperty);
 
-                var info = ItemContainerGenerator?.Containers.FirstOrDefault(match);
+                        if (searchText?.StartsWith(_textSearchTerm, StringComparison.OrdinalIgnoreCase) == true)
+                        {
+                            return true;
+                        }
+                    }
+
+                    return info.ContainerControl is IContentControl control &&
+                           control.Content?.ToString()?.StartsWith(_textSearchTerm, StringComparison.OrdinalIgnoreCase) == true;
+                }
+                
+                var info = ItemContainerGenerator?.Containers.FirstOrDefault(Match);
 
                 if (info != null)
                 {

src/Avalonia.Controls/Primitives/TextSearch.cs

@@ -0,0 +1,37 @@
+using Avalonia.Interactivity;
+
+namespace Avalonia.Controls.Primitives
+{
+    /// <summary>
+    /// Allows to customize text searching in <see cref="SelectingItemsControl"/>.
+    /// </summary>
+    public static class TextSearch
+    {
+        /// <summary>
+        /// Defines the Text attached property.
+        /// This text will be considered during text search in <see cref="SelectingItemsControl"/> (such as <see cref="ComboBox"/>)
+        /// </summary>
+        public static readonly AttachedProperty<string> TextProperty
+            = AvaloniaProperty.RegisterAttached<Interactive, string>("Text", typeof(TextSearch));
+
+        /// <summary>
+        /// Sets the <see cref="TextProperty"/> for a control.
+        /// </summary>
+        /// <param name="control">The control</param>
+        /// <param name="text">The search text to set</param>
+        public static void SetText(Control control, string text)
+        {
+            control.SetValue(TextProperty, text);
+        }
+
+        /// <summary>
+        /// Gets the <see cref="TextProperty"/> of a control.
+        /// </summary>
+        /// <param name="control">The control</param>
+        /// <returns>The property value</returns>
+        public static string GetText(Control control)
+        {
+            return control.GetValue(TextProperty);
+        }
+    }
+}