ClassicAssist.Avalonia/ClassicAssist/UI/Misc/BindableSelectedItemBehaviour.cs

using System.Windows;
using System.Windows.Controls;
using System.Windows.Interactivity;

namespace ClassicAssist.UI.Misc
{
    public class BindableSelectedItemBehavior : Behavior<TreeView>
    {
        protected override void OnAttached()
        {
            base.OnAttached();

            AssociatedObject.SelectedItemChanged += OnTreeViewSelectedItemChanged;
        }

        protected override void OnDetaching()
        {
            base.OnDetaching();

            if ( AssociatedObject != null )
            {
                AssociatedObject.SelectedItemChanged -= OnTreeViewSelectedItemChanged;
            }
        }

        private void OnTreeViewSelectedItemChanged( object sender, RoutedPropertyChangedEventArgs<object> e )
        {
            SelectedItem = e.NewValue;
        }

        #region SelectedItem Property

        public object SelectedItem
        {
            get => GetValue( SelectedItemProperty );
            set => SetValue( SelectedItemProperty, value );
        }

        public static readonly DependencyProperty SelectedItemProperty = DependencyProperty.Register( "SelectedItem",
            typeof( object ), typeof( BindableSelectedItemBehavior ),
            new UIPropertyMetadata( null, OnSelectedItemChanged ) );

        private static void OnSelectedItemChanged( DependencyObject sender, DependencyPropertyChangedEventArgs e )
        {
            if ( e.NewValue is TreeViewItem item )
            {
                item.SetValue( TreeViewItem.IsSelectedProperty, true );
            }
        }

        #endregion
    }
}