How to sort TreeView items using SortDescriptions in Xaml?

I would suggest to use converter to sort sub items. Something like this:

<TreeView Name="treeCategories" Margin="5" ItemsSource="{Binding Source={StaticResource SortedLayers}}">
<TreeView.ItemTemplate>
    <HierarchicalDataTemplate ItemsSource="{Binding Effects, Converter={StaticResource myConverter}, ConverterParameter=EffectName}">
        <TextBlock Text="{Binding Path=LayerName}" />
        <HierarchicalDataTemplate.ItemTemplate>
            <DataTemplate>
                <TextBlock Text="{Binding Path=EffectName}" />
            </DataTemplate>
        </HierarchicalDataTemplate.ItemTemplate>
    </HierarchicalDataTemplate>
</TreeView.ItemTemplate>

and converter:

public class MyConverter : IValueConverter
{
    public object Convert(object value, Type targetType, object parameter, CultureInfo culture)
    {
        System.Collections.IList collection = value as System.Collections.IList;
        ListCollectionView view = new ListCollectionView(collection);
        SortDescription sort = new SortDescription(parameter.ToString(), ListSortDirection.Ascending);
        view.SortDescriptions.Add(sort);

        return view;
    }

    public object ConvertBack(object value, Type targetType, object parameter, CultureInfo culture)
    {
        return null;
    }
}

I find using a multi converter is better:

using System;
using System.Collections;
using System.ComponentModel;
using System.Globalization;
using System.Windows.Data;

namespace Converters
{
    [ValueConversion(typeof(object[]), typeof(ListCollectionView))]
    public class IListToListCollectionViewConverter : IMultiValueConverter
    {
        public object Convert(object[] values, Type targetType, object parameter, CultureInfo culture)
        {
            var Length = values.Length;
            if (Length >= 1 && Length < 3)
            {
                var IList = values[0] as IList;

                var SortName = string.Empty;
                if (Length > 1)
                    SortName = values[1].ToString();

                var SortDirection = ListSortDirection.Ascending;
                if (Length > 2)
                    SortDirection = values[2] is ListSortDirection ? (ListSortDirection)values[2] : (values[2] is string ? (ListSortDirection)Enum.Parse(typeof(ListSortDirection), values[2].ToString()) : SortDirection);

                var Result = new ListCollectionView(IList);
                Result.SortDescriptions.Add(new SortDescription(SortName, SortDirection));
                return Result;
            }
            return null;
        }

        public object[] ConvertBack(object value, Type[] targetTypes, object parameter, System.Globalization.CultureInfo culture)
        {
            throw new NotImplementedException();
        }
    }
}