Wpf – Change TreeViewItem Template when IsSelected and two types using in TreeView

In my TreeView I use two differnt classes for binding. For example, I have a Group what can have ChildGroup and can have Items.
Example code of this classes:

using System.Collections.Generic;
using System.Collections.ObjectModel;

namespace WpfApplication1
{
public class Group
{
    public Group(string name)
    {
        Name = name;
        items = new ObservableCollection<Item>();
        groups = new ObservableCollection<Group>();
    }
    public string Name { get;
        set;
    }

    private ObservableCollection<Item> items;
    private ObservableCollection<Group> groups;

    public ObservableCollection<Item> Items
    {
        get { return items; }
    }


    public ObservableCollection<Group> Groups
    {
        get { return groups; }
    }

    public IEnumerable<object> AllItems
    {
        get
        {
            foreach (var group in groups)
            {
                yield return group;
            }
            foreach (var item in items)
            {
                yield return item;
            }
        }
    }

}

public class Item
{
    public Item(string name)
    {
        ItemName = name;
    }

    public string ItemName
    {
        get;
        set;
    } 
}
}

To bind it to TreeView I use following template

<Grid>
    <TreeView Name="treeView">
        <TreeView.Resources>
            <HierarchicalDataTemplate DataType="{x:Type WpfApplication1:Group}"
                                      ItemsSource="{Binding AllItems}">
                <TextBlock Text="{Binding Name}"/>
            </HierarchicalDataTemplate>
            <DataTemplate DataType="{x:Type WpfApplication1:Item}">
                <TextBlock Text="{Binding ItemName}" FontStyle="Italic"/>
            </DataTemplate>
        </TreeView.Resources>
    </TreeView>
</Grid>

It is easy.

The problem is that I need to change ItemTemplate when Is selected. And I need to change only then Item class selected.

I can do it if only one class use for binding. It also easy using Style and Trigger, like this:

<TreeView Name="treeView1" Grid.Column="1">
        <TreeView.Resources>
            <HierarchicalDataTemplate DataType="{x:Type WpfApplication1:Group}"
                                      ItemsSource="{Binding AllItems}"
                                      x:Key="groupTemplate">
                <TextBlock Text="{Binding Name}"/>
            </HierarchicalDataTemplate>
            <HierarchicalDataTemplate DataType="{x:Type WpfApplication1:Group}"
                                      ItemsSource="{Binding AllItems}"
                                      x:Key="selectedGroupTemplate">
                <TextBlock Text="{Binding Name}" FontStyle="Italic" FontWeight="Bold" FontSize="14"/>
            </HierarchicalDataTemplate>
        </TreeView.Resources>

        <TreeView.ItemContainerStyle>
            <Style TargetType="{x:Type TreeViewItem}">
                <Setter Property="HeaderTemplate" Value="{StaticResource groupTemplate}"/>
                <Style.Triggers>
                    <Trigger Property="IsSelected" Value="True">
                        <Setter Property="HeaderTemplate" Value="{StaticResource selectedGroupTemplate}"/>
                    </Trigger>
                </Style.Triggers>
            </Style>
        </TreeView.ItemContainerStyle>
    </TreeView>

But I have a trouble for multiclass binding.

How can I change SelectedItem template then multiclass binding using? Any ideas?

My code behind sample:

using System.Collections.ObjectModel;
using System.Windows;

namespace WpfApplication1
{
/// <summary>
/// Interaction logic for Window2.xaml
/// </summary>
public partial class Window2 : Window
{
    private ObservableCollection<Group> _groups;
    public ObservableCollection<Group> Groups
    {
        get { return _groups; }
    }

    public Window2()
    {
        InitializeComponent();

        InitGroups();

        treeView.ItemsSource = _groups;
        treeView1.ItemsSource = _groups;
    }

    private void InitGroups()
    {
        _groups = new ObservableCollection<Group>();

        Group group1 = new Group("Group1");
        group1.Groups.Add(new Group("Group1.1"));
        group1.Groups.Add(new Group("Group1.2"));
        group1.Groups.Add(new Group("Group1.3"));

        group1.Items.Add(new Item("Item1.1"));
        group1.Items.Add(new Item("Item1.2"));

        group1.Groups[1].Items.Add(new Item("Item1.2.1"));
        group1.Groups[1].Items.Add(new Item("Item1.2.2"));


        _groups.Add(group1);

        Group group2 = new Group("Group2");
        group2.Groups.Add(new Group("Group2.1"));
        group2.Groups.Add(new Group("Group2.2"));

        group2.Items.Add(new Item("Item2.1"));
        group2.Groups[0].Items.Add(new Item("Item2.1.1"));
        group2.Groups[0].Items.Add(new Item("Item2.1.1"));

        _groups.Add(group2);
    }
}
}

Result

Now I think to use TreeView.HeaderTemplateSelector, but may be exists way to use only xaml.

Thanks.