Why doesn't TreeView automatically select DataTemplates for Childnodes after defining TreeView.ItemTemplate?

Question

I don't understand what wpf does here (using .NET 3.5 with C#):

in my Application Resources I defined several DataTemplates and HierarchicalDataTemplates for different Types of ViewModels. So far this works well, and the TreeView in my Window shows the Nodes like expected. I have to say that all Childelements in every ViewModel came as ICollectionView. Now I decided to use the specialities of ICollectionView, to group, sort and filter the "Rootnodes" of the TreeView. I made the TreeView look like this:

<TreeView ItemsSource="{Binding Path=Elements.Groups}">
  <TreeView.ItemTemplate>
    <HierarchicalDataTemplate ItemsSource="{Binding Path=Items}" ItemTemplate="{StaticResource SOMEITEMTEMPLATENAME}">
      <TextBlock Text="{Binding Path=Name}"/>
    </HierarchicalDataTemplate>
  </TreeView.ItemTemplate>
</TreeView>

The ViewModel looks like this:

class RootViewModel
  : INotifyPropertyChanged
{

  public ICollectionView Elements
  {
     get
     {
       ICollectionView view = CollectionViewSource.GetDefaultView(_elementsFromModel.Select(x => new FirstChildViewModel(x));
       view.GroupDescriptions.Add(new PropertyGroupDescription("Name"));
       view.Filter = delegate(Object x) { /*do some filter stuff here*/ };

       return view;
     }
  }
}

class FirstchildViewModel
  : INotifyPropertyChanged
{
  /*some other stuff here*/

  public ICollectionView Items
  {
    get;
  }
}

The Problem now is that the TreeView doesn't select autmatically the right Templates for the deeper childelements. So what is wrong with that?

Answer 1

The best solution, after some research would be to define the TreeView like this:

<TreeView ItemsSource="{Binding Path=Elements}">
  <TreeView.GroupStyle>
    <GroupStyle>
      <GroupStyle.ContainerStyle>
        <Style TargetType="{x:Type GroupItem}">
          <Setter Property="Template">
            <Setter.Value>
              <ControlTemplate TargetType="{x:Type GroupItem}">
                <TreeViewItem>
                  <TreeViewItem.Header>
                    <TextBlock Text="{Binding Path=Name}"/>
                  </TreeViewItem.Header>

                  <ItemsPresenter Margin="-20,0,0,0"/>
                </TreeViewItem>
              </ControlTemplate>
            </Setter.Value>
          </Setter>
        </Style>
      </GroupStyle.ContainerStyle>
    </GroupStyle>
  </TreeView.GroupStyle>
</TreeView>

now the Group nodes are able to expand and collapse an the different childnodes use different DataTemplates. Thanks to @stukselbax for pointing me to the right direction.