Displaying hierarchial structure - missing/invalid datacontext

Question

I'm trying to display this hierarchial structure:

public interface IProgressIndicator
{
    CancellationToken CancellationToken { get; }
    string Name { get; set; }
}

public interface IPercentageProgressIndicator : IProgressIndicator
{
    int ProgressPercentage { get; set; }
}

public interface ICountProgressIndicator : IProgressIndicator
{
    int ProgressPercentage { get; }
    int CurValue { get; set; }
    int MaxValue { get; set; }
}

public interface ICompositeProgressIndicator : ICountProgressIndicator
{
    ObservableCollection<IProgressIndicator> SubProgressItems { get; }

    void MarkAsComplete(IProgressIndicator progress);

    IPercentageProgressIndicator CreatePercentageIndicator();
    ICountProgressIndicator CreateCountIndicator();
    ICompositeProgressIndicator CreateCompositeIndicator();
}

The view shouldn't assume the hierarchial structure is in place; i.e. a regular IProgressIndicator should be displayed (using ContentControl) and using DataTemplates display other types.

So, when IProgressIndicator is of ICompositeProgressIndicator, then the root of the whole hierarchy should be a TreeView, with root tree view item displaying information (like ProgressPercentage and Name). Then children should be displayed again as IProgressIndicator and using DataTemplates choose appropriate way to view its data. Nested ICompositeProgressIndicator objects should just add another tree view item (not whole TreeView).

Here's what I came up with. I had to use Complex Hierarchical Data Templates. Also I'm using custom DataTemplateSelector, which is fairly simple:

public class ProgressIndicatorTemplateSelector : DataTemplateSelector
{
    public override DataTemplate SelectTemplate(object item, DependencyObject container)
    {
        var element = (container as FrameworkElement);

        if (item is ICompositeProgressIndicator)
            return element.FindResource("CompositeProgressIndicatorTemplate") as DataTemplate;
        else if (item is ICountProgressIndicator)
            return element.FindResource("CountProgressIndicatorTemplate") as DataTemplate;
        else if (item is IPercentageProgressIndicator)
            return element.FindResource("PercentageProgressIndicatorTemplate") as DataTemplate;
        return null;
    }
}

Here's XAML:

<StackPanel Grid.Row="2" Margin="5" Orientation="Vertical">
    <StackPanel.Resources>
        <editorUtil:ProgressIndicatorTemplateSelector x:Key="progressIndicatorTemplateSelector" />
        <Converters:ObjectToTypeConverter x:Key="objectTypeConverter" />

        <DataTemplate x:Key="CompositeProgressIndicatorTemplateBase">
            <StackPanel Orientation="Vertical">
                <TextBlock Text="CompositeProgressIndicatorTemplateBase" />
                <TextBlock Text="{Binding Name}" />
                <TextBlock Text="{Binding SubProgressItems.Count}" />
                <TextBlock Text="{Binding Converter={StaticResource objectTypeConverter}}"  Margin="5" />
            </StackPanel>
        </DataTemplate>

        <DataTemplate x:Key="CompositeProgressIndicatorTemplate">
            <Grid>
                <Grid.RowDefinitions>
                    <RowDefinition Height="auto" />
                    <RowDefinition Height="*" />
                </Grid.RowDefinitions>
                <StackPanel Grid.Row="0" Orientation="Vertical">
                    <TextBlock Text="CompositeProgressIndicatorTemplateBase" />
                    <TextBlock Text="{Binding Name}" />
                    <TextBlock Text="{Binding SubProgressItems.Count}" />
                    <TextBlock Text="{Binding Converter={StaticResource objectTypeConverter}}"  Margin="5" />
                </StackPanel>
                <TreeView Grid.Row="1" DataContext="{Binding}">
                    <TreeView.ItemContainerStyle>
                        <Style TargetType="{x:Type TreeViewItem}">
                            <Setter Property="IsExpanded" Value="true"/>
                        </Style>
                    </TreeView.ItemContainerStyle>
                    <TreeViewItem ItemsSource="{Binding SubProgressItems}" DataContext="{Binding}"
                          ItemTemplateSelector="{StaticResource progressIndicatorTemplateSelector}" IsExpanded="True">
                        <TreeViewItem.HeaderTemplate>
                            <DataTemplate>
                                <StackPanel Orientation="Vertical">
                                    <StackPanel.DataContext>
                                        <Binding RelativeSource="{RelativeSource FindAncestor, AncestorType={x:Type coreUtil:ICompositeProgressIndicator}}"
                                                 Path="."/>
                                    </StackPanel.DataContext>
                                    <TextBlock Text="CompositeProgressIndicatorTemplate" />
                                    <ContentControl ContentTemplate="{StaticResource CompositeProgressIndicatorTemplateBase}">
                                        <ContentControl.DataContext>
                                            <Binding RelativeSource="{RelativeSource FindAncestor, AncestorType={x:Type coreUtil:ICompositeProgressIndicator}}"
                                                 Path="."/>
                                        </ContentControl.DataContext>
                                    </ContentControl>
                                </StackPanel>
                            </DataTemplate>
                        </TreeViewItem.HeaderTemplate>
                    </TreeViewItem>
                </TreeView>
            </Grid>
        </DataTemplate>

        <HierarchicalDataTemplate DataType="{x:Type coreUtil:ICompositeProgressIndicator}" ItemsSource="{Binding Path=SubProgressItems}">
            <ContentControl ContentTemplate="{StaticResource CompositeProgressIndicatorTemplateBase}" />
        </HierarchicalDataTemplate>
        <DataTemplate x:Key="CountProgressIndicatorTemplate" DataType="{x:Type coreUtil:ICountProgressIndicator}">
            <Grid>
                <ProgressBar Height="20" Value="{Binding ProgressPercentage, Mode=OneWay}" />
                <TextBlock Margin="5" HorizontalAlignment="Center" Text="{Binding ProgressPercentage, Converter={StaticResource percentageConverter}, StringFormat=P}" />
            </Grid>
        </DataTemplate>
        <DataTemplate x:Key="PercentageProgressIndicatorTemplate" DataType="{x:Type coreUtil:IPercentageProgressIndicator}">
            <Grid>
                <ProgressBar Height="20" Value="{Binding ProgressPercentage, Mode=OneWay}" />
                <TextBlock Margin="5" HorizontalAlignment="Center" Text="{Binding ProgressPercentage, Converter={StaticResource percentageConverter}, StringFormat=P}" />
            </Grid>
        </DataTemplate>
    </StackPanel.Resources>
    <ContentControl Content="{Binding ProgressIndicator}" ContentTemplateSelector="{StaticResource progressIndicatorTemplateSelector}">
    </ContentControl>
</StackPanel>

Here's how it looks at the moment:

There's missing DataContext for the root tree view item.