Binding to a cutom property of the template parent - wpf

Question

Q: How do I bind to a custom property of the template parent from a child control's style DataTrigger
I've been scratching my head over this one for a couple of days.
I have a databound TreeView which uses a Style which has a Template. The TreeView is bound to a ObservableCollection and a HierarchicalDataTemplate + DataTemplate bind to properties inside a collection item.
FontGroup -> Font(s)
<Style x:Key="ExpandCollapseToggleStyle" TargetType="{x:Type ToggleButton}">
...
<Setter Property="Template">
<Setter.Value>
<ControlTemplate TargetType="{x:Type ToggleButton}">
<Image x:Name="ExpanderImage" Source="/Typesee;component/Resources/tree_expand.png" RenderOptions.BitmapScalingMode="NearestNeighbor" />
<ControlTemplate.Triggers>
<DataTrigger Binding="??? IsItemSelected ???" Value="True">
<Setter TargetName="ExpanderImage" Property="Source" Value="/Typesee;component/Resources/tree_collapse_selected.png" />
</DataTrigger>
</ControlTemplate.Triggers>
</ControlTemplate>
</Setter.Value>
</Setter>
</Style>
<ControlTemplate x:Key="FontTreeViewTemplate" TargetType="{x:Type TreeViewItem}">
...
<ToggleButton x:Name="Expander" Style="{StaticResource ExpandCollapseToggleStyle}" ... />
...
<ControlTemplate.Triggers>
<DataTrigger Binding="{Binding IsItemSelected}" Value="True">
<!-- WORKS FINE HERE -->
</DataTrigger>
</ControlTemplate.Triggers>
</ControlTemplate>
First I tried to bind like:
Binding Path=IsItemSelected, RelativeSource={RelativeSource TemplatedParent}
Then I read that might not work so I tried (including AncestorLevel 1+3):
Binding RelativeSource={RelativeSource Mode=FindAncestor, AncestorType={x:Type TreeViewItem}, AncestorLevel=2
Have also tried combos with UpdateSourceTrigger=PropertyChanged and Mode=TwoWay
If this is a flawed design please suggest a way of doing this: I basically want to change the image of the expand toggle button based on whether the property IsItemSelected is true on the TreeViewItem -- any ideas?
Thanks so much for any help!

Answer 1

The viewmodel in all likelihood will be the DataContext, so the binding should be a RelativeSource binding with a respective path which needs to explicity target the DataContext as the new source is the RelativeSource:
{Binding DataContext.IsItemSelected,
RelativeSource={RelativeSource AncestorType=TreeViewItem}}
As noted in my comment it might be advisable to extract this logic from the ControlTemplate as this leaves its bounds. One method would be subclassing the ToggleButton and exposing a public property for the image which then can be changed via a Style.