DataTrigger in Style in HierarchicalDataTemplate not working -- TreeView - wpf

Question

I have a DataTrigger to set a TextBox's Background based on a bound property.
Here's a streamlined version of the xaml:
<TreeView >
<TreeViewItem Header="Things" >
<TreeViewItem.Resources>
<Style TargetType="{x:Type TextBlock}">
<Style.Triggers>
<DataTrigger Binding="{Binding IsDirty}" Value="True">
<Setter Property="Background" Value="LightGray" />
</DataTrigger>
</Style.Triggers>
</Style>
<HierarchicalDataTemplate DataType="{x:Type local:Type1}" ItemsSource="{Binding Children, Mode=OneWay}">
<StackPanel Orientation="Horizontal">
<TextBlock Text="{Binding Name}" Margin="6,0,6,0" />
<TextBlock Text="{Binding IsDirty}" Margin="6,0,6,0" />
<i:Interaction.Behaviors>
<dragDrop:FrameworkElementDropBehavior DragEffect="Move" />
</i:Interaction.Behaviors>
</StackPanel>
</HierarchicalDataTemplate>
</TreeViewItem.Resources>
</TreeViewItem>
I added a TextBlock to display the value of the IsDirty property; when that is true, the Background remains unchanged.
I have tried moving the Style to the HierarchicalDataTemplate.Resources, but that made no difference.
What am I overlooking?
Thanks --

Answer 1

That's because implicit styles targeting types not derived from Control do not cross the template boundary, i.e. are not applied inside templates unless they're defined within that template's scope. Here's a good post explaining how it works and why does it work this way.
In order to cross the template boundary, you should use a type deriving from Control (e.g. a Label) instead of a TextBlock and define implicit style targeting that type.
Otherwise, you could put your style in scope of the template in question by moving it into the template's resources dictionary:
<HierarchicalDataTemplate (...)>
<HierarchicalDataTemplate.Resources>
<Style TargetType="{x:Type TextBlock}">
(...)
</Style>
</HierarchicalDataTemplate.Resources>
(...)
</HierarchicalDataTemplate>