Using the Treeview HierarchicalDataTemplate.Triggers to change folder icon - wpf

Question

I'm pretty fresh to WPF and this is the closest I have come to achieving what I set out to do after reviewing many of the previously asked questions posted here.
The XAML code:
<TreeView x:Name="folderView" Grid.Column="0" Grid.Row="1" BorderThickness="0">
<TreeViewItem Header="Folders" ItemsSource="{Binding SubFolders, Source={StaticResource RootFolderDataProvider}}" Margin="5"/>
<TreeView.Resources>
<HierarchicalDataTemplate DataType="{x:Type my:FolderView}" ItemsSource="{Binding SubFolders}">
<StackPanel Orientation="Horizontal" Name="myPanel">
<Image x:Name="img" Width="16" Height="16" Source="Images/FolderClosed.png" />
<TextBlock Text="{Binding Name}" />
</StackPanel>
<HierarchicalDataTemplate.Triggers>
<DataTrigger Binding="{Binding IsExpanded}" Value="True">
<Setter TargetName="img" Property="Source" Value="Images/FolderOpen.png"/>
</DataTrigger>
</HierarchicalDataTemplate.Triggers>
</HierarchicalDataTemplate>
</TreeView.Resources>
</TreeView>
This displays the FolderClosed image on all my subfolders except the very top root folder.
The HierachicalDataTemplate trigger also fails to fire when expanded.
Any help would be appreciated.

Answer 1

If you are binding to the TreeViewItem IsExpanded property, then you will have to update your binding like:
<DataTrigger Binding="{Binding IsExpanded, RelativeSource={RelativeSource AncestorType={x:Type TreeViewItem}}}" Value="True">
<Setter TargetName="img" Property="Source" Value="Images/FolderOpen.png"/>
</DataTrigger>

Answer 2

I cannot pin point the issues. But as a first step you should check if the binding is working or not.
you can add debugging for binding as mentioned in here
eg :
<Window
xmlns:diag="clr-namespace:System.Diagnostics;assembly=WindowsBase"
xmlns:local="clr-namespace:DebugDataBindings"
x:Class="DebugDataBindings.MainWindow"
xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
Title="MainWindow" Height="350" Width="525">
<Grid x:Name="layoutRoot">
<Grid.Resources>
<local:DebugConverter x:Key="debugConverter" />
</Grid.Resources>
<TextBox
Text="{Binding Path=Customer.FirstName, diag:PresentationTraceSources.TraceLevel=High}"
Height="23" HorizontalAlignment="Left" Margin="90,18,0,0" VerticalAlignment="Top" Width="120"/>
</Grid>
</Window>