WPF Binding Issues - wpf

Question

I'm trying to customize the TreeView control. When a user selects an item in the TreeView, I need the ActualWidth of the SelectedItem to be stored in the item's Tag:
<Style x:Key="{x:Type TreeViewItem}" TargetType="{x:Type TreeViewItem}">
<!-- ... -->
<Setter Property="Template">
<Setter.Value>
<ControlTemplate TargetType="{x:Type TreeViewItem}">
<Grid ShowGridLines="True">
<Grid.ColumnDefinitions>
<ColumnDefinition Width="Auto" />
<ColumnDefinition />
</Grid.ColumnDefinitions>
<Grid.RowDefinitions>
<RowDefinition />
<RowDefinition />
</Grid.RowDefinitions>
<Rectangle x:Name="rect" />
<ContentPresenter x:Name="PART_Header" ContentSource="Header" Margin="5" />
<ItemsPresenter x:Name="ItemsHost" Grid.Row="1" Grid.Column="1" />
<!-- ... -->
</Grid>
<ControlTemplate.Triggers>
<Trigger Property="IsSelected" Value="true">
<Setter Property="Tag" Value="{Binding ElementName=rect, Path=ActualWidth}" />
</Trigger>
</ControlTemplate.Triggers>
</ControlTemplate>
</Setter.Value>
</Setter>
</Style>
Later, I listen to the SelectedItemChanged event of the TreeView:
private void views_SelectedItemChanged(object sender, RoutedPropertyChangedEventArgs<object> e)
{
TreeViewItem item = (TreeViewItem)e.NewValue;
double i = (double)item.Tag;
}
Now the problem is that item.Tag is always null. Is this a problem with my binding? Or should things be done in a different way?

Answer 1

Try that :
<Setter Property="Tag" Value="{Binding Path=ActualWidth, RelativeSource={x:Static RelativeSource.Self}}" />