TemplateBinding doesn't bind to effect's property? - wpf

Question

Imagine a control named Testko like this:
public class Testko: Control
{
public static readonly DependencyProperty TestValueProperty;
static Testko()
{
DefaultStyleKeyProperty.OverrideMetadata(typeof(Testko), new FrameworkPropertyMetadata(typeof(Testko)));
TestValueProperty = DependencyProperty.Register("TestValue", typeof(double), typeof(Testko), new UIPropertyMetadata((double)1));
}
public double TestValue
{
get { return (double)GetValue(TestValueProperty); }
set { SetValue(TestValueProperty, value); }
}
}
Nothing fancy, just an empty control with a single double property with a default value set to (double)1.
Now, image a generic style like this:
<Style TargetType="{x:Type local:Testko}">
<Setter Property="Template">
<Setter.Value>
<ControlTemplate TargetType="{x:Type local:Testko}">
<Border Background="{TemplateBinding Background}"
BorderBrush="{TemplateBinding BorderBrush}"
BorderThickness="{TemplateBinding BorderThickness}">
<StackPanel Orientation="Vertical">
<StackPanel.Effect>
<BlurEffect Radius="{TemplateBinding TestValue}" />
</StackPanel.Effect>
<Button Content="{TemplateBinding TestValue}" Margin="4" />
</StackPanel>
</Border>
</ControlTemplate>
</Setter.Value>
</Setter>
</Style>
Now, the problem is that Radius property is never bound for some reason. Wheras Button's content is properly bound to TestValue property.
I am sure I am missing something obvious. Or not?

Answer 1

If it is obvious, it is not to me :-)
My favorite book (WPF Unleashed) mentions that sometimes TemplatedBinding doesn't work (but the enumerated reasons don't match your circumstances).
But TemplatedBinding is a shortcut for:
{Binding RelativeSource={RelativeSource TemplatedParent}, Path=TestValue}
I reproduced your case, i.e. changing TestValue only has effect on the button.
After replacing the TemplatedBinding by this, I get the desired effect.