Silverlight TemplateBinding to RotateTransform - silverlight

Question

I am trying to create the simplest Silverlight templated control, and I can't seem to get TemplateBinding to work on the Angle property of a RotateTransform.
Here's the ControlTemplate from generic.xaml:
<ControlTemplate TargetType="local:CtlKnob">
<Grid x:Name="grid" RenderTransformOrigin="0.5,0.5">
<Grid.RenderTransform>
<TransformGroup>
<RotateTransform Angle="{TemplateBinding Angle}"/> <!-- This does not work -->
<!-- <RotateTransform Angle="70"/> --> <!-- This works -->
</TransformGroup>
</Grid.RenderTransform>
<Ellipse Stroke="#FFB70404" StrokeThickness="19"/>
<Ellipse Stroke="White" StrokeThickness="2" Height="16" VerticalAlignment="Top"
HorizontalAlignment="Center" Width="16" Margin="0,2,0,0"/>
</Grid>
</ControlTemplate>
Here's the C#:
using System.Windows;
using System.Windows.Controls;
namespace CtlKnob
{
public class CtlKnob : Control
{
public CtlKnob()
{
this.DefaultStyleKey = typeof(CtlKnob);
}
public static readonly DependencyProperty AngleProperty =
DependencyProperty.Register("Angle", typeof(double), typeof(CtlKnob), null);
public double Angle
{
get { return (double)GetValue(AngleProperty); }
set { SetValue(AngleProperty,value); }
}
}
}

Answer 1

Here is a workaround:
public class TemplatedControl1 : Control
{
public TemplatedControl1()
{
this.DefaultStyleKey = typeof(TemplatedControl1);
}
public override void OnApplyTemplate()
{
var transform = this.GetTemplateChild("Transform1") as RotateTransform;
transform.Angle = this.StartAngle;
base.OnApplyTemplate();
}
public static readonly DependencyProperty StartAngleProperty =
DependencyProperty.Register("StartAngle", typeof(double), typeof(TemplatedControl1), null);
public double StartAngle
{
get { return (double)GetValue(StartAngleProperty); }
set { SetValue(StartAngleProperty, value); }
}
}
And the xaml:
<Style TargetType="local:TemplatedControl1">
<Setter Property="Template">
<Setter.Value>
<ControlTemplate TargetType="local:TemplatedControl1">
<Border Background="{TemplateBinding Background}"
BorderBrush="{TemplateBinding BorderBrush}"
BorderThickness="{TemplateBinding BorderThickness}">
<Canvas>
<Polyline Fill="Black" >
<Polyline.RenderTransform>
<RotateTransform x:Name="Transform1" />
</Polyline.RenderTransform>
</Polyline>
</Canvas>
</Border>
</ControlTemplate>
</Setter.Value>
</Setter>
</Style>

Answer 2

Henrik has all ready provided the answer but I'll explain why its necessary.
Currently in Sliverlight 3 this sort of binding requires that object receiving the binding is a FrameworkElement. Its the FrameworkElement that has the SetBinding method that allows this stuff to work.
RotateTransform whilst being a DependencyObject is not a FrameworkElement and therefore cannot participate in this sort of binding. Silverlight 4 allows binding to work on DependencyObject so theoritically this sort of code ought to work in SL4. I'll have to try that to see if that is true.

Answer 3

Thanks Anthony for explaining why it doesn't work.
Thanks Henrik for the workaround, it solves half of the problem: the Angle can now be set from Xaml. However, it still can't be set programmatically, but the missing bit is now easy.
Here's the full solution:
public class CtlKnob : Control
{
public CtlKnob()
{
this.DefaultStyleKey = typeof(CtlKnob);
}
public static readonly DependencyProperty AngleProperty =
DependencyProperty.Register("Angle", typeof(double), typeof(CtlKnob),
new PropertyMetadata(AngleChanged));
public double Angle
{
get { return (double)GetValue(AngleProperty); }
set { SetValue(AngleProperty,value); }
}
public override void OnApplyTemplate()
{
base.OnApplyTemplate();
var transform = this.GetTemplateChild("RotateTransform") as RotateTransform;
transform.Angle = this.Angle;
}
private static void AngleChanged( DependencyObject dobj,
DependencyPropertyChangedEventArgs e )
{
var knob = dobj as CtlKnob;
var rotateTransform = knob.GetTemplateChild("RotateTransform") as
RotateTransform;
if (rotateTransform != null)
rotateTransform.Angle = (double)e.NewValue;
}
}

Answer 4

In Silverlight 3, you can only bind to FrameworkElements and not DependencyObjects. RotateTransform is not a FrameworkElement.
Silverlight 4 supports binding to DependencyObjects, so this works in Silverlight 4.
If you can upgrade to Silverlight 4, I'd consider it. Otherwise, a number of people have posted workarounds for Silverlight 3.