Binding to Transforms in a ControlTemplate - silverlight

Question

I'm trying to create a custom control in Silverlight that dynamically scales an element in it's ControlTemplate. First attempt of the ControlTemplate looks something like this:
<ControlTemplate TargetType="controls:ProgressBar">
<Grid>
<Rectangle x:Name="TrackPart" Fill="{TemplateBinding Background}" HorizontalAlignment="Left" />
<Rectangle x:Name="ProgressPart" Fill="Blue" >
<Rectangle.RenderTransform>
<ScaleTransform ScaleX="{TemplateBinding Progress}" />
</Rectangle.RenderTransform>
</Rectangle>
</Grid>
</ControlTemplate>
However, this forum thread states that TemplateBinding only works on derivatives of FrameworkElements. ScaleTransform is not a FrameworkElement. Is there a work around for this? Any best practices for this sort of situation out there?

Answer 1

Rather than binding the ScaleX and ScaleY properties of the RenderTransform, you can bind the RenderTransform itself.
The problem is that the source is a double value, and you need a Transform. So you need to be able to convert a double to a ScaleTransform. You can create an IValueConverter to do that:
public class TransformConverter : IValueConverter
{
public object Convert(object value, Type targetType, object parameter, CultureInfo culture)
{
if (value is double)
{
double d = (double)value;
return new ScaleTransform { ScaleY = d, ScaleX = d };
}
else
{
return new ScaleTransform();
}
}
public object ConvertBack(object value, Type targetType, object parameter, CultureInfo culture)
{
throw new NotImplementedException();
}
}
You can't specify an IValueConverter to use in a TemplateBinding, so you can use a regular Binding with RelativeSource as TemplatedParent. Like this:
<Rectangle x:Name="ProgressPart" Fill="Blue"
RenderTransform="{Binding Path=Progress, RelativeSource={RelativeSource TemplatedParent}, Converter={StaticResource converter1}}" >
and you need to place the IValueConverter in the resources of ControlTemplate's root, in scope of the Binding:
<ControlTemplate TargetType="controls:ProgressBar">
<Grid>
<Grid.Resources>
<local:TransformConverter x:Key="converter1" />
</Grid.Resources>

Answer 2

Assuming that you are always using simple items like a rectangle, you could bind the rectangle's height and width to the progress, and then use a binding converter to adjust the value accordingly