WPF: Resizing a circle, keeping the center point instead of TopLeft?

Question

I'd like to resize a circle on my canvas with the help of a slider. This circle can be moved around on the canvas by some drag&drop stuff I did in code behind, so its position is not fixed.

I have bound the slider's value to an ellipse's height and width. Unfortunately, when I use the slider, the circle gets resized with its top left point (actually the top left point of the rectangle it's sitting in) staying the same during the operation.

I would like to resize it with its center point being constant during the operation. Is there an easy way to do this in XAML? BTW, I already tried ScaleTransform, but it didn't quite do what I wanted.

Thanks a bunch! :-)

Jan

<Canvas x:Name="MyCanvas">

    <!-- this is needed for some adorner stuff I do in code behind -->
    <AdornerDecorator Canvas.Left="10"
                      Canvas.Top="10">
        <Ellipse x:Name="myEllipse"
             Height="{Binding Path=Value, ElementName=mySlider}"
             Width="{Binding Path=Value, ElementName=mySlider}"
             Stroke="Aquamarine"
             Fill="AliceBlue"
             RenderTransformOrigin="0.5 0.5">
            <Ellipse.RenderTransform>
                <RotateTransform Angle="{Binding Path=Value, ElementName=myRotationSlider}" />
            </Ellipse.RenderTransform>
        </Ellipse>
    </AdornerDecorator>

    <Slider x:Name="mySlider"
            Maximum="100"
            Minimum="0"
            Width="100"
            Value="10"
            Canvas.Left="150"
            Canvas.Top="10" />
    <Slider x:Name="myRotationSlider"
            Maximum="360"
            Minimum="0"
            Width="100"
            Value="0"
            Canvas.Left="150"
            Canvas.Top="50" />
</Canvas>

Answer 1

You can bind your Canvas.Left and Canvas.Top to your Height and Width via a ValueConverter.

Specifically (edit):
Create a property each for the Canvas.Left and Canvas.Top and bind to these.
Store the old values for Width and Heigth or the old slider value.
Whenever the slider is changed, get the incremental change "dx" by subtracting the stored value.
(Don't forget to update the stored value...)
Add dx to Width and Height property.
And, as Will said, add dx/2*-1 to Canvas.Left and Canvas.Top properties.

Does that make sense?

Answer 2

Since you're using a Canvas, the location an element has is the location. If you want the Top,Left position to change you need to do it yourself. If you were using another Panel type, like a Grid, you could change the alignment of your Ellipse to place it in the same relative location no matter what the size. You could get that effect by adding a Grid inside your AdornerDecorator and centering the Ellipse but you'd also need to set the AdornerDecorator or Grid to a fixed size because they won't stretch in a Canvas.

The best solution you could use would be a ScaleTransform applied to the RenderTransform property with a RenderTransformOrigin of 0.5,0.5. You said you had problems with ScaleTransform but not what the problem was.