How to use Microsoft.Graphics.Canvas.UI.Xaml.CanvasControl in CustomControl?

[HASPzhu]

I put a CanvasControl in a Page and used Draw function to define how to use a list of data to draw.
Now I want to make it a CustomControl. I want it have a DataSource Property to get data and use PropertyChangedCallback to trigger Invalidate().
The problem is I don't know how to define a Draw function for a CanvasControl in XAML.

    <Style TargetType="local:MyCanvasControl" >
        <Setter Property="Template">
            <Setter.Value>
                <ControlTemplate TargetType="local:MyCanvasControl">
                    <canvas:CanvasControl
                        DataSource="{TemplateBinding DataSource}"
                        Draw=""            //How to define a Draw in XAML and how to use ?.Invalidate() in CS?
                     ></canvas:CanvasControl>
                </ControlTemplate>
            </Setter.Value>
        </Setter>
    </Style>