How to get a element's position which in a UserControl - wpf

Question

I have a UserControl:
<UserControl d:DesignHeight="100" d:DesignWidth="200" ...>
<Grid>
<Grid.ColumnDefinitions>
<ColumnDefinition Width="100" />
<ColumnDefinition Width="100" />
</Grid.ColumnDefinitions>
<Ellipse Name="leftEllipse" Grid.Column="0" Width="50" Height="50" HorizontalAlignment="Center" VerticalAlignment="Center" Fill="Red" />
<Ellipse Name="rightEllipse" Grid.Column="1" Width="50" Height="50" HorizontalAlignment="Center" VerticalAlignment="Center" Fill="Green" />
</Grid>
</UserControl>
Here is my MainWindow:
<Window ...>
<Canvas Name="canvas1">
<my:MyUserControl x:Name="myUserControl1" Width="200" Height="100" Canvas.Top="100" Canvas.Left="100" />
</Canvas>
</Window>
I know how to get the position of myUserControl1 :
double x = Canvas.GetLeft(myUserControl1);
But can anyone tell me how to get the position of myUserControl1.leftEllipse?
And when myUserControl1 apply a RotateTransform, the myUserControl1.leftEllipse's position will changed, won't it?

Answer 1

Without making the generated leftEllipse field public, you could add a method to the UserControl that returns a transform object from the Ellipse's coordinates to that of an ancestor element, e.g.
public GeneralTransform LeftEllipseTransform(UIElement e)
{
return leftEllipse.TransformToAncestor(e);
}
You may then call it in your MainWindow like this:
var p = myUserControl1.LeftEllipseTransform(this).Transform(new Point());
Instead of TransformToAncestor (or TransformToVisual) you may also use TranslatePoint.
public Point GetLeftEllipsePosition(Point p, UIElement e)
{
return leftEllipse.TranslatePoint(p, e);
}
In MainWindow:
var p = myUserControl1.GetLeftEllipsePosition(new Point(), this);
Or for the center of the Ellipse (instead of its upper left corner):
var p = myUserControl1.GetLeftEllipsePosition(new Point(25, 25), this);