Why my binding does not update on UIElement.RenderSize or UIElement.ActualWidth ?

Question

I'm trying to data bind a TextBlock to the RenderSize of a Rectangle. Here is the code.

<StackPanel x:Name="root">
        <Rectangle x:Name="rect" Fill="Green" RenderTransformOrigin="0.5,0.5" Height="100" Width="100" />

        <TextBlock Text="{Binding ElementName=rect, Path=Width}"></TextBlock>
        <TextBlock Text="{Binding ElementName=rect, Path=ActualWidth}"></TextBlock>
        <TextBlock Text="{Binding ElementName=rect, Path=RenderSize}"></TextBlock>
        <Slider Value="{Binding ElementName=rect, Path=Width, Mode=TwoWay}" Maximum="200"></Slider>
        <Button Content="Manually Get RenderSize" Click="Button_Click_1"></Button>
        <TextBlock x:Name="info"></TextBlock>
    </StackPanel>

When I move the slider, rect become larger, and the first TextBox is updated correctly. However, ActualWidth and RenderSize stay to 0, and 0,0.

When I click on Button, I get the RenderSize programmatically and show it in the last TextBlock, which is different than 0,0.

private void Button_Click_1(object sender, RoutedEventArgs e)
{
    info.Text = rect.RenderSize.ToString();
}

So my binding on the RenderSize does not update the TextBox correctly... Why ? Thanks for your help,

Answer 1

I wrote an answer to a similar question here:

http://stackoverflow.com/questions/1602148/binding-to-actualwidth-does-not-work/1604662#1604662

It should be useful to you.