Binding OnwayToSource not working as expected - what alternatives?
<DataTemplate DataType="{x:Type p:Fish}">
<Border BorderBrush="Black" BorderThickness="2" >
<TextBlock FontSize="14" TextAlignment="Center" VerticalAlignment="Center"
           Width="{Binding Path=width, Mode=OneWayToSource}"
           Height="{Binding Path=height, Mode=OneWayToSource}" ...

class Fish {
    public double width { get; set; } // From DataTemplate TextBlock.Width.
    public double height { get; set; } // From DataTemplate TextBlock.Height
}

...
double rendered_width = my_fish.width; // Use the rendered width!

<TextBlock ...
     SizeObserver.Observe="True"
     SizeObserver.ObservedWidth="{Binding Width, Mode=OneWayToSource}"
     SizeObserver.ObservedHeight="{Binding Height, Mode=OneWayToSource}"

<StackPanel Orientation="Vertical">
            <Border BorderThickness="1" BorderBrush="Red">
                <TextBlock Name="tbwidthA" Text="{Binding Path=Howdy}" HorizontalAlignment="Left" Width="200"/>
            </Border>
            <TextBlock Name="tbwidthAw" Text="{Binding ElementName=tbwidthA, Path=Width}"  HorizontalAlignment="Left"/>
            <TextBlock Name="tbwidthAaw" Text="{Binding ElementName=tbwidthA, Path=ActualWidth}" HorizontalAlignment="Left" />
            <Border BorderThickness="1" BorderBrush="Red">
                <TextBlock Name="tbwidthB" Text="{Binding Path=Howdy}" HorizontalAlignment="Left" />
            </Border>
            <TextBlock Name="tbwidthBw" Text="{Binding ElementName=tbwidthB, Path=Width}" HorizontalAlignment="Left" />
            <TextBlock Name="tbwidthAbw" Text="{Binding ElementName=tbwidthB, Path=ActualWidth}" HorizontalAlignment="Left" />
            <Button Content="TBwidth" Click="Button_Click_1" Width="60" HorizontalAlignment="Left" />
        </StackPanel>

Debug.WriteLine(tbwidthB.Width.ToString());
        Debug.WriteLine(tbwidthB.ActualWidth.ToString());