Route/Tunnel property value to another property with a behavior - wpf

Question

I have a DataGrid and want to pass the AlternationIndex to the bound element (off the current object).
CellTemplate
<DataGridTemplateColumn.CellTemplate>
<DataTemplate>
<Grid>
<TextBlock HorizontalAlignment="Center" Text="{Binding Index}"/>
<i:Interaction.Behaviors>
<behaviors:RoutePropertyValue
Source="{Binding RelativeSource={RelativeSource AncestorType=DataGridRow}, Path=(ItemsControl.AlternationIndex)}"
Target="{Binding Index, Mode=TwoWay}"/>
</i:Interaction.Behaviors>
</Grid>
</DataTemplate>
</DataGridTemplateColumn.CellTemplate>
*RoutePropertyValueBehavior**
public class RoutePropertyValue : Behavior<FrameworkElement>
{
/// <summary>
/// The source property value
/// </summary>
public object Source
{
get => (object)GetValue(SourceProperty);
set => SetValue(SourceProperty, value);
}
/// <summary>
/// The <see cref="Source"/> DependencyProperty.
/// </summary>
public static readonly DependencyProperty SourceProperty = DependencyProperty.Register("Source", typeof(object), typeof(RoutePropertyValue), new PropertyMetadata(null, SourceChangedCallback));
private static void SourceChangedCallback(DependencyObject d, DependencyPropertyChangedEventArgs args)
{
if (d is RoutePropertyValue instance)
{
instance.OnSourceChanged();
}
}
protected virtual void OnSourceChanged()
{
Target = Source;
}
/// <summary>
/// The target where the value should be writetn
/// </summary>
public object Target
{
get => (object)GetValue(TargetProperty);
set => SetValue(TargetProperty, value);
}
/// <summary>
/// The <see cref="Target"/> DependencyProperty.
/// </summary>
public static readonly DependencyProperty TargetProperty = DependencyProperty.Register("Target", typeof(object), typeof(RoutePropertyValue), new PropertyMetadata(null));
}
Actually the Target is always null when debugging. Also if changing the order inside of the DataGrid, the SourceChangedCallback is not getting called again. So anyways it seems to be the wrong way to 'tunnel' property values to another property.

Answer 1

You could bind to the target object and then specify which source property you want to set. You should try to set the source property whenever any of the target properties are set. Try this:
public class RoutePropertyValue : Behavior<FrameworkElement>
{
public object Source
{
get => GetValue(SourceProperty);
set => SetValue(SourceProperty, value);
}
public static readonly DependencyProperty SourceProperty = DependencyProperty.Register(nameof(Source), typeof(object),
typeof(RoutePropertyValue), new PropertyMetadata(null, Callback));
private static void Callback(DependencyObject d, DependencyPropertyChangedEventArgs args)
{
if (d is RoutePropertyValue instance)
instance.OnAnyValueChanged();
}
protected virtual void OnAnyValueChanged()
{
if (!string.IsNullOrEmpty(TargetMember))
TargetObject?.GetType().GetProperty(TargetMember)?.SetValue(TargetObject, Source);
}
public object TargetObject
{
get => GetValue(TargetObjectProperty);
set => SetValue(TargetObjectProperty, value);
}
public static readonly DependencyProperty TargetObjectProperty = DependencyProperty.Register(nameof(TargetObject), typeof(object),
typeof(RoutePropertyValue), new PropertyMetadata(null, Callback));
public string TargetMember
{
get => (string)GetValue(TargetMemberProperty);
set => SetValue(TargetMemberProperty, value);
}
public static readonly DependencyProperty TargetMemberProperty = DependencyProperty.Register(nameof(TargetMember), typeof(string),
typeof(RoutePropertyValue), new PropertyMetadata(null, Callback));
}
XAML:
<behaviors:RoutePropertyValue
TargetObject="{Binding}"
TargetMember="Index"
Source="{Binding RelativeSource={RelativeSource AncestorType=DataGridRow}, Path=(ItemsControl.AlternationIndex)}"
/>