WPF: Evaluate XAML binding beyond top level custom object - wpf

Question

In my xaml I declare a custom object which has a property referencing another custom object:
public class CustomObjectA : FrameworkElement
{
public CustomObjectB InnerObj
{
get { return GetValue(InnerObjProperty); }
set { SetValue(InnerObjProperty, value); }
}
public static readonly DependencyProperty InnerObjProperty =
DependencyProperty.Register("InnerObj",
typeof(CustomObjectB),
typeof(CustomObjectA),
new PropertyMetadata(null));
}
public class CustomObjectB : FrameworkElement
{
public string Data
{
get { return GetValue(DataProperty); }
set { SetValue(DataProperty, value); }
}
public static readonly DependencyProperty DataProperty =
DependencyProperty.Register("Data",
typeof(string),
typeof(CustomObjectB),
new PropertyMetadata(null));
}
Now when I declare these objects in my xaml like this the binding doesn't work:
<my:CustomObjectA>
<my:CustomObjectA.InnerObj>
<my:CustomObjectB Data="{Binding someValue}" />
</my:CustomObjectA.InnerObj>
</my:CustomObjectA>
However when I declare these objects in my xaml like this the binding works:
<my:CustomObjectB x:Name="testObj" Data="{Binding someValue}" />
<my:CustomObjectA InnerObj="{Binding ElementName=testObj}" />
I'm assuming this is because the system mapping the bindings doesn't look past the top level object. My question is; is there a way to tell the system to evaluate binding expressions beyond the top level custom object so xaml like option 1 will work?

Answer 1

The DataContext is not passed down to the InnerObj. I got it to work by updating the DataContext when the InnerObj changes:
public class CustomObjectA : FrameworkElement
{
public CustomObjectB InnerObj
{
get { return (CustomObjectB)GetValue(InnerObjProperty); }
set { SetValue(InnerObjProperty, value); }
}
public static readonly DependencyProperty InnerObjProperty =
DependencyProperty.Register("InnerObj",
typeof(CustomObjectB),
typeof(CustomObjectA),
new FrameworkPropertyMetadata(OnInnerObjUpdated));
private static void OnInnerObjUpdated(DependencyObject d, DependencyPropertyChangedEventArgs e)
{
var local = (CustomObjectA)d;
var newObj = (CustomObjectB)e.NewValue;
newObj.DataContext = local.DataContext;
}
}