Xaml Trigger Parent via a specific property of any child in Collection

Question

I'm having troubles with triggers for a Child-Parent relation in my ViewModel.

Consider the following ViewModel (implementsINotifyPropertyChanged) and View:

ViewModelType
 + Items: ObservableCollection<ViewModelType>
 + IsVisible: bool
 + Text: string

ViewType
 + Visibility: Visibility
 + Header: string

I have a DataTemplate to bind the 2:

<DataTemplate DataType="{x:Type vm:ViewModelType}">
  <views:ViewType Header="{Binding Text}"/>
</DataTemplate>

And a style for the View:

<Style TargetType="{x:Type views:ViewType}">
  <Setter Property="Visibility" Value="{Binding IsVisible, Converter={StaticResource Bool2Visibility}}"/>
</Style>

This all works beautifully.

Now, what I want to achieve is that if from ViewModelTypeInstanceA all ViewModelType.Items have their IsVisible property set to false, I want ViewModelTypeInstanceA corresponding ViewType to have its Visibility property set to Visibility.Collapsed**.

I have tried DataTriggers, converters and all but I don't think I can use something like AncestorType in triggers to non referable parent? It seems impossible to trigger a parent property. Perhaps it's possible for an element to observe all it's children's IsVisisble properties?

Conditions:
1. I prefer to NOT alter the ControlTemplate (it's from a diff lib).
2. Modifying the ViewModel structure isn't an option either (compatibility issues). That means I cannot maintain a property Parent on item creation or alter the Collection type as Eli suggested.
3. I really prefer an elegant solution.

**Ofcourse depending on the children's ViewType's VisibilityProperty is also OK, if it can be set through styling.

Answer 1

Not the cleanest, but the best that I could think of I guess. I'm (ab)using the physical parent's DataContext.

1. I added a ViewModelType.HasVisibleChild-property and a EvaluateHasVisibleChildValue() method.

internal void EvaluateHasVisibleChild()
{
  foreach(ViewModelType node in Items)
    // Conditions for ultimate visibility
    if (node.IsVisible && node.HasVisibleChild)
    {
      HasVisibleChild = true;
      return;
    }

  HasVisibleChild = false;
}

2. I changed the style binding that uses a converter and provides the Visibility-dependent properties both as the physical ViewType ancestor:

 

<Style TargetType="{x:Type views:ViewType}">
  <Setter.Value>
    <MultiBinding Converter="{StaticResource ConditionalVisibilityConverter}">
      <Binding Path="IsVisible"/>
      <Binding Path="HasVisibleChild"/>
      <Binding Path="." RelativeSource="{RelativeSource Mode=FindAncestor, AncestorType={x:Type views:ViewType}}"/>
    </MultiBinding>
  </Setter.Value>
</Style>

3. I wrote a MultiBindingConverter:

public class ConditionalVisibilityConverter : IMultiValueConverter
{
  public object Convert(object[] values, Type targetType, object paramater, CultureInfo culture)
  {
    // Has no ancestor safety check
    if (values[2] != DependencyProperty.UnsetValue)
      // Sneak in an update of viewmodelparent via physical parent.
      (((values[2] as FrameworkElement).DataContext) as ViewModelType).EvaluateHasVisibleChild();

    return (bool)values[0] && (bool)values[1] ? Visibility.Visible : Visibility.Collapsed;
  }

  public object[] ConvertBack(object value, Type[] targetTypes, object paramweter, CultureInfo cultuer)
  {
    throw new NotImplementedException();
  }
}

In practise I have multiple ViewTypes as ancestors, so I added multiple Bindings for various ancestorTypes. I also considered adding an Interface to various the ViewTypes, but nuh-uh.

N.b. this obviously requires that exactly 1 ViewType is used per ViewModelType, otherwise the FindAncestorType might get unexpected results.