How to bind a MultiDataTrigger with bindings using different data contexts - wpf

Question

I have a custom user control which is in the main window of my WPF application. Within the window is an ItemsControl. I have created a style so that I can bind to an array of items which is a property of my view-model class. The array holds indexes to the position of the items control. I should add that the custom control is inherits from Shape so it has the Stroke property.
public class ViewModel
{
...
public List<int> Selections
{
get => _selections;
set
{
if (value == _selections) return;
_selections = value;
OnPropertyChanged();
}
}
public HypercombState State
{
get => _state;
set
{
if (value == _state) return;
_state = value;
OnPropertyChanged();
}
}
}
Here is the converter that is responsible for identifying whether the view-model is holding selected indexes or not. If should return true if the item is selected during databinding.
public class ArrayContainsConverter : IMultiValueConverter
{
public object Convert(object[] values, Type targetType, object parameter, CultureInfo culture)
{
if (values[0] is not int id || values[1] is not List<int> array) return null;
return array?.Contains(id);
}
...
}
<Style.Triggers>
<MultiDataTrigger>
<MultiDataTrigger.Conditions>
<Condition Value="True">
<Condition.Binding>
<MultiBinding Converter="{StaticResource ArrayContainsConverter}">
<Binding Path="(ItemsControl.AlternationIndex)" RelativeSource="
{RelativeSource AncestorType=ContentPresenter}" />
<Binding Path="DataContext.Selections" />
</MultiBinding>
</Condition.Binding>
</Condition>
...
</MultiDataTrigger.Conditions>
<Setter Property="Stroke" Value="Chartreuse"></Setter>
</MultiDataTrigger>
</Style.Triggers>
Whenever the Selections or State properties change, I would like to update the ItemsControl so that there is a Stroke as a visual cue for the item's selected state when true. I can see the Selections property changing if I use breakpoints but there the converter is not triggering when the list changes.

Answer 1

Try changing the datatype from List to ObservableCollection. There is a difference when using those 2 data types in WPF and usually I recommend using the latter for any trigger updates.
Also try updating the binding modes such as
<Binding Path="DataContext.Selections" />
To
<Binding Path="DataContext.Selections", Mode=“TwoWay” />
The difference between the two is that:
List - Implements IList interface
ObservableCollection - Implements INotifyCollectionChanged