Xaml Behavior DP not updated - xaml

Question

I have a UWP application which uses the Managed UWP Behavior SDK.
I wrote a custom behavior which has two dependency properties, one of which is a ObservableCollection.
Whenever I update an item in the collection, I make sure that PropertyChanged is called for the collection.
However, the Dependency property is not being updated.
My code:
<trigger:CustomBehavior ItemIndex="{x:Bind ItemIndex}"
Presences="{Binding ElementName=Box,
Path=DataContext.CustomCollection,
UpdateSourceTrigger=PropertyChanged, Converter={StaticResource TestConverter}}" />
My TestConverter shows me that when I update an item in the collection, the updatesource trigger is working. The dependency property in my behavior however, is not firing the Changed event. When I change the entire custom collection, the DP is updated, when I just change one item, it isn't.
Research so far says that DependencyObject.SetValue just checks to see if the object has changed and if one item changed, it will just think that the collection didn't change at all? Is this true, and if so, how can I overcome this?
Thanks

Answer 1

A collection-type dependency property should usually be declared as the most basic collection type, IEnumerable. This way you can assign a variety of actual collection types to the property, including those that implement INotifyCollectionChanged, like ObservableCollection<T>.
You would check at runtime if the collection type actually implements the interface, and possibly attach and detach a handler method for the CollectionChanged event.
public class CustomBehavior : ...
{
public static readonly DependencyProperty PresencesProperty =
DependencyProperty.Register(
"Presences", typeof(IEnumerable), typeof(CustomBehavior),
new PropertyMetadata(null,
(o, e) => ((CustomBehavior)o).OnPresencesPropertyChanged(e)));
private void OnPresencesPropertyChanged(DependencyPropertyChangedEventArgs e)
{
var oldCollectionChanged = e.OldValue as INotifyCollectionChanged;
var newCollectionChanged = e.NewValue as INotifyCollectionChanged;
if (oldCollectionChanged != null)
{
oldCollectionChanged.CollectionChanged -= OnPresencesCollectionChanged;
}
if (newCollectionChanged != null)
{
newCollectionChanged.CollectionChanged += OnPresencesCollectionChanged;
// in addition to adding a CollectionChanged handler, any
// already existing collection elements should be processed here
}
}
private void OnPresencesCollectionChanged(
object sender, NotifyCollectionChangedEventArgs e)
{
// handle collection changes here
}
}