ObservableCollection dependency property does not update when item in collection is deleted

Question

I have a attached property of type ObservableCollection on a control. If I add or remove items from the collection, the ui does not update. However if I replace the collection within with a new one the ViewModel the ui does update.

Can someone give me an example of what I need to do within the Dependency object so that it can handle changes within the collection?

Part of the dependency object is listed below:

public class RadCalendarBehavior : DependencyObject
{
private static void OnSpecialDaysChanged(DependencyObject d, DependencyPropertyChangedEventArgs e)
{
  var calendar = d as RadCalendar;
  if (e.NewValue != null)
  {
    calendar.DayTemplateSelector = new SpecialDaySelector((ObservableCollection<DateTime>)e.NewValue, GetSpecialDayTemplate(d));
  }
}

public static ObservableCollection<DateTime> GetSpecialDays(DependencyObject obj)
{
  return (ObservableCollection<DateTime>)obj.GetValue(SpecialDaysProperty);
}

public static void SetSpecialDays(DependencyObject obj, ObservableCollection<DateTime> value)
{
  obj.SetValue(SpecialDaysProperty, value);
}

public static readonly DependencyProperty SpecialDaysProperty =
    DependencyProperty.RegisterAttached("SpecialDays", typeof(ObservableCollection<DateTime>), typeof(RadCalendarBehavior), new UIPropertyMetadata(null, OnSpecialDaysChanged));
}
}

I understand that I need to register that the collection has changed, but I am unsure how to do this within the dependency property

Answer 1

A change within the collection won't trigger the OnSpecialDaysChanged callback, because the value of the dependency property hasn't changed. If you need to react to detect changes with the collection, you need to handle the event CollectionChanged event manually:

private static void OnSpecialDaysChanged(DependencyObject d, DependencyPropertyChangedEventArgs e)
{
  var calendar = d as RadCalendar;

  if (e.OldValue != null)
  {
    var coll = (INotifyCollectionChanged)e.OldValue;
    // Unsubscribe from CollectionChanged on the old collection
    coll.CollectionChanged -= SpecialDays_CollectionChanged;
  }

  if (e.NewValue != null)
  {
    var coll = (ObservableCollection<DateTime>)e.NewValue;
    calendar.DayTemplateSelector = new SpecialDaySelector(coll, GetSpecialDayTemplate(d));
    // Subscribe to CollectionChanged on the new collection
    coll.CollectionChanged += SpecialDays_CollectionChanged;
  }
}

private static void SpecialDays_CollectionChanged(object sender, NotifyCollectionChangedEventArgs e)
{
    // handle CollectionChanged
}