title: ObservableCollection dependency property does not update when item in collection is deleted
body: <p>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.</p>
  
  <p>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?</p>
  
  <p>Part of the dependency object is listed below:</p>
  
  <pre><code>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&lt;DateTime&gt;)e.NewValue, GetSpecialDayTemplate(d));
    }
  }
  
  public static ObservableCollection&lt;DateTime&gt; GetSpecialDays(DependencyObject obj)
  {
    return (ObservableCollection&lt;DateTime&gt;)obj.GetValue(SpecialDaysProperty);
  }
  
  public static void SetSpecialDays(DependencyObject obj, ObservableCollection&lt;DateTime&gt; value)
  {
    obj.SetValue(SpecialDaysProperty, value);
  }
  
  public static readonly DependencyProperty SpecialDaysProperty =
      DependencyProperty.RegisterAttached("SpecialDays", typeof(ObservableCollection&lt;DateTime&gt;), typeof(RadCalendarBehavior), new UIPropertyMetadata(null, OnSpecialDaysChanged));
  }
  }
  </code></pre>
  
  <p>I understand that I need to register that the collection has changed, but I am unsure how to do this within the dependency property</p>