title: UpdateSourceTrigger PropertyChanged on IsChecked Isn't Firing on ItemsSource of ListBox of Checkboxes
body: <p>I have a listbox in which the item source contains a List(of T) that has a SelectedFlag boolean property.  My viewmodel is set as the DataContext of my user control and everything is working as expected except I can't get the property change even when a check box is changed.</p>
  
  <p>Here is my xaml ListBox</p>
  
  <pre><code>&lt;ListBox x:Name="lstRole" ItemsSource="{Binding Path=FAccountFunctions, Mode=TwoWay, UpdateSourceTrigger=PropertyChanged}" SelectedValuePath="Id"&gt;
                  &lt;ListBox.ItemTemplate&gt;
                      &lt;DataTemplate&gt;
                          &lt;StackPanel Orientation="Horizontal"&gt;
                              &lt;CheckBox IsChecked="{Binding Path=SelectedFlag, Mode=TwoWay, UpdateSourceTrigger=PropertyChanged}" VerticalAlignment="Center" /&gt;
                              &lt;TextBlock Text="{Binding Path=FunctionDesc}" VerticalAlignment="Center" /&gt;
                          &lt;/StackPanel&gt;
                      &lt;/DataTemplate&gt;
                  &lt;/ListBox.ItemTemplate&gt;
              &lt;/ListBox&gt;
  </code></pre>
  
  <p>I need to call my Filter() function after a check box is checked and I normally would set the UpdateSourcTrigger=PropertyChanged to make this work.</p>
  
  <pre><code>Public Property FAccountFunctions As List(Of FunctionType)
          Get
              Return _faccountFunctions
          End Get
          Set(ByVal value As List(Of FunctionType))
              _faccountFunctions = value
              Filter()
          End Set
      End Property
  </code></pre>
  
  <p>The PropertyChangedEvent is getting raised on the 'SelectedFlag' property in the FAccountFunctions collection.  How can I raise an event on the items source when one of the properties SelectedFlag changes?</p>
  
  <p>Changed my FAccountFunctions property to an ObservableCollection...no luck.</p>