title: Binding.SourceUpdated not firing
body: <p>I have a binding that works. The GUI updates. </p>
  
  <pre><code>&lt;DataGridTextColumn Binding="{Binding Path=Value, 
                                NotifyOnTargetUpdated=True, 
                                NotifyOnSourceUpdated=True, 
                                Mode=OneWay}"/&gt;
  </code></pre>
  
  <p>The binding source object implements INotifyPropertyChanged and the binding property is a normal clr property.</p>
  
  <p>The <code>Binding.TargetUpdated</code> event fires, but not the <code>Binding.SourceUpdated</code> event.</p>
  
  <p>For example, the following works</p>
  
  <pre><code>&lt;EventTrigger RoutedEvent="Binding.TargetUpdated"&gt; ...
  </code></pre>
  
  <p>but </p>
  
  <pre><code>&lt;EventTrigger RoutedEvent="Binding.SourceUpdated"&gt; ...
  </code></pre>
  
  <p>does not.</p>
  
  <p>It makes no difference if I create the binding in code or XAML, use <code>EventTrigger</code> in the <code>Triggers</code> collection or an <code>EventSetter</code>. As far as I can tell the event is just not firing.</p>
  
  <p>Does anyone know how to make <code>Binding.SourceUpdated</code> fire?</p>