title: MVVM wpf TreeView how to handle Expanded event
body: <p>I'm implementing a delayed load treeview, and need to handle the Expanded event.  I would like to do it directly in the ViewModel.  I have hooked up an EventTrigger:  </p>
  
  <pre><code>&lt;i:Interaction.Triggers&gt;
      &lt;i:EventTrigger EventName="Expanded"&gt;
          &lt;interactivity:InvokeCommandAction Command="{Binding HandleExpandCmd}" TriggerParameterPath="OriginalSource"  /&gt;
      &lt;/i:EventTrigger&gt;  
  &lt;/i:Interaction.Triggers&gt;
  </code></pre>
  
  <p>While this works as expected, the OriginalSource is a TreeViewItem, and I would prefer to <em>not</em> include System.Windows.Controls in my ViewModel.<br>
  Alternatively, I could handle the event in code-behind, and pass the TreeViewItems's DataContext to the ViewModel.  (The DataContext, of course, is what the ViewModel needs to work with.)<br>
  Is there a syntax I can use in the TriggerParameterPath that passes the TVI's DataContext, instead of the TVI itself?<br>
  Thanks ---</p>