title: WPF DataTrigger never fires to Expand TreeView
body: <p>I have the following XAML which should expand my whole treeview when a specific property ExpandNodes is True, but it is never triggered.</p>
  
  <pre><code>&lt;TreeView&gt;
      &lt;TreeView.ItemContainerStyle&gt;
              &lt;Style TargetType="{x:Type TreeViewItem}"&gt;
  
                  &lt;Setter Property="IsExpanded" Value="False" /&gt;
                  &lt;Style.Triggers&gt;
                      &lt;DataTrigger Binding="{Binding ExpandNodes}" Value="True"&gt;
                          &lt;Setter Property="IsExpanded" Value="True" /&gt;
                      &lt;/DataTrigger&gt;
                      &lt;DataTrigger Binding="{Binding ExpandNodes}" Value="False"&gt;
                          &lt;Setter Property="IsExpanded" Value="False" /&gt;
                      &lt;/DataTrigger&gt;
                  &lt;/Style.Triggers&gt;
  
              &lt;/Style&gt;
          &lt;/TreeView.ItemContainerStyle&gt;
  
  // some other code
  
  &lt;/TreeView&gt;
  </code></pre>
  
  <p>In my ViewModel I set ExpandNodes to True during a specific event, yet the treeview remains non expanded. It's not an issue with my DataContext as I have other properties from the same Viewmodel that are bound and work fine.</p>
  
  <p>My Viewmodel:</p>
  
  <pre><code>private bool _expandnodes;
  public bool ExpandNodes
  {
      get
      {
          return _expandnodes;
      }
      set
      {
          _expandnodes = value;
          OnPropertyChanged("ExpandNodes");
      }
  }
  
  public event PropertyChangedEventHandler PropertyChanged;
  protected void OnPropertyChanged(string PropName)
  {
      if (PropertyChanged != null)
      {
          PropertyChanged(this, new PropertyChangedEventArgs(PropName));
      }
  }
  </code></pre>