title: WPF TreeView databinding to hide/show expand/collapse icon
body: <p>I implemented a WPF load-on-demand treeview like described in <a href="http://www.codeproject.com/KB/WPF/TreeViewWithViewModel.aspx" rel="noreferrer">this</a> (very good) article.
  In the mentioned solution a dummy element is used to preserve the expand <code>+</code> icon / treeview item behavior. The dummy item is replaced with real data, when the user clicks on the expander.</p>
  
  <p>I want to refine the model by adding a property <code>public bool HasChildren { get { ... } }</code> to my backing <code>TreeNodeViewModel</code>.</p>
  
  <p><strong>Question:</strong><br>
  How can I bind this property to hide/show the expand icon (in XAML)? I am not able to find a suitable trigger/setter combination.<br>
  (INotifyPropertyChanged is properly implemented.)</p>
  
  <p>Thanks for your time.</p>
  
  <p>Update 1:<br>
  I want to use my property <code>public bool HasChildren</code> <em>instead</em> of using the dummy element.<br>
  Determining whether or not an item has children is somewhat costly, but still much cheaper than fetching the children.</p>