//returns subcategories that should be available through the tree
public override ICollection<ShopCategory>
                           GetChildItems(ShopCategory parent)
{
  //create a filtered list
  List<ShopCategory> list = new List<ShopCategory>();
  foreach(ShopCategory category in parent.SubCategories)
  {
    if ( ... ) list.Add(category);
  }
  
  return list;
}

private Predicate<ShopCategory> filter = null;


/// <summary>
/// Defines a filter for items that are bound to the tree. Set to
/// null in order to disable filtering.
/// </summary>
public Predicate<ShopCategory> Filter
{
  get { return filter; }
  set
  {
    filter = value;

    //recreate the tree in order to apply the filter on
    //all currently visible nodes
    //-> of course, this could be optimized, but it does the job
    Refresh(GetTreeLayout());
  }
}


/// <summary>
/// Applies the filter on all child nodes.
/// </summary>
/// <param name="treeNode"></param>
protected override void OnNodeExpanded(TreeViewItem treeNode)
{
  //make sure child nodes are being created
  base.OnNodeExpanded(treeNode);

  //apply filter
  foreach (TreeViewItem childNode in treeNode.Items)
  {
    ApplyFilter(childNode, (ShopCategory)childNode.Header);
  }
}


/// <summary>
/// Immediately applies the filter on newly created items. This
/// is somewhat redundant (as we're also handling <see cref="OnNodeExpanded"/>),
/// but ensures we also consider root nodes.
/// </summary>
/// <param name="item"></param>
/// <returns></returns>
protected override TreeViewItem CreateTreeViewItem(ShopCategory item)
{
  //delegate node creation to base class
  TreeViewItem node = base.CreateTreeViewItem(item);

  //apply the filter and return the node
  ApplyFilter(node, item);
  return node;
}



/// <summary>
/// Filters categories if the <see cref="Filter"/> property
/// is set by simply setting the <see cref="TreeViewItem.Visibility"/>
/// property to <see cref="Visibility.Collapsed"/> if the item does
/// not match the filter.
/// </summary>
private void ApplyFilter(TreeViewItem node, ShopCategory item)
{
  bool visible = filter == null || filter(item);
  node.Visibility = visible ? Visibility.Visible : Visibility.Collapsed;
}