I have a List that I've bound to a TreeView. Setting TreeView.DataContext works - everything displays correctly. I then change the list (add an item to it) and set TreeView.DataContext again (to the same value) but the tree does not refresh with the new items. How do I get the treeview to refresh?

This is basically my code:

public class xItemCollection : ObservableCollection<xItem>
{
}

public class xItem : INotifyPropertyChanged
{
    xItemCollection _Items;

    public xItem()
    {
        _Items = new xItemCollection();
    }

    public xItemCollection Items {get{return _Items;}}
}

...
this.RootItem = new xItem();
treeView.DataContext = this;

Adding items to the list works until the tree is rendered for the first time. After it is rendered, adding/removing items does not refresh the tree.