void grpDiff_DataContextChanged(object sender, DependencyPropertyChangedEventArgs e)
{
var service = new MaterialService(User);
var other = e.NewValue as SiteExplorerNodeViewModel;
if (other != null) {
var differences = service.CompareSites(Destination.ElemID, other.ElemID);
lvw.ItemsSource = new ObservableCollection<SiteDifferenceViewModel>(differences.ConvertAll((m) => {
return new SiteDifferenceViewModel(m);
}));
}
}