void OnElementsAddedToGroup(Group graphGroup, IEnumerable <GraphElement> element)
        {
            var groupData = graphGroup.userData as GroupData;

            if (groupData != null)
            {
                var anyChanged = false;
                foreach (var materialNodeView in element.Select(e => e).OfType <MaterialNodeView>())
                {
                    if (materialNodeView.node.groupGuid != groupData.guid)
                    {
                        anyChanged = true;
                        break;
                    }
                }

                if (!anyChanged)
                {
                    return;
                }

                m_Graph.owner.RegisterCompleteObjectUndo(groupData.title);

                foreach (var materialNodeView in element.Select(e => e).OfType <MaterialNodeView>())
                {
                    m_Graph.SetNodeGroup(materialNodeView.node, groupData);
                }
            }
        }