public NodeGroup(VplControl hostCanvas)
: base(hostCanvas)
{
Id = Interlocked.Increment(ref counter);
Border.MouseDown += HitTestBorder_MouseDown;
Border.MouseUp += Border_MouseUp;
Border.Visibility = Visibility.Visible;
Border.Style = FindResource("VplGroupBorderStyle") as Style;
HostCanvas.NodeGroupCollection.Add(this);
SetZIndex(this, 1);
SetZIndex(Border, 1);
}