public override void OnApplyTemplate()
        {
            base.OnApplyTemplate();

            tvDnd1.DataContext = new NumberTreeViewModel("TreeView");
            lvDnd1.DataContext = NumberListViewModel.GenerateRange("VirtualStackPanel", 1, 20);
            lvDnd2.DataContext = NumberListViewModel.GenerateRange("VirtualWrapPanel", 21, 40);
            lvDnd3.DataContext = NumberListViewModel.GenerateRange("GridView", 41, 60);
            lvDnd4.DataContext = NumberListViewModel.GenerateRange("StackPanel", 61, 80);
        }
        public static NumberListViewModel GenerateRange(string displayName, int start, int end)
        {
            NumberListViewModel nvm = new NumberListViewModel(displayName);

            for (int i = start; i <= end; i++)
            {
                nvm.Items.Add(new NumberViewModel(i));
            }
            return(nvm);
        }