void ListViewTapped(object sender, ItemTappedEventArgs e)
        {
            var list = sender as ListView;
            if (list == null)
                return;
            list.SelectedItem = null;
        }

        protected override void OnAppearing()
        {
            base.OnAppearing();

            lvwUsers.ItemTapped += ListViewTapped;

        }

        protected override void OnDisappearing()
        {
            base.OnDisappearing();
            lvwUsers.ItemTapped -= ListViewTapped;
        }