Detecting grouped header key in a Listview with FastScroll

Is there any way to detect the grouped header key when i released the bar of FastScroll in a Listview?

This is my scenario:

Im using a grid with pics of employees and beside of it I have a listview with the Android "FastScroll" feature enabled. I need to swipe over the grouped Listview (bar) and when i released it, get the current key to show a "check" image over the employees pics. Searching about how to get it i found that a ViewCell is needed, can anyone explain to me how to do that?

My code:

(viewcell)

    public class GroupHeaderCell : ViewCell
        {
            public GroupHeaderCell ()
            {
                Height = 40;

                var groupKey = new Label
                {
                    FontSize = Device.GetNamedSize(NamedSize.Medium, typeof(Label)),
                    TextColor = Color.White,
                    VerticalOptions = LayoutOptions.Center
                };
                groupKey.SetBinding(Label.TextProperty, new Binding("Key"));

                StackLayout View = new StackLayout
                {
                    HorizontalOptions = LayoutOptions.FillAndExpand,
                    HeightRequest = 40,
                    BackgroundColor = Color.FromRgb(255, 153, 0),
                    Padding = 5,
                    Orientation = StackOrientation.Horizontal,
                    Children = { groupKey }
                };

                var tapGestureRecognizer = new TapGestureRecognizer();
                tapGestureRecognizer.Tapped += (s, e) => {
                    var sender = s;
                };
                View.GestureRecognizers.Add(tapGestureRecognizer);

            }
    }


(xaml.cs)
         private void CreateIndexList()
                {
                var template = new DataTemplate(typeof(TextCell));
                    template.SetBinding(TextCell.TextProperty, ".");
                    IndexList.ItemTemplate = template;
                    IndexList.GroupHeaderTemplate = new DataTemplate(typeof(GroupHeaderCell));
                    IndexList.Margin = new Thickness(0, 0, 0, 0);
                    IndexList.IsGroupingEnabled = true;
                    IndexList.GroupDisplayBinding = new Binding("Key");
                    IndexList.GroupShortNameBinding = new Binding("Key");
                    IndexList.On<Xamarin.Forms.PlatformConfiguration.Android>().SetIsFastScrollEnabled(true);
                    var source = new List<string>();
                    for (int count = 200; count > 0; --count)
                    {
                        source.Add(RandomString.Generate(5));
                    }
                    IndexList.ItemsSource = new List<IGrouping<string, string>>(
                        source.OrderBy(x => x).GroupBy(x => x[0].ToString().ToUpper())
                    );
                }

Im using this url as reference https://github.com/xamarin/Xamarin.Forms/issues/1481

I don't really need the items from the listview, the grouped header keys only to detect second names and check pics.

Any help is welcome.