gistfile1.cs

 public class ShellViewModel : Conductor<Screen>.Collection.OneActive
    {

        public int ScreenCount
        {
            get { return this.Items.Count; }
        }
        
        public ShellViewModel()
        {
        
        }


        public void NewScreen()
        {
            ActivateItem(new RandomScreenViewModel());
        }

        public override void ActivateItem(Screen item)
        {
            base.ActivateItem(item);
            NotifyOfPropertyChange(() => ScreenCount);
        }
    }