private IObservable <Unit> selectCalendar(SelectableUserCalendarViewModel calendar)
        {
            if (SelectedCalendarIds.Contains(calendar.Id))
            {
                SelectedCalendarIds.Remove(calendar.Id);
            }
            else
            {
                SelectedCalendarIds.Add(calendar.Id);
            }

            calendar.Selected = !calendar.Selected;

            return(Observable.Return(Unit.Default));
        }
 private SelectableUserCalendarViewModel toSelectable(UserCalendar calendar)
 => new SelectableUserCalendarViewModel(calendar, SelectedCalendarIds.Contains(calendar.Id));