MainPageViewModel.cs

namespace DynamicDataGroupingSample
{
    public class MainViewModel : ReactiveObject, IDisposable
    {
        public MainViewModel()
        {
            SortCommand = ReactiveCommand.CreateFromTask(ExecuteSort);
        }
        private async Task ExecuteSort()
        {
            var sort = await App.Current.MainPage.DisplayActionSheet("Sort by", "Cancel", null, buttons: new string[] { "Name", "Type" });
            if (sort != "Cancel")
            {
                SortBy = sort;
            }
        }

        private string SortBy
        {
            get => _sortBy;
            set => this.RaiseAndSetIfChanged(ref _sortBy, value);
        }
        
        public ReactiveCommand<Unit, Unit> SortCommand { get; }
        private string _sortBy;
        ...
    }
}