WPF-Demo/WpfSample/ViewModels/MainViewModel.cs

using System.Collections.ObjectModel;
using CommunityToolkit.Mvvm.ComponentModel;
using CommunityToolkit.Mvvm.Input;

namespace WpfSample.ViewModels {
    internal partial class MainViewModel: ObservableObject {
        public MainViewModel() {
            Random random = new Random();
            for (int i = random.Next(1, 10); i >= 0; --i) {
                Groups.Add(new DemoGroupViewModel() {
                    Name = $"G{i}",
                });
            }
        }

        [ObservableProperty]
        private string message = string.Empty;

        public ObservableCollection<DemoGroupViewModel> Groups { get; set; } = new ObservableCollection<DemoGroupViewModel>();

        class Test {
            public string Name { get; set; } = string.Empty;
        }

        [RelayCommand]
        private void AddItem() {
            Groups[0].AddItemCommand?.Execute(null);
        }
    }
}