presence/Presence.Desktop/ViewModels/MainWindowViewModel.cs

using DocumentFormat.OpenXml.Bibliography;
using domain.UseCase;
using ReactiveUI;
using System;
using System.Collections.Generic;
using System.Collections.ObjectModel;
using System.Linq;
using System.Reactive.Linq;

namespace Presence.Desktop.ViewModels
{
    public class MainWindowViewModel : ViewModelBase
    {
        private readonly GroupUseCase _groupUseCase;

        private List<GroupPresenter> groupPresentersDataSource = new List<GroupPresenter>();
        private ObservableCollection<GroupPresenter> _groups;
        public ObservableCollection<GroupPresenter> Groups => _groups;

        public GroupPresenter? SelectedGroupItem
        {
            get => _selectedGroupItem;
            set => this.RaiseAndSetIfChanged(ref _selectedGroupItem, value);
        }

        private GroupPresenter? _selectedGroupItem;

        public  ObservableCollection<UserPresenter> Users { get => _users; }
        private ObservableCollection<UserPresenter> _users;

        public MainWindowViewModel(GroupUseCase groupUseCase)
        {
            _groupUseCase = groupUseCase;

            foreach (var item in _groupUseCase.GetAllGroupWithStident())
            {
                GroupPresenter groupPresenter = new GroupPresenter
                {
                    Id = item.Id,
                    Name = item.Name,
                    users = item.Users?.Select(user => new UserPresenter
                    {
                        Name = user.FIO,
                        Guid = user.Guid,
                        Group = new GroupPresenter { Id = item.Id, Name = item.Name }
                    }).ToList()
                };
                groupPresentersDataSource.Add(groupPresenter);
            }
            _groups = new ObservableCollection<GroupPresenter>(groupPresentersDataSource);
            _users = new ObservableCollection<UserPresenter>();

            this.WhenAnyValue(vm => vm.SelectedGroupItem)
                .Subscribe(vm => SetUsers());
        }

        private void SetUsers()
        {
            if (SelectedGroupItem?.users == null) return;

            Users.Clear();
            foreach (var item in SelectedGroupItem.users)
            {
                Users.Add(item);
            }
        }
    }
}