desktop-mvvm

using CommunityToolkit.Mvvm.ComponentModel;
using CommunityToolkit.Mvvm.Input;

public sealed partial class OrdersViewModel(IOrderService orders, TimeProvider clock) : ObservableObject
{
    [ObservableProperty]
    [NotifyCanExecuteChangedFor(nameof(SaveCommand))]
    public partial string? CustomerName { get; set; }

    [ObservableProperty]
    public partial OrderRow? SelectedOrder { get; set; }

    public ObservableCollection<OrderRow> Orders { get; } = [];

    // Generates LoadCommand (IAsyncRelayCommand) + LoadCancelCommand
    [RelayCommand(IncludeCancelCommand = true)]
    private async Task LoadAsync(CancellationToken ct)
    {
        Orders.Clear();
        await foreach (var order in orders.StreamAsync(ct))
            Orders.Add(order); // continuation resumes on the UI thread
    }

    [RelayCommand(CanExecute = nameof(CanSave))]
    private Task SaveAsync(CancellationToken ct) =>
        orders.CreateAsync(CustomerName!, clock.GetUtcNow(), ct);

    private bool CanSave() => !string.IsNullOrWhiteSpace(CustomerName);

    partial void OnSelectedOrderChanged(OrderRow? value) =>
        CustomerName = value?.Customer; // generated hook, no setter boilerplate
}

public sealed partial class App : Application
{
    private readonly IHost host = CreateHost();

    private static IHost CreateHost()
    {
        var builder = Host.CreateApplicationBuilder(new HostApplicationBuilderSettings
        {
            ContentRootPath = AppContext.BaseDirectory // appsettings.json next to the exe, not the cwd
        });
        builder.Services.AddSingleton(TimeProvider.System);
        builder.Services.AddSingleton<IMessenger>(WeakReferenceMessenger.Default);
        builder.Services.AddHttpClient<IOrderService, OrderService>();
        builder.Services.AddTransient<OrdersViewModel>();
        builder.Services.AddSingleton<MainWindow>();
        return builder.Build();
    }

    protected override void OnStartup(StartupEventArgs e)
    {
        host.Start(); // starts IHostedService background work, if any
        host.Services.GetRequiredService<MainWindow>().Show();
        base.OnStartup(e);
    }

    protected override void OnExit(ExitEventArgs e)
    {
        // Process exit is the one place sync-over-async is acceptable (like Program.cs)
        host.StopAsync(TimeSpan.FromSeconds(5)).GetAwaiter().GetResult();
        host.Dispose();
        base.OnExit(e);
    }
}

public sealed partial class MainWindow : Window
{
    public MainWindow(OrdersViewModel viewModel)
    {
        InitializeComponent();
        DataContext = viewModel;
    }
}

// 1. Progress<T> captures the UI SynchronizationContext when constructed on the UI thread
[RelayCommand]
private async Task ExportAsync(CancellationToken ct)
{
    var progress = new Progress<int>(percent => ExportProgress = percent);
    await Task.Run(() => exporter.Export(Orders.ToList(), progress, ct), ct); // CPU work off the UI thread
}

// 2. A tiny abstraction for background-origin events, implemented per framework
public interface IUiDispatcher
{
    Task InvokeAsync(Action action);
}

public sealed class WpfUiDispatcher : IUiDispatcher
{
    public async Task InvokeAsync(Action action) =>
        await Application.Current.Dispatcher.InvokeAsync(action);
}

public sealed class AvaloniaUiDispatcher : IUiDispatcher
{
    public async Task InvokeAsync(Action action) =>
        await Avalonia.Threading.Dispatcher.UIThread.InvokeAsync(action);
}

public sealed partial class Navigator(IServiceProvider services) : ObservableObject
{
    [ObservableProperty]
    public partial ObservableObject? Current { get; set; }

    public void NavigateTo<TViewModel>() where TViewModel : ObservableObject =>
        Current = services.GetRequiredService<TViewModel>();
}

<!-- Shell window: the content area follows Navigator.Current -->
<ContentControl Content="{Binding Navigator.Current}" />

<!-- App resources: one DataTemplate per view model maps VM → View -->
<DataTemplate DataType="{x:Type vm:OrdersViewModel}">
    <views:OrdersView />
</DataTemplate>

public sealed record OrderSaved(Guid OrderId);

// Sender
messenger.Send(new OrderSaved(order.Id));

// Receiver
public sealed partial class DashboardViewModel : ObservableObject, IRecipient<OrderSaved>
{
    public DashboardViewModel(IMessenger messenger) => messenger.RegisterAll(this);

    public void Receive(OrderSaved message) => RefreshCommand.Execute(null);

    [RelayCommand]
    private Task RefreshAsync(CancellationToken ct) => Task.CompletedTask; // reload tiles
}

public sealed partial class CustomerFormViewModel : ObservableValidator
{
    [ObservableProperty]
    [NotifyDataErrorInfo]
    [Required, MaxLength(100)]
    public partial string? Name { get; set; }

    [RelayCommand]
    private void Submit()
    {
        ValidateAllProperties();
        if (HasErrors) return;
        // save
    }
}

[Fact]
public async Task SaveCommand_EmptyCustomer_CannotExecute()
{
    // Arrange
    var vm = new OrdersViewModel(new InMemoryOrderService(), new FakeTimeProvider());

    // Act
    vm.CustomerName = "  ";

    // Assert
    Assert.False(vm.SaveCommand.CanExecute(null));
}

// BAD — exceptions crash the app, button stays enabled, .Result can deadlock the UI thread
public ICommand LoadCommand => new RelayCommand(async () => await LoadAsync());
private void Refresh() => Orders = service.GetAllAsync().Result;

// GOOD — generated async command with cancellation and re-entrancy protection
[RelayCommand]
private async Task LoadAsync(CancellationToken ct) => await ReloadAsync(ct);

// BAD — ObservableCollection raises CollectionChanged off the UI thread → crash
await Task.Run(() => { foreach (var o in Fetch()) Orders.Add(o); });

// GOOD — compute in the background, mutate on the UI thread
var rows = await Task.Run(() => Fetch().ToList(), ct);
foreach (var row in rows) Orders.Add(row);

// BAD — untestable, ties the VM to WPF
if (MessageBox.Show("Delete?", "", MessageBoxButton.YesNo) == MessageBoxResult.Yes) { }

// GOOD — an interface the view layer implements
public interface IDialogService
{
    Task<bool> ConfirmAsync(string title, string message);
}
if (await dialogs.ConfirmAsync("Delete order", $"Delete {SelectedOrder!.Number}?")) { }

// BAD — 8 lines per property, typo-prone string names
private string? name;
public string? Name
{
    get => name;
    set { if (name != value) { name = value; OnPropertyChanged(nameof(Name)); } }
}

// GOOD
[ObservableProperty]
public partial string? Name { get; set; }

// BAD — click handler talks to services; nothing is testable
private async void SaveButton_Click(object sender, RoutedEventArgs e) =>
    await new OrderService().SaveAsync(NameBox.Text);

// GOOD — the view binds to a command; the view model owns behavior
// <Button Content="Save" Command="{Binding SaveCommand}" />