Adding a page

A real task to walk the conventions: add a "nozzle vacuum debug" page.

1. ViewModel

// 07.PreciSim.App/ViewModels/VacuumDebugViewModel.cs
public partial class VacuumDebugViewModel : PageViewModelBase
{
    private readonly IDigitalIo _io;
 
    public VacuumDebugViewModel(IDigitalIo io, ILogger<VacuumDebugViewModel> logger)
        : base(logger)
    {
        _io = io;
    }
 
    [ObservableProperty] private bool _vacuumOn;
    [ObservableProperty] private bool _partDetected;
 
    [RelayCommand(CanExecute = nameof(CanToggle))]
    private async Task ToggleVacuumAsync(CancellationToken ct)
    {
        await _io.WriteAsync(IoMap.VacuumValve, !VacuumOn);
        VacuumOn = !VacuumOn;
    }
 
    private bool CanToggle() => !IsBusy && ConnectionState == DeviceState.Connected;
 
    protected override void OnActivated()
    {
        // subscribe only while the page is visible; the base class unsubscribes on leave
        Subscribe(_io.Changes, OnIoChanged);
    }
 
    private void OnIoChanged(IoChange c)
    {
        if (c.Channel == IoMap.PartSensor) PartDetected = c.Value;
    }
}

Points:

CommunityToolkit.Mvvm source generators — no hand-written INotifyPropertyChanged;
commands carry CanExecute, which drives button state — no IsEnabled logic in the view;
subscribe through the base class helper. Forgetting to unsubscribe is the number one cause of WPF memory leaks.

2. View

<!-- 07.PreciSim.App/Views/VacuumDebugView.xaml -->
<UserControl x:Class="PreciSim.App.Views.VacuumDebugView"
             d:DataContext="{d:DesignInstance vm:VacuumDebugViewModel}">
  <StackPanel Margin="16">
    <ToggleButton Content="{DynamicResource Page.Vacuum}"
                  IsChecked="{Binding VacuumOn, Mode=OneWay}"
                  Command="{Binding ToggleVacuumCommand}" />
    <TextBlock Text="{Binding PartDetected, Converter={StaticResource BoolToPresence}}" />
  </StackPanel>
</UserControl>

Plain {Binding} — no binding DSL. The code-behind contains InitializeComponent() and nothing else.

3. Registration

// 07.PreciSim.App/Startup/ViewRegistration.cs
services.AddTransient<VacuumDebugViewModel>();
services.AddTransient<VacuumDebugView>();
 
// 07.PreciSim.App/Navigation/PageCatalog.cs
new PageEntry(
    id: "debug.vacuum",
    titleKey: "Page.VacuumDebug",
    icon: Icons.Vacuum,
    requiredRole: UserRole.Engineer,      // permissions declared here, not checked inside the page
    viewModel: typeof(VacuumDebugViewModel))

4. Strings

<!-- Resources/Strings.en.resx -->
<data name="Page.VacuumDebug"><value>Vacuum debug</value></data>
<!-- Resources/Strings.zh-CN.resx -->
<data name="Page.VacuumDebug"><value>真空调试</value></data>

Both resx files. A CI test compares the key sets and fails if one is missing — the same idea as the bilingual dictionary on this website.

5. Test

[Fact]
public async Task ToggleVacuum_WritesToIo()
{
    var io = new FakeDigitalIo();
    var vm = new VacuumDebugViewModel(io, NullLogger<VacuumDebugViewModel>.Instance);
 
    await vm.ToggleVacuumCommand.ExecuteAsync(null);
 
    Assert.True(io.LastWrite(IoMap.VacuumValve));
    Assert.True(vm.VacuumOn);
}

The ViewModel has no WPF dependency, so this runs instantly and needs no UI thread. That is the practical payoff of MVVM, not architectural tidiness.

Checklist

No Dispatcher or MessageBox in the ViewModel (dialogs go through IDialogService)
View code-behind is only InitializeComponent()
Both resx files updated
The page declares requiredRole
One unit test that does not touch the UI

1. ViewModel#

2. View#

3. Registration#

4. Strings#