Wpf Mvvm

Overview

Apply MVVM conventions with CommunityToolkit.Mvvm, ensuring view models are UI-agnostic and ready for the widget shell architecture.

Constraints

• Target .NET 8
• CommunityToolkit.Mvvm
• ViewModel-first navigation (custom service)
• No Prism, no heavy region manager

Core conventions

Definition of done (DoD)

• ViewModel has no WPF dependencies (Window, UserControl, etc.)
• Commands use [RelayCommand] attribute from CommunityToolkit.Mvvm
• Observable properties use [ObservableProperty] attribute
• Async commands have proper cancellation support where applicable
• ViewModel behavior is covered by unit tests

ViewModel base

• Use ObservableObject (or ObservableRecipient if messaging is required).
• Keep view models free of Window, UserControl, or DependencyObject references.

Commands

• Use [RelayCommand] for commands.
• Prefer async commands for IO or long-running work.
• Use CanExecute or CanExecute backing fields for stateful enable/disable.

State and updates

• Use [ObservableProperty] for state.
• Keep state immutable when practical; update via methods.
• Avoid direct access to Application.Current inside view models.

Validation

• Use ObservableValidator for data validation.
• Prefer INotifyDataErrorInfo via toolkit attributes.

Typical workflow

1. Create or update view model using toolkit attributes.
2. Add command handlers and validation rules.
3. Bind view to view model with XAML.
4. Add DataTemplate mapping for viewmodel-first navigation.
5. Write tests for business logic and command behavior.

Example ViewModel

public partial class WidgetViewModel : ObservableObject
{
    [ObservableProperty]
    private string _title = "Widget";

    [RelayCommand]
    private void Refresh()
    {
        // Fetch or recompute widget state.
    }
}

Binding guidance

• Bind directly to VM properties with Mode=TwoWay only when needed.
• Use UpdateSourceTrigger=PropertyChanged for live edits.
• Keep converters minimal; prefer derived properties in the VM.

References

• references/viewmodel-patterns.md for structure patterns.
• references/commands-and-async.md for command and async guidance.
• references/validation.md for validation recipes.