mvvm

# MVVM Pattern for .NET

## Trigger On

- implementing UI separation with Model-View-ViewModel
- using MVVM Toolkit (CommunityToolkit.Mvvm) for ViewModels
- designing testable UI architecture
- handling commands, property changes, and messaging
- choosing between MVVM frameworks

## Documentation

- [MVVM Toolkit Overview](https://learn.microsoft.com/en-us/dotnet/communitytoolkit/mvvm/)
- [ObservableObject](https://learn.microsoft.com/en-us/dotnet/communitytoolkit/mvvm/observableobject)
- [RelayCommand](https://learn.microsoft.com/en-us/dotnet/communitytoolkit/mvvm/relaycommand)
- [Messenger](https://learn.microsoft.com/en-us/dotnet/communitytoolkit/mvvm/messenger)
- [Source Generators](https://learn.microsoft.com/en-us/dotnet/communitytoolkit/mvvm/generators/overview)

## References

See detailed examples in the `references/` folder:
- [`patterns.md`](references/patterns.md) — ViewModel, command, navigation, and state patterns
- [`anti-patterns.md`](references/anti-patterns.md) — Common mistakes and how to fix them

## Core Concepts

| Component | Responsibility | Example |
|-----------|---------------|---------|
| **Model** | Business logic and data | `Product`, `Order`, `User` |
| **View** | UI presentation (XAML/Razor) | `ProductPage.xaml` |
| **ViewModel** | UI logic and state | `ProductViewModel` |

## Workflow

1. **Keep Views dumb** — no business logic in code-behind
2. **Use data binding** — connect View to ViewModel properties
3. **Commands for actions** — handle user interactions via ICommand
4. **Inject dependencies** — services go into ViewModel constructors
5. **Test ViewModels** — they should be unit testable without UI

## MVVM Toolkit Setup

```xml
<PackageReference Include="CommunityToolkit.Mvvm" Version="8.*" />
```

## ViewModel Patterns

### Basic ViewModel with Source Generators
```csharp
public partial class ProductViewModel(IProductService productService) : ObservableObject
{
    [ObservableProperty]
    private string _name = string.Empty;

    [ObservableProperty]
    private decimal _price;

    [ObservableProperty]
    [NotifyCanExecuteChangedFor(nameof(SaveCommand))]
    private bool _isValid;

    [RelayCommand(CanExecute = nameof(CanSave))]
    private async Task SaveAsync()
    {
        await productService.SaveAsync(new Product { Name = Name, Price = Price });
    }

    private bool CanSave() => IsValid && !string.IsNullOrEmpty(Name);
}
```

### Property Changed Notifications
```csharp
public partial class OrderViewModel : ObservableObject
{
    [ObservableProperty]
    private int _quantity;

    [ObservableProperty]
    private decimal _unitPrice;

    // Computed property - manually notify
    public decimal Total => Quantity * UnitPrice;

    partial void OnQuantityChanged(int value)
    {
        OnPropertyChanged(nameof(Total));
    }

    partial void OnUnitPriceChanged(decimal value)
    {
        OnPropertyChanged(nameof(Total));
    }
}
```

### Collection ViewModel
```csharp
public partial class ProductListViewModel(IProductService productService) : ObservableObject
{
    [ObservableProperty]
    private ObservableCollection<ProductViewModel> _products = [];

    [ObservableProperty]
    private ProductViewModel? _selectedProduct;

    [ObservableProperty]
    private bool _isLoading;

    [RelayCommand]
    private async Task LoadProductsAsync()
    {
        IsLoading = true;
        try
        {
            var items = await productService.GetAllAsync();
            Products = new ObservableCollection<ProductViewModel>(
                items.Select(p => new ProductViewModel(productService)
                {
                    Name = p.Name,
                    Price = p.Price
                }));
        }
        finally
        {
            IsLoading = false;
        }
    }

    [RelayCommand]
    private void DeleteProduct(ProductViewModel product)
    {
        Products.Remove(product);
    }
}
```

## Commands

### Async Commands with Cancellation
```csharp
public partial class SearchViewModel : ObservableObject
{
    [ObservableProperty]
    private string _searchText = string.Empty;

    [RelayCommand(IncludeCancelCommand = true)]
    private async Task SearchAsync(CancellationToken token)
    {
        await Task.Delay(500, token); // Debounce
        // Search logic with cancellation support
    }
}
```

### Command with Parameter
```csharp
public partial class NavigationViewModel : ObservableObject
{
    [RelayCommand]
    private void NavigateTo(string page)
    {
        // Navigate to page
    }

    [RelayCommand]
    private async Task OpenItemAsync(int itemId)
    {
        // Load and open item
    }
}
```

## Messenger Pattern

### Sending Messages
```csharp
// Define message
public record ProductSelectedMessage(Product Product);

// Send from one ViewModel
WeakReferenceMessenger.Default.Send(new ProductSelectedMessage(selectedProduct));
```

### Receiving Messages
```csharp
public partial class ProductDetailViewModel : ObservableRecipient
{
    public ProductDetailViewModel()
    {
        IsActive = true; // Enable message reception
    }

    protected override void OnActivated()
    {
        Messenger.Register<ProductDetailViewModel, ProductSelectedMessage>(
            this, (r, m) => r.LoadProduct(m.Product));
    }

    private void LoadProduct(Product product)
    {
        // Update UI with product details
    }
}
```

## Validation

### Using ObservableValidator
```csharp
public partial class RegistrationViewModel : ObservableValidator
{
    [ObservableProperty]
    [NotifyDataErrorInfo]
    [Required(ErrorMessage = "Email is required")]
    [EmailAddress(ErrorMessage = "Invalid email format")]
    private string _email = string.Empty;

    [ObservableProperty]
    [NotifyDataErrorInfo]
    [Required]
    [MinLength(8, ErrorMessage = "Password must be at least 8 characters")]
    private string _password = string.Empty;

    [RelayCommand(CanExecute = nameof(CanRegister))]
    private async Task RegisterAs