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name: app-ui-mvvm description: "Expert for App UI MVVM pattern and dependency injection - ObservableObject, RelayCommand, AppBuilder, service registration, and DI. Use this skill whenever the user wants to create a ViewModel, bind data to UI elements, set up dependency injection, register services, implement commands for button actions, use [ObservableProperty] or [RelayCommand] attributes, create an AppBuilder, or structure their Unity App UI project with the MVVM architecture pattern. Also trigger when the user asks about data binding in UXML, property change notifications, or service lifetimes (transient, singleton, scoped)." allowed-tools: Bash, Read, Write, Edit, Glob, Grep

App UI MVVM & Dependency Injection Expert

Overview

The MVVM (Model-View-ViewModel) pattern is a design pattern that separates user interface from business logic. Combined with Dependency Injection (DI), it creates a powerful architecture for building maintainable, testable, and scalable applications using Unity's UI Toolkit.

When to Use MVVM

Use MVVM when you need to:

• Separate UI concerns from business logic
• Make your code testable and maintainable
• Bind UI elements to data that changes dynamically
• Handle complex user interactions with clean code organization
• Enable designers and developers to work independently

Core Concepts

Model

The Model is a simple data structure containing the data used by business logic. It has no knowledge of the UI.

ViewModel

The ViewModel contains business logic and is responsible for:

• Updating the Model when the user interacts with the View
• Notifying the View when the Model changes (via property notifications)
• Handling commands (user actions like button clicks)
• Managing state and orchestrating operations

ViewModels connect to Views through data binding.

View

The View is the UI layer responsible for displaying data to the user. It should contain:

• Only UI element definitions and styling logic
• Event handlers that invoke ViewModel commands
• Data bindings to ViewModel properties

Views should never contain business logic.

Key Namespaces

• Unity.AppUI.MVVM - Core MVVM classes and DI
• Unity.Properties - Property attributes and binding support

MVVM + Dependency Injection Architecture

┌─────────────────────────────────────────────┐
│            UIToolkitAppBuilder              │
│  (Configures app and services)              │
└──────────────────┬──────────────────────────┘
                   │
        ┌──────────┴──────────┐
        │                     │
        v                     v
   ┌─────────┐         ┌──────────────┐
   │   App   │         │ Service      │
   │         │         │ Collection   │
   └────┬────┘         └──────────────┘
        │
        v
   ┌──────────────────┐
   │   Views (UI)     │◄──── Data Binding ◄─── ViewModels
   │                  │      Commands           (Observable,
   └──────────────────┘                         RelayCommand)

Common Patterns

1. Observable Properties

Mark ViewModel properties with [ObservableProperty] to automatically generate property change notifications:

[ObservableObject]
public partial class MyViewModel
{
    [ObservableProperty]
    private string _name;
}

2. Relay Commands

Use [RelayCommand] attributes to auto-generate command properties:

[ObservableObject]
public partial class MyViewModel
{
    [RelayCommand]
    private void SaveData()
    {
        // Execute command logic
    }
}

3. Service Registration

Register services in AppBuilder.OnConfiguringApp():

appBuilder.services.AddSingleton<IMyService, MyService>();
appBuilder.services.AddTransient<MyViewModel>();

4. Service Injection

Inject services via constructor or [Service] attribute:

[ObservableObject]
public partial class MyViewModel
{
    [Service]
    public IMyService MyService { get; set; }
}

5. Dependent Properties

Use [AlsoNotifyChangeFor] to notify dependent properties:

[ObservableObject]
public partial class MyViewModel
{
    [ObservableProperty]
    [AlsoNotifyChangeFor(nameof(IsNameEmpty))]
    private string _name;

    public bool IsNameEmpty => string.IsNullOrEmpty(Name);
}

Best Practices

1. Class Declaration: Classes with [ObservableObject] must be partial
2. Field Naming: Observable fields must be private with underscore prefix (_fieldName)
3. Constructor Injection: Prefer constructor injection for required dependencies
4. Property Injection: Use [Service] attribute for optional dependencies
5. Command Naming: Method names for [RelayCommand] should describe the action (e.g., SaveData, DeleteItem)
6. Separation of Concerns: Keep business logic in ViewModels, UI logic in Views
7. Service Lifetimes:
   • Use AddSingleton for stateless services (loggers, factories)
   • Use AddTransient for stateful services (ViewModels, Pages)
   • Use AddScoped for services tied to a specific scope

Command Patterns

Simple Commands

[RelayCommand]
void DoSomething() { }

Commands with Parameters

[RelayCommand]
void DoSomethingWith(string parameter) { }

Async Commands

[RelayCommand]
async Task DoSomethingAsync()
{
    await Task.Delay(1000);
}

Conditional Commands

[RelayCommand(CanExecute = nameof(CanSave))]
void Save() { }

private bool CanSave => !string.IsNullOrEmpty(Name);

Dependency Injection Lifetimes

• Transient: New instance every time it's requested
• Singleton: Same instance for the entire application lifetime
• Scoped: Same instance within a scope, new instance for different scopes

UI Binding

ViewModels expose observable properties and commands that bind to UI elements:

<ui:Button title="Save">
    <Bindings>
        <ui:DataBinding property="clickable.command" binding-mode="ToTarget" data-source-path="SaveCommand"/>
        <ui:DataBinding property="enabledSelf" binding-mode="ToTarget" data-source-path="CanSave"/>
    </Bindings>
</ui:Button>

Related Features

• State Management: Redux-like pattern for complex state
• Navigation: Structured page navigation with parameters
• Data Binding: Automatic UI updates from ViewModel changes
• Commands: User action handling via RelayCommand

Reference Documentation

Consult reference.md when you need exact API signatures for ObservableObject, RelayCommand, AppBuilder, or service container methods, full attribute parameter lists, advanced DI patterns like scoped lifetimes, or details on IDependencyInjectionListener and [CreateProperty]. See examples/ for complete MVVM app setups with multiple ViewModels and services.