Unity R3 - Modern Reactive Extensions for Unity

Overview

R3 is a modern, high-performance Reactive Extensions library for Unity developed by Cysharp (same author as UniTask), providing observable streams and reactive patterns optimized for Unity.

Library: R3 by Cysharp

R3 vs UniRx: R3 is the modern successor to UniRx with better performance, async enumerable support, and Unity 2022+ optimization. For legacy UniRx projects, see unity-unirx skill.

Foundation Required: unity-csharp-fundamentals (TryGetComponent, FindAnyObjectByType), csharp-async-patterns (async fundamentals), unity-async (Unity context)

Core Topics:

• Observable sequences and observers
• Reactive operators and transformations
• Hot vs Cold observables
• ReactiveProperty for state management
• Event-driven architecture patterns
• MVVM/MVP implementation
• UI event handling and data binding

Learning Path: C# events → Reactive patterns → Observable composition → MVVM architecture

Quick Start

Basic Observable Patterns

using R3;

// Create observable from events
button.OnClickAsObservable()
    .Subscribe(_ => Debug.Log("Button clicked!"))
    .AddTo(this);

// Property observation
this.ObserveEveryValueChanged(x => x.transform.position)
    .Subscribe(pos => Debug.Log($"Position: {pos}"))
    .AddTo(this);

// Time-based observables
Observable.Interval(TimeSpan.FromSeconds(1))
    .Subscribe(x => Debug.Log($"Tick: {x}"))
    .AddTo(this);

ReactiveProperty

// Reactive state management
public class Player : MonoBehaviour
{
    public ReactiveProperty<int> Health { get; } = new(100);
    public ReadOnlyReactiveProperty<bool> IsDead { get; }

    public Player()
    {
        IsDead = Health.Select(h => h <= 0).ToReadOnlyReactiveProperty();
    }
}

When to Use

Unity Reactive (This Skill)

• Event-driven architecture and complex event handling
• MVVM/MVP pattern implementation
• UI data binding and reactive state
• Asynchronous event streams
• Complex state management
• Real-time data flow coordination

Alternatives

• unity-unirx: Legacy UniRx library (pre-2022 projects)
• unity-async/unity-unitask: Single async operations, not event streams
• C# events: Simple event handling without composition

R3-Specific Features

• Async enumerable (IAsyncEnumerable<T>) integration
• Better performance than UniRx
• Unity 2022+ optimization
• Struct-based observers for zero allocation
• Built-in time providers for testing

Reference Documentation

Reactive Fundamentals

Core R3 concepts:

• Observable creation patterns
• Hot vs Cold observables
• Subscription lifecycle
• Marble diagrams
• Basic operators (Select, Where, DistinctUntilChanged)

Reactive Operators

Transformation and composition:

• Filtering operators (Where, Throttle, Debounce)
• Transformation operators (Select, SelectMany)
• Combination operators (CombineLatest, Merge, Zip)
• Time operators (Delay, Timeout, Sample)
• Error handling operators (Catch, Retry)

Architecture Patterns

Application patterns:

• MVVM with ReactiveProperty
• Event Aggregator pattern
• State management systems
• UI data binding
• Message broker implementation

Key Principles

1. Declarative Event Handling: Define what should happen, not how to subscribe/unsubscribe
2. Automatic Disposal: Use AddTo(this) for MonoBehaviour lifecycle management
3. Composition over Callbacks: Chain operators instead of nested callbacks
4. Hot/Cold Awareness: Understand when observables start emitting
5. Marble Diagram Thinking: Visualize data flow over time

Common Patterns

UI Event Handling

// Button with throttle to prevent spam
button.OnClickAsObservable()
    .Throttle(TimeSpan.FromSeconds(1))
    .Subscribe(_ => OnButtonClick())
    .AddTo(this);

// Input validation
inputField.OnValueChangedAsObservable()
    .Where(text => text.Length > 3)
    .Throttle(TimeSpan.FromSeconds(0.5))
    .Subscribe(ValidateInput)
    .AddTo(this);

State Management

public class GameState : MonoBehaviour
{
    public ReactiveProperty<int> Score { get; } = new(0);
    public ReactiveProperty<int> Lives { get; } = new(3);
    public ReadOnlyReactiveProperty<bool> GameOver { get; }

    public GameState()
    {
        GameOver = Lives.Select(l => l <= 0).ToReadOnlyReactiveProperty();

        GameOver.Where(over => over)
            .Subscribe(_ => OnGameOver())
            .AddTo(this);
    }
}

Multiple Stream Combination

// Combine position and health for decision making
Observable.CombineLatest(
    playerTransform.ObserveEveryValueChanged(t => t.position),
    playerHealth.Health,
    (pos, health) => new { Position = pos, Health = health }
)
.Where(state => state.Health < 30)
.Subscribe(state => FindNearestHealthPack(state.Position))
.AddTo(this);

Integration with Other Skills

• unity-unitask: Convert observables to UniTask with ToUniTask()
• unity-vcontainer: Inject ReactiveProperty as dependencies via VContainer
• unity-ui: Bind observables to UI elements for automatic updates
• unity-async: Bridge async operations with Observable.FromAsync()
• unity-unirx: For legacy projects (not recommended for new projects)

Platform Considerations

• WebGL: Full support with frame-based timing
• Mobile: Efficient for UI and event handling
• All Platforms: Zero allocation after initial setup

Best Practices

1. Always use AddTo(): Prevent memory leaks with automatic disposal
2. Throttle/Debounce user input: Prevent excessive processing
3. Use ReactiveProperty for state: Better than manual event raising
4. Understand hot vs cold: Know when subscriptions trigger work
5. Avoid nested subscriptions: Use SelectMany for flattening
6. Test with TestScheduler: Write deterministic reactive tests
7. Consider backpressure: Handle fast producers with Sample or Buffer