unity-ecs-patterns

What this skill does

# Unity ECS Patterns

Production patterns for Unity's Data-Oriented Technology Stack (DOTS) including Entity Component System, Job System, and Burst Compiler.

## When to Use This Skill

- Building high-performance Unity games
- Managing thousands of entities efficiently
- Implementing data-oriented game systems
- Optimizing CPU-bound game logic
- Converting OOP game code to ECS
- Using Jobs and Burst for parallelization

## Core Concepts

### 1. ECS vs OOP

| Aspect      | Traditional OOP   | ECS/DOTS        |
| ----------- | ----------------- | --------------- |
| Data layout | Object-oriented   | Data-oriented   |
| Memory      | Scattered         | Contiguous      |
| Processing  | Per-object        | Batched         |
| Scaling     | Poor with count   | Linear scaling  |
| Best for    | Complex behaviors | Mass simulation |

### 2. DOTS Components

```
Entity: Lightweight ID (no data)
Component: Pure data (no behavior)
System: Logic that processes components
World: Container for entities
Archetype: Unique combination of components
Chunk: Memory block for same-archetype entities
```

## Patterns

### Pattern 1: Basic ECS Setup

```csharp
using Unity.Entities;
using Unity.Mathematics;
using Unity.Transforms;
using Unity.Burst;
using Unity.Collections;

// Component: Pure data, no methods
public struct Speed : IComponentData
{
    public float Value;
}

public struct Health : IComponentData
{
    public float Current;
    public float Max;
}

public struct Target : IComponentData
{
    public Entity Value;
}

// Tag component (zero-size marker)
public struct EnemyTag : IComponentData { }
public struct PlayerTag : IComponentData { }

// Buffer component (variable-size array)
[InternalBufferCapacity(8)]
public struct InventoryItem : IBufferElementData
{
    public int ItemId;
    public int Quantity;
}

// Shared component (grouped entities)
public struct TeamId : ISharedComponentData
{
    public int Value;
}
```

### Pattern 2: Systems with ISystem (Recommended)

```csharp
using Unity.Entities;
using Unity.Transforms;
using Unity.Mathematics;
using Unity.Burst;

// ISystem: Unmanaged, Burst-compatible, highest performance
[BurstCompile]
public partial struct MovementSystem : ISystem
{
    [BurstCompile]
    public void OnCreate(ref SystemState state)
    {
        // Require components before system runs
        state.RequireForUpdate<Speed>();
    }

    [BurstCompile]
    public void OnUpdate(ref SystemState state)
    {
        float deltaTime = SystemAPI.Time.DeltaTime;

        // Simple foreach - auto-generates job
        foreach (var (transform, speed) in
            SystemAPI.Query<RefRW<LocalTransform>, RefRO<Speed>>())
        {
            transform.ValueRW.Position +=
                new float3(0, 0, speed.ValueRO.Value * deltaTime);
        }
    }

    [BurstCompile]
    public void OnDestroy(ref SystemState state) { }
}

// With explicit job for more control
[BurstCompile]
public partial struct MovementJobSystem : ISystem
{
    [BurstCompile]
    public void OnUpdate(ref SystemState state)
    {
        var job = new MoveJob
        {
            DeltaTime = SystemAPI.Time.DeltaTime
        };

        state.Dependency = job.ScheduleParallel(state.Dependency);
    }
}

[BurstCompile]
public partial struct MoveJob : IJobEntity
{
    public float DeltaTime;

    void Execute(ref LocalTransform transform, in Speed speed)
    {
        transform.Position += new float3(0, 0, speed.Value * DeltaTime);
    }
}
```

### Pattern 3: Entity Queries

```csharp
[BurstCompile]
public partial struct QueryExamplesSystem : ISystem
{
    private EntityQuery _enemyQuery;

    public void OnCreate(ref SystemState state)
    {
        // Build query manually for complex cases
        _enemyQuery = new EntityQueryBuilder(Allocator.Temp)
            .WithAll<EnemyTag, Health, LocalTransform>()
            .WithNone<Dead>()
            .WithOptions(EntityQueryOptions.FilterWriteGroup)
            .Build(ref state);
    }

    [BurstCompile]
    public void OnUpdate(ref SystemState state)
    {
        // SystemAPI.Query - simplest approach
        foreach (var (health, entity) in
            SystemAPI.Query<RefRW<Health>>()
                .WithAll<EnemyTag>()
                .WithEntityAccess())
        {
            if (health.ValueRO.Current <= 0)
            {
                // Mark for destruction
                SystemAPI.GetSingleton<EndSimulationEntityCommandBufferSystem.Singleton>()
                    .CreateCommandBuffer(state.WorldUnmanaged)
                    .DestroyEntity(entity);
            }
        }

        // Get count
        int enemyCount = _enemyQuery.CalculateEntityCount();

        // Get all entities
        var enemies = _enemyQuery.ToEntityArray(Allocator.Temp);

        // Get component arrays
        var healths = _enemyQuery.ToComponentDataArray<Health>(Allocator.Temp);
    }
}
```

### Pattern 4: Entity Command Buffers (Structural Changes)

```csharp
// Structural changes (create/destroy/add/remove) require command buffers
[BurstCompile]
[UpdateInGroup(typeof(SimulationSystemGroup))]
public partial struct SpawnSystem : ISystem
{
    [BurstCompile]
    public void OnUpdate(ref SystemState state)
    {
        var ecbSingleton = SystemAPI.GetSingleton<BeginSimulationEntityCommandBufferSystem.Singleton>();
        var ecb = ecbSingleton.CreateCommandBuffer(state.WorldUnmanaged);

        foreach (var (spawner, transform) in
            SystemAPI.Query<RefRW<Spawner>, RefRO<LocalTransform>>())
        {
            spawner.ValueRW.Timer -= SystemAPI.Time.DeltaTime;

            if (spawner.ValueRO.Timer <= 0)
            {
                spawner.ValueRW.Timer = spawner.ValueRO.Interval;

                // Create entity (deferred until sync point)
                Entity newEntity = ecb.Instantiate(spawner.ValueRO.Prefab);

                // Set component values
                ecb.SetComponent(newEntity, new LocalTransform
                {
                    Position = transform.ValueRO.Position,
                    Rotation = quaternion.identity,
                    Scale = 1f
                });

                // Add component
                ecb.AddComponent(newEntity, new Speed { Value = 5f });
            }
        }
    }
}

// Parallel ECB usage
[BurstCompile]
public partial struct ParallelSpawnJob : IJobEntity
{
    public EntityCommandBuffer.ParallelWriter ECB;

    void Execute([EntityIndexInQuery] int index, in Spawner spawner)
    {
        Entity e = ECB.Instantiate(index, spawner.Prefab);
        ECB.AddComponent(index, e, new Speed { Value = 5f });
    }
}
```

### Pattern 5: Aspect (Grouping Components)

```csharp
using Unity.Entities;
using Unity.Transforms;
using Unity.Mathematics;

// Aspect: Groups related components for cleaner code
public readonly partial struct CharacterAspect : IAspect
{
    public readonly Entity Entity;

    private readonly RefRW<LocalTransform> _transform;
    private readonly RefRO<Speed> _speed;
    private readonly RefRW<Health> _health;

    // Optional component
    [Optional]
    private readonly RefRO<Shield> _shield;

    // Buffer
    private readonly DynamicBuffer<InventoryItem> _inventory;

    public float3 Position
    {
        get => _transform.ValueRO.Position;
        set => _transform.ValueRW.Position = value;
    }

    public float CurrentHealth => _health.ValueRO.Current;
    public float MaxHealth => _health.ValueRO.Max;
    public float MoveSpeed => _speed.ValueRO.Value;

    public bool HasShield => _shield.IsValid;
    public float ShieldAmount => HasShield ? _shield.ValueRO.Amount : 0f;

    public void TakeDamage(float amount)
    {
        float remaining = amount;

        if (HasShield && _shield.ValueRO.Amount > 0)
        {
            // Shield absorbs damage first
            remaining = math.max(0, amount - _shield.ValueRO.Amount);
        }

        _health.ValueRW.Current = math.max(0, _health.ValueRO.Current - remaining);
    }