Unity Ecs Patterns

# unity-ecs-patterns — detailed patterns and worked examples

## 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 buf