Unity DOTS学习记录(六)-一些常用操作

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环境：
Unity：6000.2.10f1
Entities：1.4.3

参考：
https://github.com/Unity-Technologies/EntityComponentSystemSamples

实例化预设Prefab

在DOTS中，实例化预设的Prefab通常通过Authoring组件和Baker来实现。以下是一个炮塔射击系统的示例，展示了如何在Baker中引用Prefab实体，并在Job中实例化它。

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public class TurretRotationAuthoring : MonoBehaviour
{
    public GameObject bulletPrefab;

    class TurretRotationBaker : Baker<TurretRotationAuthoring>
    {
        public override void Bake(TurretRotationAuthoring authoring)
        {
            // 该实体将被移动
            var entity = GetEntity(TransformUsageFlags.Dynamic);
            // 为实体添加组件数据
            AddComponent(entity, new TurretShootComponent()
            {
                BulletPrefab = GetEntity(authoring.bulletPrefab, TransformUsageFlags.Dynamic)
            });
        }
    }
}

public struct TurretShootComponent : IComponentData
{
    public Entity BulletPrefab;
}

public void OnUpdate(ref SystemState state)
{
    var ecb =
        SystemAPI
            .GetSingleton<EndSimulationEntityCommandBufferSystem.Singleton>()
            .CreateCommandBuffer(state.WorldUnmanaged).AsParallelWriter();
    state.Dependency = new SpawnJob
    {
        ECB = ecb
    }.ScheduleParallel(state.Dependency);
}

[BurstCompile]
public partial struct TurretShootJob : IJobEntity
{
    public float DeltaTime;
    public EntityCommandBuffer.ParallelWriter ECB;

    void Execute([EntityIndexInQuery] int sortKey, ref LocalTransform transform, ref TurretShootComponent shootComponent, Entity entity)
    {
        // ECB实例化实体
        Entity bullet = ECB.Instantiate(sortKey, shootComponent.BulletPrefab);
        // 给实体设置组件（假设component已定义）
        ECB.SetComponent(sortKey, bullet, component);
    }
}

实体/组件操作

主线程中：

  • 获取实体单例：

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    Entity _entity;
    SystemAPI.TryGetSingletonEntity<CharacterAgentComponent>(out _entity);

    Entity e = SystemAPI.GetSingletonEntity<MyComponent>();

  • 获取实体上的组件：

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    LocalTransform playerTransform = SystemAPI.GetComponent<LocalTransform>(_entity);

  • 根据组件获取实体：

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    EntityQuery query = SystemAPI.QueryBuilder()
        .WithAll<MyComponent>()
        .Build();
    // 转化为列表
    using var entities = query.ToEntityArray(Allocator.Temp);

或者使用foreach：

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foreach (var (comp, entity) in
         SystemAPI.Query<RefRO<MyComponent>>()
                  .WithEntityAccess())
{
    // entity 就是挂着 MyComponent 的实体
}

Job/子线程中

  • ECB操作

ECB（Entity Command Buffer）是ECS的事务系统（执行失败会报错，但不影响其他命令）。在Job中提交命令，在帧末统一执行。

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// Update中
void OnUpdate(ref SystemState state)
{
    var ecb = SystemAPI
                    .GetSingleton<EndSimulationEntityCommandBufferSystem.Singleton>()
                    .CreateCommandBuffer(state.WorldUnmanaged).AsParallelWriter();
    new BulletMoveJob
    {
        DeltaTime = SystemAPI.Time.DeltaTime
    }.ScheduleParallel();
}

[BurstCompile]
public partial struct BulletMoveJob : IJobEntity
{
    public float DeltaTime;
    public EntityCommandBuffer.ParallelWriter ECB;

    void Execute([EntityIndexInQuery] int sortKey, ref LocalTransform transform, ref BulletMoveComponent moveComponent, Entity entity)
    {
        // ECB实例化实体
        Entity bullet = ECB.Instantiate(sortKey, shoot.BulletPrefab);
        // 给实体设置组件
        ECB.SetComponent(sortKey, bullet, component);
        // 添加组件
        ECB.AddComponent(sortKey, entity, new Health { Value = 100 });
        // 删除组件
        ECB.RemoveComponent<DeadTag>(sortKey, entity);
        // 销毁实体
        ECB.DestroyEntity(sortKey, entity);
    }
}

注意：ECB.AddComponent添加组件后，不能马上读到这个组件，需要等到下一帧。ECB是ECS的事务系统：Job里只提交命令，命令在当前帧的ECB System Playback时统一执行，Playback之后World立即改变，但只有在Playback之后运行的系统（通常是下一帧）才能观察到这些变化。

  • 获取实体上的组件
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    void OnUpdate(ref SystemState state)
    {
        var ltLookup = SystemAPI.GetComponentLookup<LocalTransform>(true);
        // 将ltLookup传入Job中
        new TurretShootJob
        {
            ltLookup = SystemAPI.GetComponentLookup<LocalTransform>(true)
        }.ScheduleParallel();
    }

    [BurstCompile]
    public partial struct TurretShootJob : IJobEntity
    {
        [ReadOnly] public ComponentLookup<LocalTransform> ltLookup;

        void Execute([ChunkIndexInQuery] int sortKey, ref TurretShootComponent shoot, in LocalToWorld muzzleLtw)
        {
            // 判断entity有没有LocalTransform
            if (ltLookup.HasComponent(entity))
            {
                LocalTransform lt = ltLookup[entity];
            }
        }
    }