Unity DOTS系列之Filter Baking Output与Prefab In Baking核心分析

最近DOTS发布了正式的版本, 我们来分享一下DOTS里面Baking核心机制，方便大家上手学习掌握Unity DOTS开发。今天给大家分享的Baking机制中的Filter Baking Output与Prefab In Baking。

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Filter Baking Output 机制

在默认情况下,Baking会为每个GameObject生成的Entity与Component, 这些entity都会被创建到Conversion World里面。然后在创作的时候不是所有的GameObject都需要被转换成Entity。例如: 在一个样条曲线上，一个控制点在创作的时候被用到了，但是bake成ecs数据后可能就再也没有用了，所以不需要把这个控制点Bake成entity。

为了不把这个GameObject Bake产生的Entity输出到World并保存到entity scene里面,我们可以给这个Entity添加一个BakingOnlyEntity tag Component。当你添加了这个tag component后，Baking系统就不会把你这个entity存储到entity scene里面，也不会把这个entity生成到运行的main World里面。我们可以直接在Baker函数里面添给entity 添加一个BakingOnlyEntity的组件数据,也可以在Authoring GameObject里面添加一个 BakingOnlyEntityAuthoring的组件,这两种方式都可以达到同样的效果没有区别。

你也可以给组件添加注解[BakingType],[TemporaryBakingType] attributes过滤掉掉组件component,让它不输出到entity中。

[TemporaryBakingType]:被这个attributes注记的Component会在Baking Output的时候不会输出到entity。这个Component只会存活在Baker过程中,Baker结束以后就会销毁。

Prefab In Baking机制

生成entity prefab到entity scene以后，我们就可以像使用普通的Prefab创建GameObject一样来创建entity到世界。但是使用enity prefab之前一定要确保它已经被Baker到了entity scene。当预制体实例化到Authoring Scene中的时候，Baking把它当作普通的GameObject来进行转换，不会把它当作预制体。

生成一个entity prefab你需要注册一个Baker,在Bake函数里面添加一个依赖关系，让这个依赖于Authoring GameObject Prefab。然后Prefab将会被bake出来。我们搞一个组件保存了entity prefab的一个引用，那么unity就会把这个entity prefab 序列化到subscene中。当需要使用这个entity prefab的时候就能获取到。代码如下:

public struct EntityPrefabComponent : IComponentData
 {
 public Entity Value;
 }
 public class GetPrefabAuthoring : MonoBehaviour
 {
 public GameObject Prefab;
 }
 public class GetPrefabBaker : Baker
 {
 public override void Bake(GetPrefabAuthoring authoring)
 {
 // Register the Prefab in the Baker
 var entityPrefab = GetEntity(authoring.Prefab, TransformUsageFlags.Dynamic);
 // Add the Entity reference to a component for instantiation later
 var entity = GetEntity(TransformUsageFlags.Dynamic);
 AddComponent(entity, new EntityPrefabComponent() {Value = entityPrefab});
 }
 }
    #endregion

在Baking的时候，当我们需要引用一个entity prefab, 可以使用EntityPrefabReference。这个会把它序列化到entity scene文件里面去。运行的时候直接load进来，就可以使用了。这样可以防止多个subscene不用重复拷贝生成同一个Prefab。

#region InstantiateLoadedPrefabs
 public partial struct InstantiatePrefabReferenceSystem : ISystem
 {
 public void OnStartRunning(ref SystemState state)
 {
 // Add the RequestEntityPrefabLoaded component to the Entities that have an
 // EntityPrefabReference but not yet have the PrefabLoadResult
 // (the PrefabLoadResult is added when the prefab is loaded)
 // Note: it might take a few frames for the prefab to be loaded
 var query = SystemAPI.QueryBuilder()
                .WithAll()
                .WithNone().Build();
 state.EntityManager.AddComponent(query);
 }
 public void OnUpdate(ref SystemState state)
 {
 var ecb = new EntityCommandBuffer(Allocator.Temp);
 // For the Entities that have a PrefabLoadResult component (Unity has loaded
 // the prefabs) get the loaded prefab from PrefabLoadResult and instantiate it
 foreach (var (prefab, entity) in
 SystemAPI.Query>().WithEntityAccess())
 {
 var instance = ecb.Instantiate(prefab.ValueRO.PrefabRoot);
 // Remove both RequestEntityPrefabLoaded and PrefabLoadResult to prevent
 // the prefab being loaded and instantiated multiple times, respectively
 ecb.RemoveComponent(entity);
 ecb.RemoveComponent(entity);
 }
 ecb.Playback(state.EntityManager);
 ecb.Dispose();
 }
 }
    #endregion
实例化entity prefab可以使用EntityManager与entity command buffer。
#region InstantiateEmbeddedPrefabs
 public partial struct InstantiatePrefabSystem : ISystem
 {
 public void OnUpdate(ref SystemState state)
 {
 var ecb = new EntityCommandBuffer(Allocator.Temp);
 // Get all Entities that have the component with the Entity reference
 foreach (var prefab in
 SystemAPI.Query>())
 {
 // Instantiate the prefab Entity
 var instance = ecb.Instantiate(prefab.ValueRO.Value);
 // Note: the returned instance is only relevant when used in the ECB
 // as the entity is not created in the EntityManager until ECB.Playback
 ecb.AddComponent(instance);
 }
 ecb.Playback(state.EntityManager);
 ecb.Dispose();
 }
 }
    #endregion
实例化EntityPrefabReference,可以使用如下代码:
 #region InstantiateLoadedPrefabs
 public partial struct InstantiatePrefabReferenceSystem : ISystem
 {
 public void OnStartRunning(ref SystemState state)
 {
 // Add the RequestEntityPrefabLoaded component to the Entities that have an
 // EntityPrefabReference but not yet have the PrefabLoadResult
 // (the PrefabLoadResult is added when the prefab is loaded)
 // Note: it might take a few frames for the prefab to be loaded
 var query = SystemAPI.QueryBuilder()
                .WithAll()
                .WithNone().Build();
 state.EntityManager.AddComponent(query);
 }
 public void OnUpdate(ref SystemState state)
 {
 var ecb = new EntityCommandBuffer(Allocator.Temp);
 // For the Entities that have a PrefabLoadResult component (Unity has loaded
 // the prefabs) get the loaded prefab from PrefabLoadResult and instantiate it
 foreach (var (prefab, entity) in
 SystemAPI.Query>().WithEntityAccess())
 {
 var instance = ecb.Instantiate(prefab.ValueRO.PrefabRoot);
 // Remove both RequestEntityPrefabLoaded and PrefabLoadResult to prevent
 // the prefab being loaded and instantiated multiple times, respectively
 ecb.RemoveComponent(entity);
 ecb.RemoveComponent(entity);
 }
 ecb.Playback(state.EntityManager);
 ecb.Dispose();
 }
 }
    #endregion

在实例化EntityPrefabReference之前，Unity必须要先加载对应的entity prefab,然后才能使用它，添加RequestEntityPrefabLoaded组件能确保entity prefab被加载。Unity会PrefabLoadResult加载到带有RequestEntityPrefabLoaded同一个entity上。

 #region PrefabsInQueries
 // This query will return all baked entities, including the prefab entities
 var prefabQuery = SystemAPI.QueryBuilder()
                .WithAll().WithOptions(EntityQueryOptions.IncludePrefab).Build();
#endregion
使用EntityManager与entity command buffer也可以销毁一个预制体节点,代码如下:
            #region DestroyPrefabs
 var ecb = new EntityCommandBuffer(Allocator.Temp);
 foreach (var (component, entity) in
 SystemAPI.Query>().WithEntityAccess())
 {
 if (component.ValueRO.RadiansPerSecond <= 0)
 {
 ecb.DestroyEntity(entity);
 }
 }
 ecb.Playback(state.EntityManager);
 ecb.Dispose();
            #endregion

今天的Baking 系列就分享到这里了，关注我学习更多的最新Unity DOTS开发技巧。