实例化Entity Prefab

创建Player Prefab

在“TestEntitySubScene”下新建一个空对象，命名为“PlayerSpawner”。

在Scripts文件夹中新建“PlayerSpawnerComponent”

using Unity.Entities;

public struct PlayerSpawnerComponent : IComponentData
{
    public Entity playerPrefab;
}

再新建“PlayerSpawnerAuthoring”

using Unity.Entities;
using UnityEngine;

public class PlayerSpawnerAuthoring : MonoBehaviour
{
    public GameObject playerPrefab;
}
public class PlayerSpawnerBaker : Baker<PlayerSpawnerAuthoring>
{
    public override void Bake(PlayerSpawnerAuthoring authoring)
    {
        AddComponent(new PlayerSpawnerComponent
        {
            playerPrefab = GetEntity(authoring.playerPrefab),//使用GetEntity方法将Go转化
        });
    }
}

我们将“TestEntitySubScene”中的“GameObject”重命名为“Player”，并拉取成为一个Prefab，在场景中删掉它

在PlayerSpawner上挂载PlayerSpawnerAuthoring，并将Player Prefab拖进去

创建PlayerSpawnerSystem

我们使用SystemBase来创建

using Unity.Entities;

public partial class PlayerSpawnerSystem : SystemBase
{
    protected override void OnUpdate()
    {
        
    }
}

想要高效地获取到Player Entity，我们需要给Player Entity添加一种Component，这种Component没有任何参数，它们用来标记一种Entity，称为“Tag Component”

创建“PlayerTag”和“PlayerTagAuthoring”文件

using Unity.Entities;

public struct PlayerTag : IComponentData
{
    
}

using Unity.Entities;
using UnityEngine;

public class PlayerTagAuthoring : MonoBehaviour
{
    
}
public class PlayerTagBaker : Baker<PlayerTagAuthoring>
{
    public override void Bake(PlayerTagAuthoring authoring)
    {
        AddComponent(new PlayerTag());
    }
}

在Project窗口中选中“Player” prefab，将PlayerTagAuthoring挂载上。

在PlayerSpawnerSystem中添加代码

using Unity.Entities;

public partial class PlayerSpawnerSystem : SystemBase
{
    protected override void OnUpdate()
    {
        EntityQuery playerEntityQuery = EntityManager.CreateEntityQuery(typeof(PlayerTag));//通过PlayerTag查找所有Entities
        PlayerSpawnerComponent playerSpawnerComponent = SystemAPI.GetSingleton<PlayerSpawnerComponent>();//因为只需要读取PlayerSpawnerComponent，所以不用RefRW
        int playerSpawnCount = 2;
        if(playerEntityQuery.CalculateEntityCount() < playerSpawnCount)
        {
            EntityManager.Instantiate(playerSpawnerComponent.playerPrefab);
        }
    }
}

Struct EntityManager | Entities | 1.0.0-pre.44 (unity3d.com)

Struct EntityQuery | Entities | 1.0.0-pre.44 (unity3d.com)

点击运行，这时会有两个Entity行动。

Entity CommandBuffer

当我们实例化或者销毁Entities的时候，或者通过增减Component来修改archetype的时候，需要注意时机，当有些System在轮询Entities的时候另一个System发生了Entity增删操作，很容易出现问题。

Structural changes concepts | Entities | 1.0.0-pre.44 (unity3d.com)

为了解决这个问题，我们需要使用Entity Command Buffer System，这个系统先将增删操作缓存起来，确保操作无害。

Entity CommandBufferSystem

Class BeginSimulationEntityCommandBufferSystem | Entities | 1.0.0-pre.44 (unity3d.com)

修改PlayerSpawnerSystem，不使用EntityManager.Instantiate

using Unity.Entities;

public partial class PlayerSpawnerSystem : SystemBase
{
    protected override void OnUpdate()
    {
        EntityQuery playerEntityQuery = EntityManager.CreateEntityQuery(typeof(PlayerTag));
        PlayerSpawnerComponent playerSpawnerComponent = SystemAPI.GetSingleton<PlayerSpawnerComponent>();
        RefRW<RandomComponent> randomComponent = SystemAPI.GetSingletonRW<RandomComponent>();

        EntityCommandBuffer entityCommandBuffer = SystemAPI.GetSingleton<BeginSimulationEntityCommandBufferSystem.Singleton>().CreateCommandBuffer(World.Unmanaged);

        int playerSpawnCount = 20;
        if(playerEntityQuery.CalculateEntityCount() < playerSpawnCount)
        {
            Entity spawnedEntity = entityCommandBuffer.Instantiate(playerSpawnerComponent.playerPrefab);
            //给每一个spawnedEntity添加Speed Component，并且给Speed的value赋随机值
            entityCommandBuffer.SetComponent(spawnedEntity,new Speed
            {
                value = randomComponent.ValueRW.random.NextFloat(1f, 5f)
            });
        }
    }
}

World concepts | Entities | 1.0.0-pre.44 (unity3d.com)

Class World | Entities | 1.0.0-pre.44 (unity3d.com)

Entities Moving

GameObject引用Entity

我们不在Entity SubScene，而是在正常的Scene里面创建一个3D-Cube，去除Box Collider

在Scripts窗口中新建PlayerVisual脚本：

using Unity.Entities;
using UnityEngine;
using Unity.Collections;
using Unity.Transforms;

public class PlayerVisual : MonoBehaviour
{
    private Entity targetEntity;
    private void LateUpdate()
    {
        if(Input.GetKeyDown(KeyCode.Space)) { targetEntity = GetRandomEntity(); }

        if (targetEntity != Entity.Null)
        {
            //EntityManager通过访问LocalToWorld Component来获取World Position
            Vector3 followPosition = World.DefaultGameObjectInjectionWorld.EntityManager.GetComponentData<LocalToWorld>(targetEntity).Position;
            transform.position = followPosition;
        }
    }
    private Entity GetRandomEntity() {
        //在GameObject环境下使用World.DefaultGameObjectInjectionWorld来访问EntityManager，因为此Go和Entity是一个World，和PlayerSpawnerSystem一样，先CreateEntityQuery查询
        EntityQuery playerTagEntityQuery = World.DefaultGameObjectInjectionWorld.EntityManager.CreateEntityQuery(typeof(PlayerTag));

        //将playerTagEntityQuery转换为NativeArray。需要使用Unity.Collections
        //我们只需要在EntityQuery中随机找一个，所以使用Allocator.Temp，这样分配的内存会接着被清除。
        NativeArray<Entity> entityNativeArray = playerTagEntityQuery.ToEntityArray(Unity.Collections.Allocator.Temp);

        if (entityNativeArray.Length > 0)
        {
            return entityNativeArray[Random.Range(0, entityNativeArray.Length)];
        }
        else
        {
            return Entity.Null;
        }
    }
}