Unity的ECS与JobSystem初探

大概学习一下。

传统方式：

问题：（1）数据分散

stuff过于混乱

（2）存在冗余数据

当move的时候，只需要speed，position和rotation，但transform含有lot of stuff

（3）串行方式

所以带来了JobSystem

• CPU利用率提高，一般游戏都是2core
• 传统多线程的问题：race condition, context switching
• thread safe

ECS需要改变思考方式。

Entity：不是game object，只是reference to data，purely date

System：functionality, 关键是一个Filter

例如，一个Render System有一个Render和Position的Filter，那么就会找到所有包含Render和Position的data component构成的Entity

ECS的优势：

• 不容易写出表现差的代码，原生带有良好的性能
• 可重用性高
• 能更充分的利用CPU
• achetypes在内存中紧密打包，访问速度提高

下面的一个job:

[ComputeJobOptimization]
public struct MovementJob : IJobParallelForTransform {
    public float moveSpeed;
    public float topBound;
    public float bottomBound;
    public float deltaTime;
    // index: id, TransformAccess包含Transform的元数据，除去引用数据
    public Execute(int index, TransformAccess transform) {
        // 逻辑差距不打
        Vector3 pos = transform.position;
        pos += moveSpeed * deltaTime * (transform.rotation * new Vector3(0,0,1));
        if (pos.z < bottomBound) pos.z = topBound;
        transform.position = pos;
    }
}

GameManager：

public class GameManager : MonoBehavior {
    #region GAME_MANAGER_STUFF
        //...
    #endregion
    TransformAccessArray transforms;
    MovementJob moveJob;
    JobHandle moveHandle;
    
    void OnDisable() {
        moveHandle.Complete();
        transforms.Dispose(); // 手动GC
    }
    
    void Update() {
        moveHandle.Complete(); //等待完成
        moveJob = new MovementJob() { // 创建信息
            moveSpeed = enemySpeed,
            topBound = topBound,
            bottomBound = bottomBound, 
            deltaTime = Time.deltaTime
        };
        moveHandle = moveJob.Schedule(transforms); // 传入参数，传递给handle
        JobHandle.ScheduleBatchedJobs(); // 执行任务
    }
    
    void AddShips(int amount) {
        moveHandle.Complete();
        transform.capacity = transform.length + amount;
        for (int i = 0; i < amount; ++i) {
            float xV = Random.Range(leftBound, rightBound);
            float zV = Random.Range(0f, 10f);
            Vector3 pos = new Vector3(xVal, 0f, zV + topBound);
            Quanternion rot = Quanternion.Euler(0f, 180f, 0f);
            var obj = Instantiate<GameObject>(prefab, pos, rat);
            transforms.Add(obj.transform);
        }
        count += amount;
        fps.SetElementCount(count);
    }
    
}

这种做法基本保持了原有的Unity方案。但我们希望用完整的ECS策略，还需要把Entity分出来。

[Serializable] public struct MoveSpeed : IComponentData {
    public float Value;
}
public class MoveSpeedComponent : ComponentDataWrapper<MoveSpeed> {}

接下来创建一个System来处理：

public class MovementSystem : JobComponentSystem {
    [ComputeJobOptimization]
	public struct MovementJob : IJobParallelForTransform<Position, Rotation, MoveSpeed> { // data inject
        public float topBound;
        public float bottomBound;
        public float deltaTime;
        public Execute(ref Position position, [ReadOnly] ref Rotation rotation, [ReadOnly] ref MoveSpeed speed) {
            // 逻辑差距不大
            float3 pos = position.Value;
            pos += speed.Value * deltaTime * math.forward(rotation.Value);
            if (pos.z < bottomBound) pos.z = topBound;
            position.Value = pos;
        }
    }
    protected override JobHandle OnUpdate(JobHandle inputDeps) {
        MovementJob moveJob = new MovementJob {
            topBound = GameManager.GM.topBound,
            bottomBound = GameManager.GM.bottomBound,
            deltaTime = Time.deltaTime
        };
        JobHandle moveHandle = moveJob.Schedule(this, 64, inputDeps);
        return moveHandle;
    }
}

在GameManager中：

public class GameManager : MonoBehavior {
    #region GAME_MANAGER_STUFF
        //...
    #endregion
    EntityManager manager;
    
    void OnDisable() {
        moveHandle.Complete();
        transforms.Dispose(); // 手动GC
    }
    
    void Start() {
        fps = GetComponent<FPS>();
        manager = World.Active.GetOrCreateManager<EntityManager>();
        AddShips(enemyShitCount);
    }
    
    void Update() {
        if (Input.GetKeyDown("space"))
            AddShips(enemyShipIncrement);
    }
    
    void AddShips(int amount) {
        NativeArray<Entity> entities = new NativeArray<Entity>(amount, Allocator.Temp); // new type collections
        manager.Instantiate(enemyShipPrefab, entities); // create entities with component data
        for (int i = 0; i < amount; ++i) {
            float xV = Random.Range(leftBound, rightBound);
            float zV = Random.Range(0f, 10f);
            manager.SetComponentData(entities[i], new Position {Value = new float3(xVal, 0f, topBound + zVal)});
            manager.SetComponentData(entities[i], new Rotation { Value = new quanternion(0,1,0,0) });
            manager.SetComponentData(entities[i], new MoveSpeed { Value = enemySpeed });
        }
        
        entities.Dispose(); // GC
        count += amount;
        fps.SetElementCount(count);
    }
    
}

如果使用burst compiler，很多运算会交给GPU去进行，运算可以进一步优化。