DOTS笔记

keyle_xiao

Baker 简单理解：Mono脚本为Authoring，通过Baker将一个Authoring与Data关联。

// They constitute the inputs for the baking systems which generates ECS data.
class TurretAuthoring : UnityEngine.MonoBehaviour
{
    // Bakers convert authoring MonoBehaviours into entities and components.
    class TurretBaker : Baker<TurretAuthoring>
    {
        public override void Bake(TurretAuthoring authoring)
        {
            AddComponent<Turret>();
        }
    }
}

// An empty component is called a "tag component".
struct Turret : IComponentData
{
}

再从这段代码反思ecs的设计模式，组件与属性相互绑定，Turret 不应添加诸如Speed类参数，动态Speed应该由 MoveSystem 控制。Turret内属于一些专属不会变的属性，如MaxSpeed，MaxBullet 之类的不会变动的参数。

keyle_xiao

ISystem 的OnUpdate是一直刷的，在这里通过Query找到对应的Turret对其Rotate进行改变。

using Unity.Burst;
using Unity.Entities;
using Unity.Mathematics;
using Unity.Transforms;

// Unmanaged systems based on ISystem can be Burst compiled, but this is not yet the default,
// so we have to explicitly opt into Burst compilation with the [BurstCompile] attribute.
// It has to be added on BOTH the struct AND the OnCreate/OnDestroy/OnUpdate functions to be
// effective.
[BurstCompile]
partial struct TurretRotationSystem : ISystem
{
    // Every function defined by ISystem has to be implemented even if empty.
    [BurstCompile]
    public void OnCreate(ref SystemState state)
    {
    }

    // Every function defined by ISystem has to be implemented even if empty.
    [BurstCompile]
    public void OnDestroy(ref SystemState state)
    {
    }

    // See note above regarding the [BurstCompile] attribute.
    [BurstCompile]
    public void OnUpdate(ref SystemState state)
    {
        // The amount of rotation around Y required to do 360 degrees in 2 seconds.
        var rotation = quaternion.RotateY(SystemAPI.Time.DeltaTime * math.PI);

        // The classic C# foreach is what we often refer to as "Idiomatic foreach" (IFE).
        // Aspects provide a higher level interface than directly accessing component data.
        // Using IFE with aspects is a powerful and expressive way of writing main thread code.

        // WithAll adds a constraint to the query, specifying that every entity should have such component.
        //var result = SystemAPI.Query<TransformAspect>().WithAll<Turret>();//error SGFE001: Invocations of `SystemAPI.Query<T>() are currently supported only if they take place inside `foreach` statements.
        foreach (var transform in SystemAPI.Query<TransformAspect>().WithAll<Turret>())
            transform.RotateWorld(rotation);
    }
}

第37行尝试暂存Query出来的QueryEnumerable<TransformAspect>，报错如下：
SGFE001: Invocations of SystemAPI.Query<T>() are currently supported only if they take place insideforeach` statements，
大意就是只能在内foreach使用。

keyle_xiao

需要注意的是这里用的SystemBase没有用ISystem，导致其不能使用BurstCompile进行优化。好处有可以使用托管类型的API，Entities就是SystemBase内建API。可以通过它轻松的遍历指定类型。
简而言之：Struct系统类使用SystemAPI的Query，Class系统使用SystemBase内建的Entities。
有一定局限，这里的Foreach必须使用Lambda，使用Action或者局部函数替换报错。

using Unity.Burst;
using Unity.Entities;
using Unity.Mathematics;
using Unity.Transforms;

// Contrarily to ISystem, SystemBase systems are classes.
// They are not Burst compiled, and can use managed code.
partial class TankMovementSystem : SystemBase
{
    protected override void OnUpdate()
    {
        // The Entities.ForEach below is Burst compiled (implicitly).
        // And time is a member of SystemBase, which is a managed type (class).
        // This means that it wouldn't be possible to directly access Time from there.
        // So we need to copy the value we need (DeltaTime) into a local variable.
        var dt = SystemAPI.Time.DeltaTime;

        // Entities.ForEach is an older approach to processing queries. Its use is not
        // encouraged, but it remains convenient until we get feature parity with IFE.
        Entities
            .WithAll<Tank>()
            .ForEach((TransformAspect transform) =>
            {
                // Notice that this is a lambda being passed as parameter to ForEach.
                var pos = transform.LocalPosition;

                // Unity.Mathematics.noise provides several types of noise functions.
                // Here we use the Classic Perlin Noise (cnoise).
                // The approach taken to generate a flow field from Perlin noise is detailed here:
                // https://www.bit-101.com/blog/2021/07/mapping-perlin-noise-to-angles/
                var angle = (0.5f + noise.cnoise(pos / 10f)) * 4.0f * math.PI;

                var dir = float3.zero;
                math.sincos(angle, out dir.x, out dir.z);
                transform.LocalPosition += dir * dt * 5.0f;
                transform.LocalRotation = quaternion.RotateY(angle);

                // The last function call in the Entities.ForEach sequence controls how the code
                // should be executed: Run (main thread), Schedule (single thread, async), or
                // ScheduleParallel (multiple threads, async).
                // Entities.ForEach is fundamentally a job generator, and it makes it very easy to
                // create parallel jobs. This unfortunately comes with a complexity cost and weird
                // arbitrary constraints, which is why more explicit approaches are preferred.
                // Those explicit approaches (IJobEntity) are covered later in this tutorial.
            }).ScheduleParallel();
    }
}

keyle_xiao

前文的SystemBase写法同样可以改为ISystem写的写法，这两段是等价的：

[BurstCompile]
partial struct TankMovementSystem : ISystem
{
    public void OnCreate(ref SystemState state)
    {
    }

    public void OnDestroy(ref SystemState state)
    {
    }

    [BurstCompile]
    public void OnUpdate(ref SystemState state)
    {
        //var dt = state.World.Time.DeltaTime;
        var dt = SystemAPI.Time.DeltaTime;

        foreach (TransformAspect transform in SystemAPI.Query<TransformAspect>().WithAll<Tank>())
        {
            var pos = transform.LocalPosition;
            var angle = (0.5f + noise.cnoise(pos / 10f)) * 4.0f * math.PI;
            var dir = float3.zero;
            math.sincos(angle, out dir.x, out dir.z);
            transform.LocalPosition += dir * dt * 5.0f;
            transform.LocalRotation = quaternion.RotateY(angle);
        }
    }
}

这里关注下 GetComponentLookup 函数，通过SystemAPI.GetComponentLookup找到当前世界的 WorldTransform 队列。同样可以通过state.GetComponentLookup获取。

[BurstCompile]
public void OnCreate(ref SystemState state)
{
    // ComponentLookup structures have to be initialized once.
    // The parameter specifies if the lookups will be read only or if they should allow writes.
    m_WorldTransformLookup = SystemAPI.GetComponentLookup<WorldTransform>(true);
    //m_WorldTransformLookup = state.GetComponentLookup<WorldTransform>(true);
}

由于OnUpdate需要每一帧都刷新所以 m_WorldTransformLookup 这个结构不能在这个函数中每一帧创建，就在OnCreate中获取一次，在OnCreate函数中使用SystemState进行计算。

    // A ComponentLookup provides random access to a component (looking up an entity).
    // We'll use it to extract the world space position and orientation of the spawn point (cannon nozzle).
    ComponentLookup<WorldTransform> m_WorldTransformLookup;
    
    public void OnUpdate(ref SystemState state)
    {
        // ComponentLookup structures have to be updated every frame.
        m_WorldTransformLookup.Update(ref state);

        // Creating an EntityCommandBuffer to defer the structural changes required by instantiation.
        var ecbSingleton = SystemAPI.GetSingleton<BeginSimulationEntityCommandBufferSystem.Singleton>();
        var ecb = ecbSingleton.CreateCommandBuffer(state.WorldUnmanaged);
        // Creating an instance of the job.
        // Passing it the ComponentLookup required to get the world transform of the spawn point.
        // And the entity command buffer the job can write to.
        var turretShootJob = new TurretShoot
        {
            WorldTransformLookup = m_WorldTransformLookup,
            ECB = ecb
        };

        // Schedule execution in a single thread, and do not block main thread.
        turretShootJob.Schedule();
    }

对结构的增删改要放在buff内，buff又分三大Group，可以world中获取以下CommandBufferSystem（对应到不同周期），创建EntityCommandBuffer之后在其内可以对Entity进行操作。
下面是几个可以获取到的 EntityCommandBufferSystem：
BeginInitializationEntityCommandBufferSystem
EndInitializationEntityCommandBufferSystem
BeginSimulationEntityCommandBufferSystem
EndSimulationEntityCommandBufferSystem
BeginPresentationEntityCommandBufferSystem

keyle_xiao

世界System的Group执行流程：