我们使用ISystem来创建System

创建MovingISystem

在Scripts文件夹中新建MovingISystem文件：

using Unity.Entities;

public partial struct MovingISystem : ISystem
{
    public void OnCreate(ref SystemState state)
    {
    }

    public void OnDestroy(ref SystemState state)
    {
    }

    public void OnUpdate(ref SystemState state)
    {
        var randomComponent = SystemAPI.GetSingletonRW<RandomComponent>();
        foreach (MoveToPositionAspect moveToPositionAspect in SystemAPI.Query<MoveToPositionAspect>())
        {
            moveToPositionAspect.Move(SystemAPI.Time.DeltaTime, randomComponent);
        }
    }
}

我们让MovingISystem实现和MovingSystemBase相同的功能，可以看到ISystem和SystemBase相比多了两个方法：

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public void OnCreate(ref SystemState state)

public void OnDestroy(ref SystemState state)

其实SystemBase也能重载OnCreate和OnDestroy，只不过它只需要强制重载OnUpdate。

进入运行模式，可以看到我们的Entity以两倍的速度运动，这是因为MovingISystem和MovingSystemBase同时起效，我们只需要在Entities——Systems窗口中将“Moving System Base”停止即可。

引入Jobs

ISystem可以让我们引入Jobs。

我们希望可以有很多的Entities执行Move方法，这些方法运行在多个work thread中，而不是像之前MovingSystemBase在主线程里通过foreach轮询。

修改MoveToPositonAspect

目前的Move方法是在MoveToPositonAspect实现的：

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

public readonly partial struct MoveToPositionAspect : IAspect
{
    //...
    
    public void Move(float deltaTime,RefRW<RandomComponent> randomComponent)
    {
        float3 direction = math.normalize(targetPosition.ValueRW.value - transformAspect.WorldPosition);

        transformAspect.WorldPosition += direction * deltaTime * speed.ValueRO.value;

        float reachedTargetDistance = .5f;
        if( math.distance(transformAspect.WorldPosition,targetPosition.ValueRW.value) < reachedTargetDistance)
        {
            targetPosition.ValueRW.value = GetRandomPosition(randomComponent);
        }
    }

    //...
}

这个方法有一个问题，就是对RefRW<RandomComponent>的引用，在多线程程序中，单例的引用必须警惕，我们这里先分离这个方法：

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

public readonly partial struct MoveToPositionAspect : IAspect
{
    private readonly Entity entity;

    private readonly TransformAspect transformAspect;
    private readonly RefRO<Speed> speed;
    private readonly RefRW<TargetPosition> targetPosition;
    
    public void Move(float deltaTime)
    {
        float3 direction = math.normalize(targetPosition.ValueRW.value - transformAspect.WorldPosition);

        transformAspect.WorldPosition += direction * deltaTime * speed.ValueRO.value;

        
    }
    public void TestReachedTargetPosition(RefRW<RandomComponent> randomComponent)
    {
        float reachedTargetDistance = .5f;
        if (math.distance(transformAspect.WorldPosition, targetPosition.ValueRW.value) < reachedTargetDistance)
        {
            targetPosition.ValueRW.value = GetRandomPosition(randomComponent);
        }
    }
    private float3 GetRandomPosition(RefRW<RandomComponent> randomComponent)
    {
        return new float3(
            randomComponent.ValueRW.random.NextFloat(0f, 15f), 
            0, 
            randomComponent.ValueRW.random.NextFloat(0f, 15f)
            );
    }
}

这样其中的Move方法就是一个完全可以多线程运行的方法了。

添加MoveJob和TestReachedTargetPositionJob

修改MovingISystem文件：

//...
//添加
public partial struct MoveJob : IJobEntity
{
    public float deltaTime;
    public void Execute(MoveToPositionAspect moveToPositionAspect)
    {
        moveToPositionAspect.Move(deltaTime);
    }
}
public partial struct TestReachedTargetPositionJob : IJobEntity
{
    public RefRW<RandomComponent> randomComponent;
    public void Execute(MoveToPositionAspect moveToPositionAspect)
    {
        moveToPositionAspect.TestReachedTargetPosition(randomComponent);
    }
}

Unity.Entities提供了IJobEntity和IJobChunk两个接口，前者能够轮询Entity，后者能够轮询每个JobChunk

只要继承的这些接口有Execute方法，就会执行此方法

public void Execute(MoveToPositionAspect moveToPositionAspect)Execute方法可以使用Aspect、Component作为参数，注意添加其他参数会导致此Job无法在多线程运行（只能调用Run、不能调用Schedule或ScheduleParallel

需要是“partial struct”，在Unity DOTS中，partial的目的是为了更好地使用源生成器生成代码。

修改MovingISystem文件：

去除foreach，使用创建Job的方式来执行Move

using Unity.Burst;
using Unity.Collections.LowLevel.Unsafe;
using Unity.Entities;
using Unity.Jobs;

[BurstCompile]
public partial struct MovingISystem : ISystem
{
    [BurstCompile]
    public void OnCreate(ref SystemState state)
    {
    }
    [BurstCompile]
    public void OnDestroy(ref SystemState state)
    {
    }
    [BurstCompile]
    public void OnUpdate(ref SystemState state)
    {
        var randomComponent = SystemAPI.GetSingletonRW<RandomComponent>();
        float deltaTime = SystemAPI.Time.DeltaTime;
        JobHandle jobHandle = new MoveJob { deltaTime = deltaTime }.ScheduleParallel(state.Dependency);
        jobHandle.Complete();
        new TestReachedTargetPositionJob { randomComponent = randomComponent }.Run();
    }
}
[BurstCompile]
public partial struct MoveJob : IJobEntity
{
    public float deltaTime;
    [BurstCompile]
    public void Execute(MoveToPositionAspect moveToPositionAspect)
    {
        moveToPositionAspect.Move(deltaTime);
    }
}
[BurstCompile]
public partial struct TestReachedTargetPositionJob : IJobEntity
{
    [NativeDisableUnsafePtrRestriction] public RefRW<RandomComponent> randomComponent;
    [BurstCompile]
    public void Execute(MoveToPositionAspect moveToPositionAspect)
    {
        moveToPositionAspect.TestReachedTargetPosition(randomComponent);
    }
}

MoveJob是完全多线程的，所以我们使用ScheduleParallel，并且使用ScheduleParallel(JobHandle dependsOn)重载来返回一个jobhandle。调用jobHandle.Complete();是为了保证“MoveJob”先运行才能运行TestReachedTargetPositionJob。

TestReachedTargetPositionJob需要运行在主线程上（直接调用Run），因为它引用了RefRW<RandomComponent>，而且此引用还需要添加NativeDisableUnsafePtrRestriction特性

在Job的构造函数中不能直接调用SystemAPI，所以我们要写float deltaTime = SystemAPI.Time.DeltaTime;

我们在ISystem和IJobEntity上添加BurstCompile特性，并且在它们各自的方法内添加BurstCompile特性