I want to make a HitBrick game with ECS and new Physics,but I met some problems

I use the ConverToEntity,Physics Shape,Physics Body, the collision between ball and bricks work well.Then I wrote a "ColllisionJob" derived from ICollisionEventsJob. I'm going to destroy the brick and instantiate a new ball in this code.But it reports an error "UnityException: scope can only be called from the main thread.". So I add a component named "InstantiateBall" to the ball, and then write a system named "InstantiateBallSystem", which is derived from the ComponentSystem, and I get all entities with "InstantiateBall" component, and then create a new ball and remove the "InstantiateBall" component, but it reports" type of component: InstantiateBall has been added to the entity."What's more, it creates a lot of balls.How do I modify my code?

 

I think you need to be using a command buffer so that you can ensure the instantiation only occurs on the main thread.

Could it be that a ball in a collision is the point where you add new balls which is where they are probably going to be in a collision state as well.

You may want to look at the order of your systems e.g. CreateBall might be before DestroyBrick and especially where the Collision Checking occurs.

Also posting your code can really help developers help you.

 

Thank you for response. Maybe you are right.The code is shown below

 

You can just past the code into the forum e.g.

Code (CSharp):

1. public class InstiateBallSystem : ComponentSystem
2. {
3.     public EntityCommandBufferSystem bufferSystem;
4.     protected override void OnCreate()
5.     {
6.         bufferSystem = World.Active.GetOrCreateSystem<EntityCommandBufferSystem>();
7.     }
8.     protected override void OnUpdate()
9.     {
10.         var commandBuffer = bufferSystem.CreateCommandBuffer();
11.         var time = Time.time;
12.         Entities.ForEach((Entity entity, ref Translation trans, ref InstantiateBall ball) =>
13.         {
14.             var newball = commandBuffer.Instantiate(entity);
15.             commandBuffer.RemoveComponent<InstantiateBall>(entity);
16.             commandBuffer.RemoveComponent<InstantiateBall>(newball);
17.             commandBuffer.SetComponent(newball, new PhysicsVelocity
18.             {
19.                 Linear = math.normalize(new float3(UnityEngine.Random.Range(-100f, 100f), UnityEngine.Random.Range(0f, 100f), 0)) * 40f
20.             });
21.  
22.         });
23.  
24.     }
25. }
26.  
27. public class CollisionSystem : JobComponentSystem
28. {
29.     EndSimulationEntityCommandBufferSystem endSimCommandBuffer;
30.     BuildPhysicsWorld buildPhysicsWorldSystem;
31.     StepPhysicsWorld stepPhysicsWorldSystem;
32.     protected override void OnCreate()
33.     {
34.         endSimCommandBuffer = World.GetOrCreateSystem<EndSimulationEntityCommandBufferSystem>();
35.         buildPhysicsWorldSystem = World.GetOrCreateSystem<BuildPhysicsWorld>();
36.         stepPhysicsWorldSystem = World.GetOrCreateSystem<StepPhysicsWorld>();
37.         base.OnCreate();
38.     }
39.     protected override JobHandle OnUpdate(JobHandle inputDeps)
40.     {
41.         var job = new CollisionJob
42.         {
43.             commandBuffer = endSimCommandBuffer.CreateCommandBuffer(),
44.             balls = GetComponentDataFromEntity<BallTag>(false),
45.             bricks = GetComponentDataFromEntity<BrickTag>(false),
46.             Instiates = GetComponentDataFromEntity<InstantiateBall>(true),
47.         }.Schedule(stepPhysicsWorldSystem.Simulation, ref buildPhysicsWorldSystem.PhysicsWorld, inputDeps);
48.         endSimCommandBuffer.AddJobHandleForProducer(job);
49.         job.Complete();
50.         return job;
51.     }
52.     public struct CollisionJob : ICollisionEventsJob
53.     {
54.         [ReadOnly] public ComponentDataFromEntity<BrickTag> bricks;
55.         [ReadOnly] public ComponentDataFromEntity<BallTag> balls;
56.         [ReadOnly] public ComponentDataFromEntity<InstantiateBall> Instiates;
57.         [ReadOnly] public EntityCommandBuffer commandBuffer;
58.         public float time;
59.         public void Execute(Unity.Physics.CollisionEvent collisionEvent)
60.         {
61.             EntityPair pair = collisionEvent.Entities;
62.             if (Instiates.Exists(pair.EntityA) || Instiates.Exists(pair.EntityB))
63.             {
64.                 return;
65.             }
66.             if (bricks.Exists(pair.EntityA) && balls.Exists(pair.EntityB))
67.             {
68.                 commandBuffer.DestroyEntity(pair.EntityA);
69.                 if (Instiates.Exists(pair.EntityA) || Instiates.Exists(pair.EntityB))
70.                 {
71.                     return;
72.                 }
73.                 commandBuffer.AddComponent(pair.EntityB, new InstantiateBall { });              
74.             }
75.             else if (balls.Exists(pair.EntityA) && bricks.Exists(pair.EntityB))
76.             {
77.                 commandBuffer.DestroyEntity(pair.EntityB);
78.                 if (Instiates.Exists(pair.EntityA) || Instiates.Exists(pair.EntityB))
79.                 {
80.                     return;
81.                 }
82.                 commandBuffer.AddComponent(pair.EntityA, new InstantiateBall { });
83.             }
84.         }
85.     }
86. }
87.  
88.  

 

I think it is because the command buffer is consumed after the job, in this case at the EndSimBarrier. You check if the entities already has a component on them but it wont take the ones already added this frame in the command buffer in consideration. If an entity is colliding more than once a frame it will add the component multiple times.

Edit: Removed my old answer because I didn't fully read the question.

 

OK，I got it.

 

In the InstiateBallSystem class you are trying to get EntityCommandBufferSystem but that is an abstract class. I'm not sure how this is handled but my guess is that it will return the first barrier system that is found. Instead since you are using a ComponentSystem you should use PostUpdateCommands which will execute all the commands for it's command buffer just after the update function is called instead of at the barrier.

 

I have done it, but it doesn't work.

 

Just wanted to say huge thanks for this post - I'd been really struggling to get my head around collisions in this system, and this really helped unblock me!