Entities.ForEach cannot use component access method HasComponent with the same type

Hey,
I want to calculate elliptical orbits for my celestials around an arbitrary center of mass. So imagine a binary system where both stars orbit around a center of mass entity (which has just a position and a tag). Also some planets orbit around the same center. One planet orbits around one of the stars (which is also a center of mass then). And a moon orbits around that planet. So I just want to specify for a given entity which other entity it orbits around. I do this by storing the entity in the OrbitParameterData : IComponentData.

However, I have problems retriving the Translation component of the center of mass entity with GetComponent as I get the following error:

I understand the error that since I access the translation component to set the new position of my current entity I cannot (somehow) access the translation component of another entity. The reason for this is beyond my understanding.
Here is the offending code:

Code (csharp):

1.  
2. public class OrbitSystem : SystemBase
3. {
4.     protected override void OnUpdate()
5.     {
6.        Entities.ForEach((ref Translation translation, ref OrbitParameterData orbit) =>
7.         {
8.             // the orbit calculation happens here, it gives me a float3 which should be added to the center of mass's position and written to this entity's position
9.  
10.             // Check to make sure the target Entity still exists and has the needed component source: https://docs.unity3d.com/Packages/com.unity.entities@0.14/manual/ecs_lookup_data.html
11.             if (!HasComponent<Translation>(orbit.centerOfMassEntity))   // <- this is complained about
12.                 return;
13.             Translation cOMTransform = GetComponent<Translation>(orbit.centerOfMassEntity);    // this too when I remove HasComponent above
14.             float3 cOMPosition = cOMTransform.Value;
15.  
16.             translation.Value = cOMPosition + math.mul( rotation, orbiterpos );
17.         }).ScheduleParallel();
18.     }
19. }
20.  

I have also tried ComponentDataFromEntity<Translation> allTranslations = GetComponentDataFromEntity<Translation>(true); outside of the foreach but it suffers from the same problem. And I have searched the error text but got no information which help me resolve this.

So my questions are:
How can I refer to another entities translation component in my system which shall set the translation of each entity?
Am I thinking "the wrong (OOP)" way?
Is there a better approach obvious?
Is this restriction "justified"? I don't see what could go wrong. The error remains when I do Schedule instead of ScheduleParallel so even if one thread writes to the data is forbidden? Can you give me an example why this is forbidden?
Since I only need read access of the center of mass's entity translation component can I somehow convince Unity to give me at least that?

So I'm pretty stumped and hope someone has insights for me. Anyway, many thanks in advance.

 

You can't use ComponentDataFromEntity with a component being passed in the in Query (Get/Set/HasComponent just generates CDFE code for you when using Schedule/ScheduleParallel).

Here is the fixed code (may have typos):

Code (CSharp):

1. public class OrbitSystem : SystemBase
2. {
3.     protected override void OnUpdate()
4.     {
5.        Entities.ForEach((Entity entity, ref OrbitParameterData orbit) =>
6.         {
7.             if (!HasComponent<Translation>(orbit.centerOfMassEntity))
8.                 return;
9.             Translation cOMTransform = GetComponent<Translation>(orbit.centerOfMassEntity);
10.             float3 cOMPosition = cOMTransform.Value;
11.             SetComponent<Translation>(entity, new Translation() { Value = cOMPosition + math.mul( rotation, orbiterpos ) });
12.         }).Schedule();
13.     }
14. }

 

Thanks Bruno.
That was fast. And more importantly, it works. But when I use ScheduleParallel instead of schedule it does not work anymore. Since most entities in the game will be orbiters I would really like this system to run multithreaded.

With ScheduleParallel the "test Entity" does not move anymore and I get this error consecutively in playmode:

Which is the same error I got when using ComponentDataFromEntity<Translation> allTranslations = GetComponentDataFromEntity<Translation>(true); method outside of ForEach. Maybe I should mention that I'm a total noob in ECS and am just starting with it. And I'm on Linux Editor 2020.1 with latest packages (Entities 0.14.0 - preview 18).

So how can I declare the translation (of the referenced Entity) readonly to satisfy the compiler?
What does CDFE mean?
Does the "return" only cancel the current iteration (like a continue in a for loop)? So are other Entities still processed when one is missing?
Where can I learn such "restrictions" from? Documentation is sparse. Is reacting to errors and warnings the "way to go" for ECS still?

Thanks

 

See that link: https://docs.unity3d.com/Packages/c.../ecs_entities_foreach.html#supported-features

CDFE is ComponentDataFromEntity (Has/Set/GetComponent uses it under the hood inside Entities.ForEach.Schedule/ScheduleParallel) and ScheduleParallel can't write to it (SetComponent, in your case).

What you can do is split the job into 2, this way you could do something like that:

Code (CSharp):

1.             NativeArray<float3> positions = new NativeArray<float3>(_entityQuery.CalculateEntityCount(), Allocator.TempJob, NativeArrayOptions.UninitializedMemory);
2.  
3.             Entities
4.                .WithNativeDisableParallelForRestriction(positions)
5.                .WithStoreEntityQueryInField(ref _entityQuery)
6.                .ForEach((int entityInQueryIndex, ref OrbitParameterData orbit) =>
7.                 {
8.                     if (!HasComponent<Translation>(orbit.centerOfMassEntity))
9.                         return;
10.  
11.                     Translation cOMTransform = GetComponent<Translation>(orbit.centerOfMassEntity);
12.                     float3 cOMPosition = cOMTransform.Value;
13.                     positions[entityInQueryIndex] = cOMPosition + math.mul(rotation, orbiterpos);
14.                 })
15.                .ScheduleParallel();
16.  
17.             Entities
18.                .WithReadOnly(positions)
19.                .WithDeallocateOnJobCompletion(positions)
20.                .ForEach((int entityInQueryIndex, ref Translation translation, in OrbitParameterData orbit) =>
21.                 {
22.                     if (!HasComponent<Translation>(orbit.centerOfMassEntity))
23.                         return;
24.  
25.                     translation.Value = positions[entityInQueryIndex];
26.                 })
27.                .ScheduleParallel();

EDIT: untested, may have flaws there, but I think that it should be a good starting point

 

About that, yes, you can see "return" being like "continue" in a for loop.

 

Thanks alot Bruno. I had to "wrestle" the example code a bit. But that was very helpful and insightful. Now I understand the syntax and workings of ECS a bit better. And my system is working properly and multithreaded too. Now I can start to procedurally generate some solar systems which will probably spawn more questions.
Thanks again.

 

Ugh.... that's so ugly... but it works. I wish there was a cleaner way to solve the problem.