Adding multiple items to DynamicBuffer causing Structural Changes error?

Not sure what I'm doing wrong here but i'm 100% certain its my fault. Can someone help me understand?

I am creating a bunch of new entities and adding them into a DynamicBuffer on an already existing Entity. But only the first item gets added, and on the second it throws an exception saying:

InvalidOperationException: Attempted to access ArchetypeChunkBufferType<EconSim.ItemReferenceElement> which has been invalidated by a structural change.

Code (CSharp):

1.        
2.         [Button]
3.         public List<Entity> CreateItemsInContainer(int itemID, Entity container, int itemCount)
4.         {
5.             if (itemID >= items.Count) return null;
6.             if (container.Equals(Entity.Null)) return null;
7.             if (!entityManager.Exists(container)) return null;
8.             if (!entityManager.HasComponent<VolumeCapacityData>(container)) return null;
9.            
10.             var itemDefinition = items[itemID];
11.             var containerVolume = entityManager.GetComponentData<VolumeCapacityData>(container).value;
12.             var totalItemVolume = itemDefinition.shippingVolume * itemCount;
13.          
14.             if (totalItemVolume > containerVolume) return null;
15.  
16.             var createdEntities = new List<Entity>(itemCount);
17.             var itemBuffer = entityManager.GetBuffer<ItemReferenceElement>(container);
18.  
19.             for (var i = 0; i < itemCount; i++)
20.             {
21.                 var entity = entityManager.CreateEntity(itemArchetype);
22.                 entityManager.AddComponentData(entity, new ItemIDData { value = itemID });
23.                 entityManager.AddComponentData(entity, new ItemPhysicalData { currentDurability = itemDefinition.durability });
24.                 entityManager.AddComponentData(entity, new ItemCargoData
25.                 {
26.                     cargoClass = itemDefinition.cargoClass,
27.                     massOfShippingVolume =  itemDefinition.massOfShippingVolume,
28.                     shippingVolume = itemDefinition.shippingVolume,
29.                 });
30.                 entityManager.AddComponentData(entity, new ContainerReferenceData { value = container });
31.                
32.                 createdEntities.Add(entity);
33.                 itemBuffer.Add(entity);
34.             }
35.            
36.             // Update the container's remaining volume
37.             entityManager.SetComponentData(container, new VolumeCapacityData { value = containerVolume - totalItemVolume });
38.            
39.             return createdEntities;
40.         }
41.  

 

If I change my code to get the buffer each time before adding, it seems to work but that feels horribly inefficient. Is that really neccesary?

 

https://docs.unity3d.com/Packages/com.unity.entities@0.11/manual/sync_points.html

Solution: Write it to an ECB then play it back.

 

If adding/removing items from a buffer is going to cause a sync point with with every change then I'm very concerned that using them will be incredibly inefficient.

EDIT: Also I didn't realize sync points even existed when doing standard main thread code in the first place (IE this isn't a job, just a normal function on a GO), so clearly I understand even less than I thought

 

Adding to the buffer is not causing the sync point.

Using EntityManager is causing the sync point (AddComponentData, CreateEntity) which invalidates the buffer.

 

The buffer is on a different entity than the one that is being modified with EntityManager though. Modifying one entity invalidates all the others?

Hmm.

How do I get an ECB outside of a Job? The documentation only shows getting one inside of a Job/System.

 

Ok I just refactored things to do all the adds after everything else is done.

I will look more into the ECBs but up until now I thought those were only neccesary inside Jobs/Systems.

Code (CSharp):

1.             var itemBuffer = entityManager.GetBuffer<ItemReferenceElement>(container);
2.             foreach (var createdEntity in createdEntities)
3.             {
4.                 itemBuffer.Add(createdEntity);
5.             }
6.  

 

Been searching for a bit now and I can't find anything about using ECB/ECBSystems outside of a Job.

The example in the docs on using ECB within a job, at the end passes the job to the ECBSystem:

m_EndSimulationEcbSystem.AddJobHandleForProducer(this.Dependency);

So I am unsure of what to do if I don't have a job. Do I need to do something else to register my additions so that they get run later?

 

You can just create an ecb and play it back yourself.

var ecb= new EntityCommandBuffer(Allocator.TempJob);

ecb.AddCompoent(entity, new Component());
ecb.AddCompoent(entity2, new Component());
ecb.AddCompoent(entity3, new Component());

ecb.PlayBack(EntityManager);
ecb.Dispose();

this way you can make a bunch of changes without invalidating your buffer.

any structural change call to EM will invalidate everything.

 

Thank you very much for that.

I've reworked things to use an ECB. Took a bit to figure out the buffer handling, but the key was to set a NEW buffer through the ECB and then add to that.

I'm not sure how I would handle this if I needed to append to an existing one though, but in this case I don't.

One problem I do have though is this method is supposed to return a List<Entity> of the newly created entities, which it can't do because the ECB creates them, and if I try to store the references the indices are all invalid.

This isn't KEY, and I can currently do without that List but again not sure how I would handle it if I needed it.

This is what I ended up with.

Code (CSharp):

1.  
2. var ecb = new EntityCommandBuffer(Allocator.Temp);
3. var itemBuffer = ecb.SetBuffer<ItemReferenceElement>(container);
4.  
5.  
6. for (var i = 0; i < itemCount; i++)
7. {
8.     // create the new item entity
9.     var entity = ecb.CreateEntity(itemArchetype);
10.     ecb.AddComponent(entity, new ItemIDData { value = itemID });
11.     ecb.AddComponent(entity, new ItemIDData { value = itemID });
12.     ecb.AddComponent(entity, new ItemPhysicalData { currentDurability = itemDefinition.durability });
13.     ecb.AddComponent(entity, new ItemCargoData
14.     {
15.         cargoClass = itemDefinition.cargoClass,
16.         massOfShippingVolume =  itemDefinition.massOfShippingVolume,
17.         shippingVolume = itemDefinition.shippingVolume,
18.     });
19.     ecb.AddComponent(entity, new ContainerReferenceData { value = container });
20.    
21.     // Add the item to the return array as well as the container's buffer
22.     createdEntities.Add(entity);
23.     itemBuffer.Add(entity);
24. }
25.  
26. // Update the container's remaining volume
27. ecb.SetComponent(container, new VolumeCapacityData { value = containerVolume - totalItemVolume });
28. ecb.Playback(entityManager);
29. ecb.Dispose();
30.  

 

if only i could add FrozenRenderSceneTag with ecb