Request: ISystemStateBufferElementData

It would be good to have a system state version of IBufferElementData. A use case would be to destroy all children of an entity when it is destroyed. I am storing children in an IBufferElementData.

 

To suggest an alternative, this is a system I use to destroy my child entities. It uses TransformSystem's ParentToChildTree MultiHashMap, which tracks all children for a parent:

Code (CSharp):

1. [UpdateAfter(typeof(TransformSystem))]
2. [UpdateBefore(typeof(EndFrameBarrier))]
3. public class DestroyHierarchySystem : JobComponentSystem {
4.     [Inject] EndFrameBarrier barrier;
5.     bool isInitialized;
6.     NativeMultiHashMap<Entity, Entity> parentToChildTree;
7.    
8.     void Init() {
9.         //Using reflection to get the existing ParentToChildTree HashMap of TransformSystem.
10.         var transformSystem = World.GetExistingManager<TransformSystem>();
11.         var parentToChildInfo = typeof(TransformSystem).GetField(
12.             "ParentToChildTree", BindingFlags.NonPublic | BindingFlags.Instance);
13.         parentToChildTree = (NativeMultiHashMap<Entity, Entity>)parentToChildInfo.GetValue(transformSystem);
14.         isInitialized = true;
15.     }
16.  
17.     [BurstCompile]
18.     struct DestroyHierarchyJob : IJobProcessComponentDataWithEntity<DestroyEntityTag> {
19.         public EntityCommandBuffer.Concurrent CommandBuffer;
20.         [ReadOnly] public NativeMultiHashMap<Entity, Entity> ParentToChildTree;
21.         [ReadOnly] public ComponentDataFromEntity<DestroyEntityTag> DestroyEntityTags;
22.        
23.         public void Execute(Entity entity, int jobIndex, [ReadOnly] ref DestroyEntityTag tag) {
24.             DestroyTreeIterative(jobIndex, entity);
25.         }
26.        
27.         void DestroyTreeIterative(int jobIndex, Entity parent) {
28.             CommandBuffer.DestroyEntity(jobIndex, parent);
29.             if(!ParentToChildTree.TryGetFirstValue(parent, out Entity foundChild, out var iterator)) {
30.                 return;
31.             }
32.             do {
33.                 DestroyTreeIterative(jobIndex, foundChild);
34.                 //Check that child does not already have a tag so it is not deleted twice.
35.                 if(!DestroyEntityTags.Exists(foundChild)) {
36.                     CommandBuffer.DestroyEntity(jobIndex, foundChild);
37.                 }
38.             } while(ParentToChildTree.TryGetNextValue(out foundChild, ref iterator));
39.         }
40.     }
41.  
42.     protected override JobHandle OnUpdate(JobHandle inputDeps) {
43.         if(!isInitialized) {
44.             Init();
45.         }
46.         return new DestroyHierarchyJob {
47.             CommandBuffer = barrier.CreateCommandBuffer().ToConcurrent(),
48.             ParentToChildTree = parentToChildTree,
49.             DestroyEntityTags = GetComponentDataFromEntity<DestroyEntityTag>(isReadOnly:true)
50.         }.Schedule(this, inputDeps);
51.     }
52. }

I just add DestroyEntityTag component to the parent in the hierarchy where I want to destroy, when TransformSystem runs again it cleans up the hashmap for me.

 

I appreciate it but I have already implemented an alternative. It was just an example use case to justify the request. There will always be alternate ways to do things but I think it’s a valid request and fits well with the existing API. I think dynamic buffers should have all of the same features as component data.