Feedback Rename/improve the API for EntityCommandBuffer.SetBuffer

So I only recently found out we can directly add references to entities created via an EntityCommandBuffer to Dynamic Buffers. When I first tried it I would do this:

Code (CSharp):

1.            Entities
2.                 .ForEach((int entityInQueryIndex, Entity e, ref DynamicBuffer<LinkedEntityGroup> group) =>
3.             {
4.                 var newA = ecb.CreateEntity(entityInQueryIndex, arch);
5.                 group.Add(newA);
6.             }).ScheduleParallel();

This just stores an invalid reference in the dynamic buffer. So I created a weird workaround where I had a separate system add the reference on the next frame, and it was just a mess. I only today learned from helpful folks on the discord that what you actually have to do is this:

Code (CSharp):

1.  
2.             Entities
3.                 .ForEach((int entityInQueryIndex, Entity e) =>
4.             {
5.                 var newA = ecb.CreateEntity(entityInQueryIndex, arch);
6.                 var buffer = ecb.SetBuffer<LinkedEntityGroup>(entityInQueryIndex, e);
7.                 buffer.Add(newA);
8.             }).ScheduleParallel();

So you can't actually use the dynamic buffer that you pass into the lambda if you're adding a newly created entity reference to it. Not intuitive at all. I understand there's a good reason for this but it's just a bit confusing. The API naming is strange too - you're calling "SetBuffer" but you're getting back a buffer to operate on.

I'm not sure exactly how to improve it - I think just renaming "SetBuffer" to something like "GetPlaybackBuffer" or something would help a bit.

 

It is worth noting however, that SetBuffer will cause the existing buffer to be reset (at least from my testing). So if you either keep long-term stuff in a buffer (LinkedEntityGroup anyone?) or have multiple systems writing to the same buffer using the same *EntityCommandBufferSystem (they'll overwrite each other) this will not suffice. Some way of adding to an existing buffer using an entity command buffer would be lovely indeed.

 

SetBuffer works exactly the same way as SetComponent. If you set a component on a command buffer it will replace the existing version in its entirety. When working SetComponent or SetBuffer if you want to preserve any of the existing data you have to manually copy that data across. When working with a buffer the flow looks like so:

Call SetBuffer on the command buffer to return a fresh buffer that will override the current one sitting on the entity. Copy the entities from the original buffer to the new one. Add the additional entities that have been newly created through the command buffer. Once the command buffer resolves the new entity buffer will contain all the original entities as well as the remapped versions of the ones you just created.

 

Hey, I appreciate the reply but I think you might have misunderstood the original post. Whether the LinkedEntityGroup buffer is attached to an entity during conversion or at any time else, this code doesn't work:

Code (CSharp):

1.            Entities
2.                 .ForEach((int entityInQueryIndex, Entity e, ref DynamicBuffer<LinkedEntityGroup> group) =>
3.             {
4.                 var newA = ecb.CreateEntity(entityInQueryIndex, arch);
5.                 group.Add(newA);
6.             }).ScheduleParallel();

The reference added to the LinkedEntityGroup is invalid, it will point to index -1. What @Brendon_Smuts described will do what I want, but it still seems pretty awkward compared to the snippet above which one would intuitively expect to "just work".

 

Hmm, that's a cool workaround! But it does involve extra copying around of data. Would be nice if that could be avoided, given that buffers can, depending on the use case, hold a lot of data.

 

Had the same problem today

Unfortinately this workaround would not work if there are two systems, which want to add Entity to dynamic buffer using ecb. We definetely need some way of adding to an existing buffer using an entity command buffer.

 

I don't understand the failure case you are describing but this API exists in the latest Entities. ECB has an AppendToBuffer method now.

 

I didn't know about this, I am still on 0.9 because 0.10 is too unstable. But this is fantastic news.

 

As Brendon_Smuts said, If you want to adding new LinkedEntityGroup elems to an existing buffer using an entity command buffer. This is feasible:

Entities.ForEach((int entityInQueryIndex, Entity entity, in DynamicBuffer<LinkedEntityGroup> linkedEntityGroups) =>
{
var newEntity = commandBuffer.CreateEntity(entityInQueryIndex);
var buffer = commandBuffer.SetBuffer<LinkedEntityGroup>(entityInQueryIndex,entity);
var curLEG = linkedEntityGroups.ToNativeArray(Allocator.Temp);
buffer.AddRange(curLEG);
buffer.Add(newEntity);
curLEG.Dispose();
}).ScheduleParallel(Dependency);