Question Questions around EntityCommandBuffer.SetBuffer and dynamic buffer sync points

Two questions:

1. How are you supposed to use

EntityCommandBuffer.SetBuffer

? There's only a default constructor on a

DynamicBuffer

and otherwise I'm not seeing a way to create a "temporary local buffer" to pass into that function. On the other hand, if you modify (e.g. clear, which the ECB doesn't have a function to simulate) a real buffer, then there's no need to call

SetBuffer

at all because those changes will be applied directly?

2. The docs about sync points don't suggest that dynamic buffer modifications could trigger a sync point. Confirming that this is the case?

3. If Q2 is confirmed, then what's the point of using

EntityCommandBuffer.SetBuffer

at all? I suppose there are the "fringe benefits" of replay-ability, and I guess it's needed if you want to set a buffer on an entity that's being created in an ECB, but then per Q1 I wouldn't know how to actually use

EntityCommandBuffer.SetBuffer

in that instance.

 

Pertaining to Q1: Ok I think I misunderstood what

SetBuffer

actually does and assumed too much meaning from its name. Turns out you don't pass a buffer into it, rather it gives you a (new?) buffer for a given entity, which you can then use the other ECB commands to fill in.

I found this snippet in a markdown file in Entities source:

Code (CSharp):

1. // SetBuffer is like AddBuffer, but safety checks will throw an exception at playback if
2. // the entity doesn't already have a MyElement buffer.
3. DynamicBuffer<MyElement> otherBuf = ecb.SetBuffer<MyElement>(otherEntity);
4.  
5. // Records a MyElement value to append to the buffer. Throws an exception at
6. // playback if the entity doesn't already have a MyElement buffer.
7. ecb.AppendToBuffer<MyElement>(otherEntity, new MyElement { Value = 12 });

Need to find out what happens if a buffer already exists on that entity - will it be overwritten with a new one, or returned as is?

 

AddBuffer will add or overwrite. SetBuffer will overwrite or throw if a buffer isn't already there. AppendToBuffer is like SetBuffer but instead of overwriting, it appends a single element.

 

Code (CSharp):

1.                 {
2.                     var b = mgr.GetBuffer<RewardChoiceBufferData>(self._singletonEnt);
3.                     UnityEngine.Debug.Log($"{b.Length}");
4.                 }
5.                 {
6.                     var cb = new EntityCommandBuffer(Allocator.Temp);
7.                     var b = cb.SetBuffer<RewardChoiceBufferData>(self._singletonEnt);
8.                     cb.AppendToBuffer(self._singletonEnt, new RewardChoiceBufferData());
9.                     cb.AppendToBuffer(self._singletonEnt, new RewardChoiceBufferData());
10.                     cb.AppendToBuffer(self._singletonEnt, new RewardChoiceBufferData());
11.                     cb.Playback(state.EntityManager);
12.                 }
13.                 {
14.                     var b = mgr.GetBuffer<RewardChoiceBufferData>(self._singletonEnt);
15.                     UnityEngine.Debug.Log($"{b.Length}");
16.                 }
17.                 {
18.                     var cb = new EntityCommandBuffer(Allocator.Temp);
19.                     var b = cb.SetBuffer<RewardChoiceBufferData>(self._singletonEnt);
20.                     cb.AppendToBuffer(self._singletonEnt, new RewardChoiceBufferData());
21.                     cb.AppendToBuffer(self._singletonEnt, new RewardChoiceBufferData());
22.                     cb.Playback(state.EntityManager);
23.                 }
24.                 {
25.                     var b = mgr.GetBuffer<RewardChoiceBufferData>(self._singletonEnt);
26.                     UnityEngine.Debug.Log($"{b.Length}");
27.                 }

Log shows 0 -> 3 -> 2, so

SetBuffer

overrides, and Q1 is now answered!

AddBuffer

exhibits the same behaviour, which leads to question what the difference between the two (noting this on the

AddBuffer

docs: "Will throw an error ... if the entity doesn't have a DynamicBuffer<T> component storing elements of type T").

 

Thanks @DreamingImLatios , didn't see your response before I posted that.

However, the docs suggest your comment about

AddBuffer

adding or overwriting is wrong - although I've not tested. See the last sentence in my above post. Maybe the docs are wrong? Or my interpretation of either is wrong

 

I think those docs are wrong.

 

It looks like indeed doc is wrong.
DynamicBuffer is added, when is missing.
Can you guys use report button on the doc, to correct the information.