Question About GetAllUniqueSharedComponentData

Hey there, I've noticed that when you call

Code (CSharp):

1. EntityManager.GetAllUniqueSharedComponentData(...)

you will always get a default constructed instance of the ISharedComponentData that you request.

The implementation confirms this:

        public void GetAllUniqueSharedComponents<T>(List<T> sharedComponentValues)

            where T : struct, ISharedComponentData

        {

            sharedComponentValues.Add(default(T));

            for (var i = 1; i != m_SharedComponentData.Count; i++)

            {

                var data = m_SharedComponentData[i];

                if (data != null && data.GetType() == typeof(T))

                    sharedComponentValues.Add((T) m_SharedComponentData[i]);

            }

        }

As ISharedComponentData can be used with reference types, it can require you to null check the fields of each element in the populated lists.

Is there any reason why this is done? This signature of the function doesn't suggest this would be the behaviour.

 

If you never use the default (eg you have a reference in it), just start your indexing at 1, the first result is always the default .

 

I mean yes - there are workarounds. The question was more geared toward a mismatch of expectations of what a function says it does and what it actually does.

It feels like a code smell that you have to be aware of this quirk and work around it.

 

I feel like there should be a bool includeDefault as a parameter to the function. I can see where the logic is though, default is a datum value, and it's technically always used, if a shared component is added via archetype, it'll be the default value if no other changes are applied.

 

IMO, that function should do what it says.. it should give me all the unique shared component data that actually exists.

If there is an Entity that has been created with the default value of the component, then it should give it to me. But if there isn't, then it should not.

---

The problem I have with the current behavior... is that GetAllUniqueSharedComponentData() returns the default value even if there are no components that contain the default value..

For example, I have to do a null check in this code.. because it's giving me back a default value that I've never created, and I will never create.

Code (csharp):

1. var clothOwners = new List<ClothChild>();
2.       EntityManager.GetAllUniqueSharedComponentData<ClothChild>(clothOwners);  
3.  
4.  
5.       foreach (var clothOwner in clothOwners) {
6.         if (clothOwner.ClothEntity == Entity.Null) { continue; }
7.         var cloth = clothView[clothOwner.ClothEntity];

 

I fully support that.
I just fell into this trap and it took me a few hours to figure out what was wrong:
https://forum.unity.com/threads/orphaned-sharedcomponentdata.987417/

 

Please fix this really unexpectable behavior…

 

Fixing it will broke all existing codebase, where we start iteration at 1 index.

 

That's why we are having 'preview'.
We must fix this terrible behaviour before its release.
ECS already have so many breaking API changes.

 

Just got tripped up in this as well. Because the doc specifically says:

My first reaction was to figure out if my own code added an extra version somewhere. I strongly feel this is an unnecessary timesink that should be fixed.