Multiple archetypes in one system?

Unity Engine
,
1

I’m pretty new to ECS and most of the example systems I see work on one type of entity defined by a single archetype. Is it bad practice for a system to work with two distinct sets of entities.

What functions cover this and is it normal behaviour? (Maybe code examples I’ve seen are a little dated)

Thanks

2

No, it’s totally fine and very common.

Just make sure the scope of your system doesn’t expand beyond where it belongs.

3

Aye thanks. I see that as the danger, if you let it get out of hand. Are there any official examples I can reference that use two multiple groups in a system?

4

Not sure about specific examples, but you can always look at the source of systems that Unity has developed. For example the ParentSystem uses 4 queries

protected override void OnCreate()
        {
            m_NewParentsGroup = GetEntityQuery(new EntityQueryDesc
            {
                All = new ComponentType[]
                {
                    ComponentType.ReadOnly<Parent>(),
                    ComponentType.ReadOnly<LocalToWorld>(),
                    ComponentType.ReadOnly<LocalToParent>()
                },
                None = new ComponentType[]
                {
                    typeof(PreviousParent)
                },
                Options = EntityQueryOptions.FilterWriteGroup
            });
            m_RemovedParentsGroup = GetEntityQuery(new EntityQueryDesc
            {
                All = new ComponentType[]
                {
                    typeof(PreviousParent)
                },
                None = new ComponentType[]
                {
                    typeof(Parent)
                },
                Options = EntityQueryOptions.FilterWriteGroup
            });
            m_ExistingParentsGroup = GetEntityQuery(new EntityQueryDesc
            {
                All = new ComponentType[]
                {
                    ComponentType.ReadOnly<Parent>(),
                    ComponentType.ReadOnly<LocalToWorld>(),
                    ComponentType.ReadOnly<LocalToParent>(),
                    typeof(PreviousParent)
                },
                Options = EntityQueryOptions.FilterWriteGroup
            });
            m_DeletedParentsGroup = GetEntityQuery(new EntityQueryDesc
            {
                All = new ComponentType[]
                {
                    typeof(Child)
                },
                None = new ComponentType[]
                {
                    typeof(LocalToWorld)
                },
                Options = EntityQueryOptions.FilterWriteGroup
            });
        }