Multiple archetypes in one system?

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

 

No, it's totally fine and very common.

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

 

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?

 

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

Code (CSharp):

1. protected override void OnCreate()
2.         {
3.             m_NewParentsGroup = GetEntityQuery(new EntityQueryDesc
4.             {
5.                 All = new ComponentType[]
6.                 {
7.                     ComponentType.ReadOnly<Parent>(),
8.                     ComponentType.ReadOnly<LocalToWorld>(),
9.                     ComponentType.ReadOnly<LocalToParent>()
10.                 },
11.                 None = new ComponentType[]
12.                 {
13.                     typeof(PreviousParent)
14.                 },
15.                 Options = EntityQueryOptions.FilterWriteGroup
16.             });
17.             m_RemovedParentsGroup = GetEntityQuery(new EntityQueryDesc
18.             {
19.                 All = new ComponentType[]
20.                 {
21.                     typeof(PreviousParent)
22.                 },
23.                 None = new ComponentType[]
24.                 {
25.                     typeof(Parent)
26.                 },
27.                 Options = EntityQueryOptions.FilterWriteGroup
28.             });
29.             m_ExistingParentsGroup = GetEntityQuery(new EntityQueryDesc
30.             {
31.                 All = new ComponentType[]
32.                 {
33.                     ComponentType.ReadOnly<Parent>(),
34.                     ComponentType.ReadOnly<LocalToWorld>(),
35.                     ComponentType.ReadOnly<LocalToParent>(),
36.                     typeof(PreviousParent)
37.                 },
38.                 Options = EntityQueryOptions.FilterWriteGroup
39.             });
40.             m_DeletedParentsGroup = GetEntityQuery(new EntityQueryDesc
41.             {
42.                 All = new ComponentType[]
43.                 {
44.                     typeof(Child)
45.                 },
46.                 None = new ComponentType[]
47.                 {
48.                     typeof(LocalToWorld)
49.                 },
50.                 Options = EntityQueryOptions.FilterWriteGroup
51.             });
52.         }