Multiple queries in one system

[Danon5]

Because systems can be shared across multiple stores and therefore have a list of queries, one for each store, what's the intended way to set up a second query within a system? For example:

public sealed class SomeSystem : QuerySystem<SomeComponent> {
   private ArchetypeQuery m_someQuery;
   
   protected override void OnUpdate() {
      // How would I use m_someQuery in here?
      Query.ForEachEntity((ref SomeComponent someComponent, Entity e) => {
         // m_someQuery.ForEachEntity...
      }
   }
}

There are many examples where this kind of thing is useful, but an easy example could be iterating over all network objects and then doing a nested iteration loop over all clients to send them information about the objects.

It seems I would need to manage a list or hashmap of my custom query to account for multiple stores and account for when the system is removed / added to stores, but that sounds like a lot of work for something so simple—assuming I didn't want to recreate the query every frame (sounds wasteful).

[Etlay]

I agree, I think it's must have feature. Let me show an example of API from some different ECS, how it's possible to implement and in what situation it may be useful:

public class TriggerWhenOwnerHealthBelowSystem : SystemBase
{

    // several queries (groups) in system
		private Group _triggerQuery;
		private Group _charactersQuery;

		protected override void Initialize()
		{
                         // initialization of queries 
			_triggerQuery = CreateQuery()
				.RequireTag<TriggerWhenOwnerHealthBelow>()
				.Require<TargetCharacterLink>()
				.EnsureNotEmpty();   // execute method wan't be called if query is empty (performance trick remove overhead for virtual calls if no data to process)

                        _charactersQuery = CreateQuery()
  				.Require<Character>()
  				.Require<HealthFraction>()
  				.EnsureNotEmpty();
		}

		protected override void Execute()
		{
                   var entities = _triggerQuery.Entities; // get query entities
                   var whenBelow = _triggerQuery.GetComponents<TriggerWhenOwnerHealthBelow>(); // get query Component data
                   var triggerTargets = _triggerQuery.GetComponents<TargetCharacterLink>().Reinterpret<Entity>(); 

                   var players = _charactersQuery.Entities; 

                    var healthFractions = _charactersQuery.GetComponentsExtended(Arena, HealthFraction.Max);

                   var filter = NativeMemoryPool.Take<bool>(entities.Length);

        
                    for (int i = 0, length = _triggerQuery.Length; i != length; ++i)
                    {
                          var playerIndex = players.BinarySearchIndexUnsigned(triggerTargets[i]); // find entity index in array of player entities

                          // calculate condition for each trigger and store result into array
                          filter[i] = healthFractions[playerIndex].Value < whenBelow[i].Value; 
                      }

                   //batch mark all triggers with tag TriggerWhenOwnerHealthBelow as active
                   World.AddTags<ActionTriggerIsActiveTag>(entities, filter);
		}
	}

[friflo]

Late answer caused by focus of final release v3.0.0 - sorry.
But anyway.

I added an example to the documentation which enables execution of nested queries.
See: https://friflo.gitbook.io/friflo.engine.ecs/documentation/systems#custom-system

So for the given code snippet it would kook like:

public sealed class SomeSystem : QuerySystem<SomeComponent> {
    private ArchetypeQuery<Position> m_someQuery;   
    
    protected override void OnAddStore(EntityStore store) {
        m_someQuery = store.Query<Position>();
        base.OnAddStore(store); // must be called to ensure execution of OnUpdate()
    }
    
    /// Executes m_someQuery and the base class Query in nested loops.
    protected override void OnUpdate() {
      Query.ForEachEntity((ref SomeComponent someComponent, Entity e) => {
         m_someQuery.ForEachEntity(...);
      }
    }
}