Tag support in QuerySystem generic parameters?

[Danon5]

Getting started with this library, and the very first system I tried to make seems to require a custom system setup. All I want is a system that can query entities with a certain tag, but QuerySystem seems to only support components as generic arguments (and you can't have zero generic arguments). It seems like I would have to make my own system base type, or make tags into components so I can query them normally... but both of these options feel wrong conceptually (since there are obvious abstractions for both in the library). Am I missing something? Queries themselves can have zero arguments and just be filtered by tag, but not in QuerySystems.

I thought I could use BaseSystem to do what I want while retaining the whole "System" abstraction that currently exists, but there is no way to build / iterate over a simple query since no "current EntityStore" is passed into OnUpdateGroup, so the system would never know which EntityStore to run against. If I am approaching this all wrong, I would love to gain some insight. I think what is needed is the abstraction of QuerySystem but without the generic parameters. It would just let users provide a query directly.

It might seem strange to iterate over entities with a tag without any data involved, but imagine a simple scenario like wanting entities to be destroyed at the end of the frame in a game engine. You could just throw a "DestroyAtEndOfFrameTag" on them and have a system take care of it. In my case, I simply want to know if any entities exist with a certain tag, but I would like to do it with the perf-debug features of BaseSystem.

For now I will just make my own system base class, but maybe this could be considered.

[friflo]

To filter the result set of a QuerySystem<> with tags you can use its Filter property. E.g.

struct Pulsating : ITag { }

class PulseSystem : QuerySystem<Scale3>
{   
    public PulseSystem() => Filter.AnyTags(Tags.Get<Pulsating>());
    
    protected override void OnUpdate() {
        // ...
    }
}

In case you want to create a custom system by extending BaseSystem you can access the stores with SystemRoot.Stores. E.g.

class CustomSystem : BaseSystem
{   
    protected override void OnUpdate() {
        foreach (var store in SystemRoot.Stores) {
            // iterate all entities in a store
            foreach (var entity in store.Entities) { }
        }
    }
}

You are right. A QuerySystem without any generic components is a valid use case.
This is currently not available. I will add this in a future release.

[friflo]

A QuerySystem class without generic component parameters is now available in release
https://www.nuget.org/packages/Friflo.Engine.ECS/3.0.0-preview.13

Example:

public class AllEntitiesSystem : QuerySystem
{
    protected override void OnUpdate() {
        foreach (var entity in Query.Entities) {
            ...
        }
    }
}