Advanced Component Queries

[roo-oliv]

If I'm not missing something, the only way to query Entities is via the presence (or absence) of Components. I find this limitation makes my code a little too loose and harder to read. Let me explain.

Say I have a Collidable component to indicate which entities should be checked for collisions. Now, I don't want to check every collidable entity against each other as it would be inefficient but rather I just want to check active/moving entities against all collidable entities. To do that I would have to create a new component, say CollisionActor, to then use this in the system's query. And this can get more and more complicated, I could have a collision cool down period which I would have to implement via the temporary deletion of the Collidable component or via another component IgnoreCollision.

I think it would be nice to be able to do a component like:

public struct Collidable
{
    public bool IsActor;
    public int GracePeriod;
    public int CollisionGroup;

    public Collidable(bool isActor = true, int gracePeriod = 0, int collisionGroup = 0)
    {
        IsActor = isActor;
        GracePeriod = gracePeriod;
        CollisionGroup = collisionGroup;
    }
}

And since there are no custom filtering lambdas I can pass over to DefaultECS I'd have to filter the entities myself:

    protected override void Update(GameState state, ReadOnlySpan<Entity> entities)
    {
        foreach (var entity in entities)
        {
            if(!entity.IsActor || entity.gracePeriod > 0 || entity.collisionGroup == 2) continue;
            ...

But this "if" would cause a branch divergence as far as I know and I'd lose performance. Also, I realize that maybe it's impractical to implement a filtering functionality within DefaultECS since you'd give up on using spans... unless you manage to create a more complex system of declaring member properties of a component as indexes I think.

Am I better off just creating new Components for these "properties" so I can filter my entities or is there a better way?

[Doraku]

You can actually query on the component value too using the With(ComponentPredicate) api. Only thing to keep in mind when using it, you need to Notify explicitly (doing an entity.Set(newValue) does it implicitely, using entity.Get<T>() = newValue does not) that the value has changed so the systems can recheck the entity.

[roo-oliv]

Oh, I missed this completely! Thanks for pointing it out!