Re: [Vala] Treating a uint as a reference type?

[Alan Manuel Gloria <almkglor gmail com>]

On Fri, Jul 31, 2015 at 1:00 AM, Al Thomas <astavale yahoo co uk> wrote:

> > From: Alan Manuel Gloria <almkglor gmail com>
> > Semantically, an Entity is a reference to a row in an entity component
> > system; it just happens to be implemented using uint.
>
>
> This kind of sounds like referential integrity rules in
> a relational database. I presume you not using a database
> for performance and/or memory issues?
Yes.


> > I want to do this> because when I delete a row in the ECS table, I want
> the row to be retained
> > as "deleted" until all references to it are dropped, so that I avoid
> > the "missile suddenly chases a new entity" problem.
>
> By 'retained as "deleted"' do you mean changing the state of the row so it
> can
> no longer be used when a new entity is created? Would it cause too much of
> a
> performance hit if the row had an 'allow new references' boolean that
> caused
> a newly created entity to skip it if the row was closed?

The refcount would serve as something like that; if the refcount drops to
0, the row can be reused, otherwise not.


> Another thought is GLib signals, these are Vala signals, with a return
> value.
> So you would emit an I am about to delete xyz referenced component and if
> anything returns no don't do that the component is kept. I've not used
> return
> values so not sure how this would work and is a bit more heavy weight
> compared
> to reference counting.
>
> Just a few thoughts. I got the impression it could be a design issue rather
> than an implementation one.

I'd prefer to keep it automated; keeping a reference to the Entity should
be enough to keep the row alive, and in each subsystem that is interested,
query the liveness.

class TrackingMissile: Component {
  public Entity target;
}
class TrackingMissileSys: Subsystem {
  protected override
  void run() {
    foreach(Entity e in component_column<TrackingMissile>()) {
      TrackingMissile tm = e.get<TrackingMissile>();
      if (!tm.target.is_live()) {
        // missile loses its ability to track anything once target is dead.
        e.detach<TrackingMissile>();
        continue;
      }
      // ... track target here ...
    }
  }
}

Of course, maybe signals would also work; maybe I could add hooks for
attaching and detaching components to entities.

> All the best,
>
>
>
> Al
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