Semi-automatic registration

[ItzShiney]

It is possible to make struct registration semi-automatic, using inventory crate. The solution simplifies registration a lot and does not require a centralized registration function with all the structs listed.

A feature (which may not be enabled by default) can be introduced to toggle this, since it requires an additional dependency which may not be wanted.

It can be achieved using these three things:

1. A helper struct:

pub struct Register(pub fn(&mut World));

2. An attribute #[register] for impl Trait for Struct, which generates:

inventory::submit!(bevy_trait_query::Register(|world| {
    use bevy_trait_query::RegisterExt;
    world.register_component_as::<dyn Trait, Struct>();
}));

3. A global registration function, which should be called manually (that's why the solution is semi-automatic):

fn register_traits(world: &mut World) {
    inventory::collect!(Register);
    for callback in inventory::iter::<Register> {
        callback(world);
    }
}

[RobWalt]

I think we can look into that. As far as I'm aware, inventory isn't supported on WASM though, so this would probably be feature gated. Nice find!

See bevyengine/bevy#3936 for a reference in upstream, links and discussion on the topic on a broader range

[joseph-gio]

Another option I looked into a while ago is moving trait registration into a method on the Component trait. This would still require you to manually register each trait for each component type (likely using an attribute on the component derive macro) but we could make it a compile-time error if you forget to do so.

When I tried this I ran into issues because currently we rely on all trait components being registered at app startup, whereas with the approach I'm describing each component would only be registered the first time it's used in the World. There's likely a way to work around this, though

I'm on my phone rn but I can go into more detail later if needed

[RobWalt]

Uhhh, sounds jnteresting as well. I would be interested. But it would need to happen upstream, right? Anyways, I'd probably even try to make it happen there.

[joseph-gio]

We'd need to add a default method to the Component trait and then add a bevy-trait-query::Component derive macro that would implement it. Its a very small upstream change and the method doesn't need to be for trait queries specifically