multiple queries / SystemParam from `DeferredWorld`

[DasLixou]

What problem does this solve or what need does it fill?

There is currently no easy way of getting multiple queries from a DeferredWorld.

What solution would you like?

Add DeferredWorld::queries over a generic amount of queries and return if they have no violations.

[SkiFire13]

This should be made to work in a generic way with any SystemParam/with SystemState. Then multiple queries would be supported as (Query<A>, Query<B>) or SystemState<(Query<A>, Query<B>)

[DasLixou]

Ok so: Unsure if this should be allowed in DeferredWorld because I think there could be system params that break its promises. Also, seems like SystemState is that what I was searching for. I think we should remove World::query and either link more agressively to or add a helper method for system states. DeferredWorld on the other side, unsure if we can do anything more than multi-query, really.. Thoughts?

[SkiFire13]

unsure if we can do anything more than multi-query

AFAIK no SystemParam yet modifies the World structurally, also because I don't think the access mechanism allows to safely express that. So currently any SystemParam should be ok to allow. If this ever change there are at least these SystemParams that don't perform structural changes:

• Res/ResMut and Options of them;
• ParamSet of allowed SystemParams;
• given a separate SystemState stuff like EventWriter/EventReader could also work;
• &Archetypes, &Bundles and other shared references to world metadata;
• probably some other less used SystemParams.

[DasLixou]

But System param has &mut World, right? So a third party system param might do that.

[SkiFire13]

Ah I see my mistake: I was assuming this is just about fetching the system parameter (which takes a UnsafeWorldCell and need to respect the access API), however there's also the problem of initializing the SystemParam, and that takes &mut World and can do pretty much anything.

It could however work if the SystemState was already initialized though. I wonder if guaranteeing that is possible.

[DasLixou]

So having SystemState still take an &mut World but then have system_params (bikeshed) functions on both World and DeferredWorld? That would also make finding it easier.

[SkiFire13]

So having SystemState still take an &mut World but then have system_params (bikeshed) functions on both World and DeferredWorld?

The issue is that currently SystemState is lazily initialized the first time get is called. This cannot happen with DeferredWorld, so something else needs to initialize it before that point.

I just noticed however that component hooks are simple fns, so they can't hold on to an initialized system param.

[DasLixou]

Sorry, what do component hooks have to do with that? And I thought SystemState::new initializes the parameters? It at least calls Param::init_state..

[SkiFire13]

Sorry, what do component hooks have to do with that?

They are the main users of DeferredWorld, so that's probably where you want to get system states/parameters.

And I thought SystemState::new initializes the parameters?

You're right, I think have confused them with systems (which initialize their SystemState on either an explicit call to initialize or lazily when run is called).

That doesn't really solve the problem since you have to create them somewhere though.