Extension trait to add methods to World

[sunjay]

It is kind of awkward sometimes to call group.create(&mut world). Some people might prefer the inverse of that API: world.create_group(group). This is possible via an extension trait:

trait WorldGroupExt {
    fn first_group<G: ComponentGroup>(&self) -> Option<G>;
    fn fetch_group<G: ComponentGroup>(&self, entity: Entity) -> G;
    fn create_group<G: ComponentGroup>(&mut self, group: G) -> Entity;
    fn update_group<G: ComponentGroup>(&mut self, entity: Entity, group: G) -> Result<(), <G as ComponentGroup>::UpdateError>;
    fn remove_group<G: ComponentGroup>(&mut self, entity: Entity) -> G;
}

impl WorldGroupExt for World {
    fn first_group<G: ComponentGroup>(&self) -> Option<G> {
        G::first_from_world(self)
    }
    fn fetch_group<G: ComponentGroup>(&self, entity: Entity) -> G {
        G::from_world(self, entity)
    }
    fn create_group<G: ComponentGroup>(&mut self, group: G) -> Entity {
        group.create(self)
    }
    fn update_group<G: ComponentGroup>(&mut self, entity: Entity, group: G) -> Result<(), <G as ComponentGroup>::UpdateError> {
        group.update(self, entity)
    }
    fn remove_group<G: ComponentGroup>(&mut self, entity: Entity) -> G {
        G::remove(self, entity)
    }
}

This extension trait needs to be imported every time it is used. (Not a huge deal at all.)

ComponentGroup trait	WorldGroupExt trait
MyGroup::first_from_world(&world)	let group: MyGroup = world.first_group()
MyGroup::from_world(&world, entity)	let group: MyGroup = world.fetch_group(entity)
group.create(&mut world)	world.create_group(group)
group.update(&mut world, entity)	world.update_group(entity, group)
MyGroup::remove(&mut world, entity)	let group: MyGroup = world.remove_group(entity)