Component groups and get_many/get_many_mut for EntityRef/EntityMut.

[mintlu8]

Objective

Give a generic group of components similar functionality as if they were a single component. This is achieved with access to ergonomic filter queries and with get function support for EntityRef/EntityMut.

Use case:

trait SomeBehavior {
    type Read: ComponentGroup;
    type Modify: ComponentGroup;
    type Require: ComponentGroup;
    
    fn modify_behavior(
        write: &mut <Self::Modify as ComponentGroup>::WriteQuery, 
        read: &<Self::Read as ComponentGroup>::ReadQuery
    );
}

fn behavior_system<B: SomeBehavior>(query: Query<
    (Entity, <B::Read as ComponentGroup>::ReadQuery, <B::Modify as ComponentGroup>::WriteQuery), 
    (<B::Read as ComponentGroup>::Changed, <B::Require as ComponentGroup>::With),
>) {
     for (r, mut w) in query.iter() {
         SomeBehavior::modify_behavior(&mut w, r);
     }
}

Solution

Added trait ComponentGroup implemented on components and tuple Bundles. The trait has GATs that converts the tuple into queries, With queries, Has queries, Added queries, Changed queries, etc.

Or and its new counterpart And can now be used on the data side as boolean combinators to support this change.

---

Changelog

• Added aforementioned ComponentGroup.
• Added the capability of And and Or to function on the data side.
  And and Or can be used with filter-only queries on the data side. This currently causes no issue but do keep this in mind.
• Since Changed and Added recently got removed from the data side due to their confusing nature, defined Is<T> = And<(T,)> as a way to use these queries on the data side.
• Added primitives Always and Never as default cases for some queries.
• Implemented get_many and get_many_mut on component groups.

Migration Guide

I don't think there is a breaking change here.

[james7132]

I'm not seeing the clear benefit of having this when the QueryData and QueryFilter macros exist. They seem to do the same thing.

[mintlu8]

I'm not seeing the clear benefit of having this when the QueryData and QueryFilter macros exist. They seem to do the same thing.

It is designed for generic type projection in traits, specifically for library authors, as shown in the example. The macros only work for specific components.

What this trait means is 'Any number of components that act as a single component'. The analogues to ComponentGroup in bevy right now are WorldQuery and Bundle, but none of them support type projection, and both of them have quirks that makes type projection an awkward fit to implement on them.

Edit: Updated PR to show more intent.

[mintlu8]

Edit: Added new major functionality of get_many/get_many_mut for EntityRef/EntityMut.

[janhohenheim]

Triage: has merge conflicts
@mintlu8 do you want to update this PR or should I tag it S-Needs-Adoption? :)

[mintlu8]

Triage: has merge conflicts @mintlu8 do you want to update this PR or should I tag it S-Needs-Adoption? :)

This PR is designed with ideas before the QueryData/QueryFilter split. I'd imagine starting a new PR would be better. That being said categorizing it asS-Needs-Adoption is fine.