Manual SystemState + SystemMeta ref methods

[ecoskey]

• expose API for manually initializing system param states (dynamic queries)
• no existing code should be affected
• SystemMeta methods for exposing reference fields (custom System impls)

Objective

Currently SystemMeta's private fields and the extensive unsafe code for dealing with SystemParams make custom system implementations unwieldy. One particular area this pops up is when trying to use dynamic queries with normal systems.

Solution

Allow users to specify parameters in SystemState that require explicit state initialization, and expose some readonly SystemMeta fields.

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Changelog

• Add additional type parameter Extra to SystemState

• add new_with_extra(), get_with_extra(), get_mut_with_extra(), get_unchecked_with_extra(), get_unchecked_mut_with_extra(), and fetch_with_extra() to SystemState (names verbose to not conflict with existing code)

• add component_access(&self), archetype_component_access(&self), and raw_name(&self) to SystemMeta

[ecoskey]

Also this issue is blocking my renderer abstraction stuff so it would be cool to merge this :)

[ItsDoot]

Interested in an example usage.

[ecoskey]

For some motivation:

Dynamic queries (and possibly other future {Param}Builders) don't make sense for normal function systems (possibly without some new API), since you have to manually initialize the system state, which intuitively requires a struct. One example of this usecase might be a new system wrapper, which could give access to a single dynamic query param:

struct WithDynamicQuery<I, O, Q, P, F: FnMut(I, Query<Q>, SystemParamItem<P>) -> O> {
  query_state: QueryState<Q>,
  f: F,
}

struct IsDynamicQuerySystem;

struct DynamicQuerySystemI, O, Q, P, F: FnMut(I, Query<Q>, SystemParamItem<P>) -> O> {
  state: SystemState<P, Query<Q>>,
  f: F
}

impl IntoSystem for WithDynamicQuery<...> { ... }
impl System for DynamicQuerySystem<...> { ... }

Though this is the only example I can think of where this makes sense (besides the renderer stuff I'm working on), I think exposing this functionality in the most low-level way possible while maintaining safety is worth it.