Objective

Fixes #22885

Solution

I added associated function to SystemParam which returns all of the types that the param uses as shared state. It returns a static slice of SharedStateVTable which contains type-erased functions for working with the single instances of each type of shared state.

I've also added a new struct SharedStates which is used to manage the type-erased shared states. The SystemParam::init_state function accepts a reference to the shared states and implementers can use it to get typed pointers which it can store as part of the state. SystemParam::init_state is now unsafe to call. It is required to make sure the newly constructed state does not outlive the SharedStates.

Each type that is to be used as shared state must implement SystemParamSharedState which is used internally to make the vtables.

Open questions

How should DynSystemParam work?

Currently SystemParam::shared is an associated function so DynSystemParam can't forward to the concrete type. For now, I've added "Don't use shared state" to the safety requirements of DynSystemParam::new. Another possibility is to make it not share with anything else, but still share internally and log a warning.

Todo

• redo impl of SystemParam for Commands to share their InternaCommandQueues
• implement SystemParamSharedState in the SystemParam derive macro
• include the pointers to shared states in derived implementation of SystemParam

Testing

I added a test case that confirms the shared state is actually shared.

Showcase

See the test case for basic usage. It shares an Arc<AtomicBool>. https://github.com/Person-93/bevy/blob/4cb020a0c9065f5bf363316677d49d60b1de4d51/crates/bevy_ecs/src/system/system_param.rs#L3529-L3602