Tracking: Atomically Staged Components

[ElliottjPierce]

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

Lots of planned and wishlist features require (or would benefit from) mutably sharing components between threads. These notably include assets as entities (#11266), multi-world bevy, readonly queries and other readonly system parameters, pre-registering components, and scripting.

Roadmap

• Prototype. See Synchronized component registration #17569.
• Decouple registration from storage. See Isolate component registration #17671.
• Fix required components merging. See Use register_dynamic for merging #18028.
• Implement generalized atomic staging. See Generalized and atomic staging utils #17701.
• Implement atomic staging for components. See Staged Components #17871.
• Use staged components in World.
• Change signatures of internal use of Components to use staged components.
• Allow readonly queries and system parameters to be initialized with only &World.
• Allow creating and running read-only systems with only &World.
• Remove try_query API (no longer needed).
• Expose components to users via a system parameter. Maybe.
• After final interfaces are decided, do a final pass for code quality.
• Related: Make ComponentInfo fully immutable in an arc or something. This mainly benefits code quality.

[ElliottjPierce]

Noting for the record this conversation on discord. It discusses alternative design, that, at the time of writing is better IMO.

[alice-i-cecile]

I think we should close this and open a tracking issue for that design :)

[ElliottjPierce]

I think we should close this and open a tracking issue for that design :)

Absolutely.