Pipe dream: ability to do total rewind of some world state

[Architector4]

This isn't really a feature request or an "idea" or anything such, because I'm pretty certain that this is impossible with current architecture of Bevy and is probably unlikely to be supported in the future. I just want to put the thought out and maybe discuss it.

One thing I'm dreaming about is for the ability to do total rewind of some world state (some component/resource types, some systems, and some entities) within Bevy.

For components/resources that would involve also snapshotting and rewinding their change detection tick values, as well as added/removed component signals. For systems, that would involve snapshotting and rewinding their Local params, including the cursors in EventReaders. For entities, that would involve snapshotting and rewinding which entities exist, including the exact entity IDs and generations.

Obviously, some world state is best not rewinded. It'd be a bad idea to rewind fancy components and resources, like Window components (at least not unintentionally), some random technical components with raw pointers, or systems that interface with the OS and need some of their Locals actually untouched. Or caches of various kinds. But still.

For example, I want to be able to perform a series actions from an exclusive system over &mut World:

1. Run a schedule A. Systems in that schedule spawn/remove/modify some entities and components and produce various events. An entity with an ID like 10v10 is spawned.
2. Snapshot world state as it is right now.
3. Run a schedule B that only has systems that respond to events and change detection. Systems in that schedule respond to change detection from events of system A, read the incoming events, do their stuff. Entity 10v10 is deleted, and another entity with the same ID but new generation, 10v11 is spawned.
4. Run a schedule B again. It only responds to stuff that was done in step 3.
5. Rewind state to that saved in step 2. Entity 10v10 exists again.
6. Run schedule B. Systems in it perform all the same things they did in step 3.

This is obviously impossible with current Bevy, as nothing can touch Local params except the system that has them itself (and even then there's nothing that can check if they are clonable at all), nothing can undespawn entities with their previously existing components, nothing can rewind entity generation IDs, and nothing can swallow or generate components added/removed signals.

I just think that, if Bevy was radically different (and more complex) and supported such chicanery, it'd be very cool. I don't think this is reasonable to request, so I don't, I just wanted to braindump.

[nicopap]

Check this out, bevy implements system stepping:

• System Stepping implemented as Resource #8453

Not exactly rollback, but half way there!

Also check out all the rollback mutliplayer plugins for bevy. Rollback requires the ability to rewind the world state, and they implement this as a necessity. So if you are curious, you can check out their implementations!