Ergonomic way to get exited state in OnExit system

[tmstorey]

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

If you have to run the same logic when exiting multiple states, you might want to reuse the same system in multiple OnExit schedules. Unfortunately, Res<State<_>> returns the state that is being entered, not the one that was just exited, so the only way to get the previous state is to read the StateTransitionEvent.

What solution would you like?

Either have Res<State<_>> return the state being exited, or add something like PreviousState.

What alternative(s) have you considered?

As I said above, I'm currently using the StateTransitionEvent, but it's a bit clumsy.

Additional context

None that I'm aware of.

[alice-i-cecile]

I prefer PreviousState I think :)

[benfrankel]

Agreed that this would be useful.

The approach taken in pyri_state is to have NextState always store the next state instead of only when the user intends to change the state. Then during state transitions, State is the "state being exited" and NextState is the "state being entered" (State gets updated with NextState's value after the user's systems run).

[janhohenheim]

Implemented by #21995