StateTransitionEvent

[SIGSTACKFAULT]

Objective

• Make it possible to react to arbitrary state changes
• this will be useful regardless of the other changes to states currently being discussed

Solution

• added StateTransitionEvent<S> struct
• previously, this would have been impossible:

#[derive(States, Eq, PartialEq, Hash, Copy, Clone, Default)]
enum MyState {
  #[default]
  Foo,
  Bar(MySubState),
}

enum MySubState {
  Spam,
  Eggs,
}

app.add_system(Update, on_enter_bar);

fn on_enter_bar(trans: EventReader<StateTransition<MyState>>){
  for (befoare, after) in trans.read() {
    match before, after {
      MyState::Foo, MyState::Bar(_) => info!("detected transition foo => bar");
      _, _ => ();
    }
  }
}

---

Changelog

• Added
  • StateTransitionEvent<S> - Fired on state changes of S

Migration Guide

N/A no breaking changes

[Nilirad]

Interesting move. Just a couple of terminology nits.

[SIGSTACKFAULT]

just making sure y'all aren't waiting for me to do something

[Nilirad]

Looks almost right. Just a small doc nit I forgot. For the rest it's ok.

[jannik4]

I know I'm a bit late on this, but @SIGSTACKFAULT could you maybe give #10334 a look. I think it also does cover your use case and in addition to that it allows to react to arbitrary state changes at the "right" time (in between enter/exit).