for reusing code between states, did you try impl blocks that are generic over the state type parameter? for struct State<T> { state: T } with possible s

for reusing code between states, did you try impl blocks that are generic over the state type parameter? for struct State<T> { state: T } with possible states struct A { id: Uuid } struct B { id: Uuid } use a trait trait HasId { fn id_mut(&mut self) -> &mut Uuid; } now you can impl over both A and B impl<T: HasId> for State<T> { fn new_id(&mut self) { *self.state.id_mut() = Uuid::new_v4() } } simplistic example but enough to communicate the idea hopefully. generally, macros makes this kind of thing ergonomic so you can generate the trait implementations and so on. otherwise it's a lot of typing. don't understand what you mean about making callbacks to other APIs during transitions.