use bevy::prelude::*;

#[derive(Clone, Copy, Debug, PartialEq, Eq, Hash)]
enum AppState {
    A,
    B,
}

fn main() {
    App::new()
        .add_state(AppState::A)
        .add_system_set(SystemSet::on_update(AppState::A).with_system(change_state))
        .add_system_set(SystemSet::on_exit(AppState::A).with_system(change_state))
        .run();
}

fn change_state(mut state: ResMut<State<AppState>>) {
    println!("Changing state to B");
    state.set(AppState::B).unwrap();
}