Manage app state

Derive a States type in an AppState enum:

#[derive(Debug, Clone, Eq, PartialEq, Hash, Default, States)]
enum AppState {
  #[default]
  MainMenu,
  InGame,
  Paused,
}

Using NextState you can transition the apps state between the enum values:

fn start_game(mut next_state: ResMut<NextState<AppState>>) {
  next_state.set(AppState::InGame);
}

fn pause_game(
  mut next_state: ResMut<NextState<AppState>>,
  current_state: Res<State<AppState>>,
  input: Res<ButtonInput<KeyCode>>,
) {
  if input.just_pressed(KeyCode::Escape) {
    next_state.set(AppState::MainMenu);
  }
}

You can then use the state of your app to run systems conditionally:

fn main() {
  App::new()
    // Add our state to our app definition
    .init_state::<AppState>()
    // We can add systems to trigger during transitions
    .add_systems(OnEnter(AppState::MainMenu), spawn_menu)
    // Or we can use run conditions
    .add_systems(Update, play_game.run_if(in_state(AppState::InGame)))
    .add_systems(Update, pause_game)
    .run();
}