Explore how to make APIs play nicer with Plugins about iyes_loopless HOT 5 OPEN

// instead of `impl Plugin` ...

pub fn add_enter_systems(stage: &mut SystemStage) {
    stage.add_system(thing);
    stage.add_system(thing2);
}
pub fn add_exit_systems(stage: &mut SystemStage) {
    stage.add_system(cleanup);
}
pub fn add_fixed_timestep_systems(stage: &mut SystemStage) {
    stage.add_system(fixed_thing);
}
pub fn add_general_systems(app: &mut App) {
    // add other things
}

fn main() {
    // ... set up other things ...
    // add our things, using our helper functions
    // (instead of `.add_plugin`)
    module::add_general_systems(&mut app);
    module::add_fixed_timestep_systems(&mut fixed_stage);
    module::add_enter_systems(&mut state_enter_stage);
    module::add_exit_systems(&mut state_exit_stage);
}

pub struct WorldPlugin {
    pub with_map: bool,
    pub with_creatures: bool,
    pub with_physics: bool,
}

            //Main time stage
        let mut main_stage = FixedTimestepStage::from_stage(
            Duration::from_millis((1000.0 / BASE_FPS) as u64),
            SystemStage::parallel().with_system_set(
                ConditionSet::new()
                    .run_in_state(GameState::Gaming)
                    .with_system(sync_game_speed)
                    .into(),
            ),
        );
        //Add map stage
        if self.with_map {
            app.add_plugin(MapPlugin);
            main_stage.add_stage(MapPlugin::get_time_stage());
        }

        //Add creature stages
        if self.with_creatures {
            app.add_plugin(CreaturePlugin);
            for stage in CreaturePlugin::get_stages() {
                main_stage.add_stage(stage);
            }
        }

        //Add physics stages
        if self.with_physics {
            app.add_plugin(
                RapierPhysicsPlugin::<NoUserData>::default().with_default_system_setup(false),
            );
            main_stage.add_stage(
                SystemStage::parallel().with_system_set(
                    RapierPhysicsPlugin::<NoUserData>::get_systems(PhysicsStages::SyncBackend),
                ),
            );
            main_stage.add_stage(
                SystemStage::parallel().with_system_set(
                    RapierPhysicsPlugin::<NoUserData>::get_systems(PhysicsStages::StepSimulation),
                ),
            );
            main_stage.add_stage(
                SystemStage::parallel().with_system_set(
                    RapierPhysicsPlugin::<NoUserData>::get_systems(PhysicsStages::Writeback),
                ),
            );

            // NOTE: we run sync_removals at the end of the frame, too, in order to make sure we don’t miss any `RemovedComponents`.
            main_stage.add_stage(
                //TODO run this outside in the correct stage?
                SystemStage::parallel().with_system_set(
                    RapierPhysicsPlugin::<NoUserData>::get_systems(PhysicsStages::DetectDespawn),
                ),
            );
        }

        app.add_stage_after(CoreStage::Update, GameStages::WorldTopStage, main_stage);