use bevy_ecs::{lifecycle::HookContext, prelude::*, system::SystemId, world::DeferredWorld};

fn main() {
    let mut world = World::new();

    let system_cleanup = world.spawn_empty().id();
    let system_id = world.register_system(move |mut commands: Commands| {
        commands.entity(system_cleanup).despawn();
    });
    world
        .entity_mut(system_cleanup)
        .insert(SystemCleanup(system_id));

    world.run_system(system_id).unwrap();
}

#[derive(Component)]
#[component(on_replace = on_replace)]
struct SystemCleanup(SystemId);

fn on_replace(mut world: DeferredWorld, context: HookContext) {
    let system = world.get_mut::<SystemCleanup>(context.entity).unwrap().0;
    world.commands().unregister_system(system);
}

        if let Ok(mut entity) = self.get_entity_mut(id.entity)
            && !entity.is_despawned()
        {
            entity.insert::<RegisteredSystem<I, O>>(RegisteredSystem {
                initialized,
                system,
            });
        }