Add SystemIdTemplate to allow one-shot system in bsn!

[hxYuki]

Objective

• Fixes Allow registering one-shot system in bsn! #24002

Solution

• Add SystemIdTemplate

  pub struct SystemIdTemplate(Arc<Mutex<Option<SystemOrId>>>);
  
  enum SystemOrId {
      BoxedSystem(BoxedSystem<(), ()>),
      SystemId(SystemId<(), ()>),
  }

• Add a helper function system_value<M> to convert a system to SystemIdTemplate

• When a SystemIdTemplate is built, record its TypeId for the SystemId in HashMap and spawn a "relationship pair" between the template entity and the registered system entity.

• Each cloned template built will get the inserted SystemId.

• Each new built template refers to same system will get the SystemId by system type.

• When a template item despawns, despawn all its "relationship pairs" entities, which triggers a hook on the relationship component to check if the system entity is still referenced. If not, unregister it and remove the record.

Testing

codes

fn main() {
    fn run_callbacks(mut commands: Commands, callbacks: Query<(Entity,&Callback)>) {
        callbacks.iter().for_each(|(entity, callback)| {
            commands.run_system(callback.0);

            // It will print "1,2,1,2" for the scene entity is despawned after
            // called. The system is unregistered then re-registered.
            // Comment this line you will get "1,2,3,4,5,6".
            commands.entity(entity).despawn();
        });
    }
    App::new()
        .add_plugins(DefaultPlugins)
        .add_systems(Startup, (callback_scene.spawn(), run_callbacks, callback_scene.spawn(), run_callbacks).chain())
        .run();
}

#[derive(Component, FromTemplate)]
struct Callback(bevy_ecs::system::SystemId);

#[derive(Component, FromTemplate)]
struct SomeStruct{
        value: u32,
        callback: bevy_ecs::system::SystemId,
}
fn callback_scene() -> impl SceneList {
    bsn_list! {
        Name("template1") Callback(system_value(callback_system)),
        Name("template2") Callback(system_value(callback_system)) SomeStruct{value: 42, callback: system_value(another_callback_system)},
    }
}
fn callback_system(mut call_counter:Local<u32>){
    *call_counter += 1;
    println!("Hello from the system! Called: {}", *call_counter);
}

fn another_callback_system(mut call_counter:Local<u32>){
    *call_counter += 1;
    println!("Hello from another system! Called: {}", *call_counter);
}

---

Showcase

#[derive(Component, FromTemplate)]
struct Callback(SystemId);

fn callback_scene() -> impl SceneList {
    bsn_list! {
        Callback(system_value(|| {
            println!("Hello from the callback!");
        })),
        Callback(system_value(callback_system)),
    }
}

fn callback_system(){
    println!("Hello from the system!");
}

[github-actions]

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[hxYuki]

Not sure where to place these codes. I've searched impl FromTemplate for in repo and got only Entity here.

[hxYuki]

What should be the default value for a SystemId?

[SkiFire13]

IMO this is becoming pretty complex and I'm kinda losing track of how everything fits together.