Add 'observe_target' method to 'RelatedSpawner'

[Glory2Antares]

Objective

Add a way to spawn an observer targeting the 'target_entity()' of a 'RelatedSpawner'. This can be useful to react to Trigger's targeting a parent entity, and then making changes to children.

This enables a pattern very similar to signals' in Godot.

Solution

Use the 'world' field of 'RelatedSpawner' to spawn the 'Observer'.

Showcase

Use case:

Click to view showcase

fn before(world: &mut World) {
    world.spawn(Name::new("Parent")).with_children(|builder| {
        let parent_id = builder.target_entity();

        let mut child = builder.spawn(Name::new("Child"));
        let child_id = child.id();

        child.world_scope(move |world| {
            world.entity_mut(parent_id).observe(change_name(child_id));
        });
    });
}

fn after(mut commands: Commands) { // note now we can use commands instead of world
    commands.spawn(Name::new("Parent")).with_children(|builder| {
        let child_id = builder.spawn(Name::new("Child")).id();

        builder.observe_target(change_name(child_id));
    });
}

fn change_name(entity: Entity) -> impl Fn(Trigger<MyEvent>, Query<&mut Name>) {
    move |_: Trigger<MyEvent>, mut query: Query<&mut Name>| {
        if let Ok(mut name) = query.get_mut(entity) {
            name.set("New Name!");
        }
    }
}

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[Glory2Antares]

For tracking it may be necessary to spawn the observer as so:

    pub fn observe_target<E: Event, B: Bundle, M>(
        &mut self,
        observer: impl IntoObserverSystem<E, B, M>,
    ) -> EntityWorldMut<'_> {
        self.world
        self.assert_not_despawned();
        self.world
            .spawn_with_caller(Observer::new(observer).with_entity(self.target), MaybeLocation::caller());
        self.world.flush();
        self.update_location();
        self
}

But I am not sure.