Allow `Scene` to overwrite entities in a `World`

[dmyyy]

What problem does this solve or what need does it fill?

Right now we can write to a world from a scene using

let instance = scene.write_to_world_with(world, app_type_registry) .unwrap();

Where the returned instance is a mapping from entities in the scene world to entities to newly created entities in the world we pass in.

However, in some cases we want to overwrite entities in the world (instead of allocating new ones). Say I have many snapshots of my world in-memory that I've saved using Scene::clone_with. When I apply them to the world I don't want to allocate new entities each time - instead I want to be passing in the scene instance entity map I got back from my write_to_world_with call and overwriting the entities that way.

What solution would you like?

Sample API

impl Scene {
    ...
    /// Overwrites entities specified in the scene that have an existing mapping to an entity in world
    pub fn overwrite(&self, world: &mut World, instance_info: InstanceInfo) {
        let entity_map = instance_info.0;
        ...
    }
    ...
}

This can be used as so

#[derive(Component)]
pub struct A(u32);

let mut world = World::default();
let scene = Scene::new(world);

let mut entity_mut = scene.world.spawn(A(0));
let snapshot = scene.clone_with(app_type_registry).unwrap();

entity_mut.insert(A(1));

let instance = scene.write_to_world_with(main_world, app_type_registry) .unwrap();

// load an older snapshot but reuse all the same entities
snapshot.overwrite(world, instance);

Additional considerations

We could also consider removing entities that are in the InstanceInfo but not in the scene from the world, but a straightforward overwrite would work for my use case.

[ua-kxie]

I implemented rollbacks using bevy_save. I used a component type to mark entities which I wanted to save and deleted all such entities just before loading. Perhaps this is applicable to you?

[dmyyy]

I implemented rollbacks using bevy_save. I used a component type to mark entities which I wanted to save and deleted all such entities just before loading. Perhaps this is applicable to you?

bevy_save seems like a nice fully featured solution - it was the first thing I reached for when starting. I still think basic scene diffing/overwriting is something bevy_scene should provide out of the box.