(De) serialize resources in scenes

[Carbonhell]

Objective

Co-Authored-By: davier bricedavier@gmail.com
Fixes #3576.
Adds a resources field in scene serialization data to allow de/serializing resources that have reflection enabled.

Solution

Most of this code is taken from a previous closed PR: #3580. Most of the credit goes to @Davier , what I did was mostly getting it to work on the latest main branch of Bevy, along with adding a few asserts in the currently existing tests to be sure everything is working properly.

This PR changes the scene format to include resources in this way:

(
  resources: {
    // List of resources here, keyed by resource type name.
  },
  entities: [
    // Previous scene format here
  ],
)

An example taken from the tests:

(
  resources: {
    "bevy_scene::serde::tests::MyResource": (
      foo: 123,
    ),
  },
  entities: {
    // Previous scene format here
  },
)

For this, a resources fields has been added on the DynamicScene and the DynamicSceneBuilder structs. The latter now also has a method named extract_resources to properly extract the existing resources registered in the local type registry, in a similar way to extract_entities.

---

Changelog

Added: Reflect resources registered in the type registry used by dynamic scenes will now be properly de/serialized in scene data.

Migration Guide

Since the scene format has been changed, the user may not be able to use scenes saved prior to this PR due to the resources scene field being missing. To preserve backwards compatibility, I will try to make the resources fully optional so that old scenes can be loaded without issue.

TODOs

• I may have to update a few doc blocks still referring to dynamic scenes as mere container of entities, since they now include resources as well.
• I want to make the resources key optional, as specified in the Migration Guide, so that old scenes will be compatible with this change. Since this would only be trivial for ron format, I think it might be better to consider it in a separate PR/discussion to figure out if it could be done for binary serialization too.
• I suppose it might be a good idea to add a resources in the scene example so that users will quickly notice they can serialize resources just like entities.

[MrGVSV]

Just a few remaining comments, but overall LGTM!

[mockersf]

I'm not sure writing a scene to a world should always write the resources. Unlike entities, this would overwrite existing resources. This seems like a desirable behaviour sometimes but not always.

[MrGVSV]

I'm not sure writing a scene to a world should always write the resources. Unlike entities, this would overwrite existing resources. This seems like a desirable behaviour sometimes but not always.

Theoretically this should only overwrite the resources selected by the user in their passed AppTypeRegistry, right? Is there a reason they would choose to serialize a resource but not have it be deserialized? I guess if you want to write just the entities to a particular world, but I'm not sure 🤔

[mockersf]

Yes, but that would mean having multiple registries depending on what you would want to do. It's easier to think with one registry per scene.

I am not sure having the same behavior for entities and resources makes sense, writing the entities only create new ones, writing resources replace them. Having that being implicit doesn't feel nice, I may be wrong.

A reason to not want to rewrite them would be for a reusable scene that can need a few resources on its first spawn, but can reuse them after

[MrGVSV]

A reason to not want to rewrite them would be for a reusable scene that can need a few resources on its first spawn, but can reuse them after

Yeah this is what I think this is really needed for. For scenes that are more like save files, this really helps restore the full world state (well, more or less).

But I think this type of scene will generally be treated much differently than a transient one (e.g. networking) or ones that are written to a world multiple times (e.g. prefabs). So in these cases, we can expect them to rely on different filters. And this can be made a lot nicer with something like #6793 where we have an actual filter type rather than multiple registries.

So imo this PR would be really nice to have.

[Carbonhell]

I'm not sure writing a scene to a world should always write the resources. Unlike entities, this would overwrite existing resources. This seems like a desirable behaviour sometimes but not always.

Sorry for the late answer. I understand this might not always be the desired behaviour, but I think it's an acceptable default behaviour. The reason for this is that resources represent globally unique data in a Bevy app, therefore it seems natural to me that deserialization implies overwriting the current values with the loaded ones. The only other ideas that I can think of would be either manually handling deserializing a resource, or having some sort of scene-scoped resource.

1. The manual deserialization doesn't seems to be doable right now in an easy way, if I understood correctly. There needs to be some sort of hook prior to writing a dynamic scene to a world, to allow the user to be able to access both the old resource data and the loaded one, to decide what the final data should be. Since DynamicScene.write_to_world_with has a mutable ref to World, this should be doable I think.
2. Scene scoped resources sound complex to do and I'm honestly not sure if it would be worth it and what changes it would imply for bevy_ecs.

As @MrGVSV stated, using a specific AppTypeRegistry is the most obvious answer to circumvent this problem. I think this issue is similar to the one I had in my project, where using the world AppTypeRegistry caused the resulting serialized scene data to be bloated with components which the user may want to omit to, instead, recreate them on load manually (like in my case for my app). With my limited use of registries, I also think it should be unique and creating one for each use case within a single app feels confusing. In my opinion, the registry should specify which reflected types are registered, not which ones the user might be interested in de/serializing at a specific time.
For this reason. I believe moving the latter functionality to a filter type like @MrGVSV suggested in his draft PR (#6793) is the best solution for both problems (component serialization bloat and specifying which resources to de/serialize). Maybe manual deserialization for resources could also be implemented so that the user can implement some sort of trait (or reuse something like FromReflect?) to specify exactly what data a loaded resource should have.

[Carbonhell]

I resolved the conflicts, added a test in the dynamic_scene_builder.rs to test the extract_method in detail and removed the world argument in favor of self.original_world to be consistent with the extract_entities method. The PR should be ready, I believe.

[nicopap]

LGTM.

I agree with François, implicitly adding resources seems like a potential footgun. But with the current bevy tools, there is simply no way of not doing it this way.

The ability to load resources from scenes is necessary first before being capable of developing tools to filter resources when loading them, if we wait the tools before adding loading, then the snake bites the tail of the chicken's egg and we will wait forever.

[cart]

The implementation is great and I agree with the sentiment expressed here: this is a good first step into the space. I strongly suspect we will need more involved and flexible resource workflows in our scenes (ex: we need to support inline assets, which can't just overwrite the current asset state), but this is a reasonable first step.