add ReflectAsset and ReflectHandle (bevyengine#5923)

# Objective
![image](https://user-images.githubusercontent.com/22177966/189350194-639a0211-e984-4f73-ae62-0ede44891eb9.png)

^ enable this

Concretely, I need to
- list all handle ids for an asset type
- fetch the asset as `dyn Reflect`, given a `HandleUntyped`
- when encountering a `Handle<T>`, find out what asset type that handle refers to (`T`'s type id) and turn the handle into a `HandleUntyped`

## Solution

- add `ReflectAsset` type containing function pointers for working with assets
```rust
pub struct ReflectAsset {
    type_uuid: Uuid,
    assets_resource_type_id: TypeId, // TypeId of the `Assets<T>` resource

    get: fn(&World, HandleUntyped) -> Option<&dyn Reflect>,
    get_mut: fn(&mut World, HandleUntyped) -> Option<&mut dyn Reflect>,
    get_unchecked_mut: unsafe fn(&World, HandleUntyped) -> Option<&mut dyn Reflect>,
    add: fn(&mut World, &dyn Reflect) -> HandleUntyped,
    set: fn(&mut World, HandleUntyped, &dyn Reflect) -> HandleUntyped,
    len: fn(&World) -> usize,
    ids: for<'w> fn(&'w World) -> Box<dyn Iterator<Item = HandleId> + 'w>,
    remove: fn(&mut World, HandleUntyped) -> Option<Box<dyn Reflect>>,
}
```
- add `ReflectHandle` type relating the handle back to the asset type and providing a way to create a `HandleUntyped`
```rust
pub struct ReflectHandle {
    type_uuid: Uuid,
    asset_type_id: TypeId,
    downcast_handle_untyped: fn(&dyn Any) -> Option<HandleUntyped>,
}
```
- add the corresponding `FromType` impls
- add a function `app.register_asset_reflect` which is supposed to be called after `.add_asset` and registers `ReflectAsset` and `ReflectHandle` in the type registry
---

## Changelog

- add `ReflectAsset` and `ReflectHandle` types, which allow code to use reflection to manipulate arbitrary assets without knowing their types at compile time

crates/bevy_animation/src/lib.rs

@@ -17,7 +17,7 @@ use bevy_ecs::{
 };
 use bevy_hierarchy::Children;
 use bevy_math::{Quat, Vec3};
-use bevy_reflect::{Reflect, TypeUuid};
+use bevy_reflect::{FromReflect, Reflect, TypeUuid};
 use bevy_time::Time;
 use bevy_transform::{prelude::Transform, TransformSystem};
 use bevy_utils::{tracing::warn, HashMap};
@@ -31,7 +31,7 @@ pub mod prelude {
 }
 
 /// List of keyframes for one of the attribute of a [`Transform`].
-#[derive(Clone, Debug)]
+#[derive(Reflect, FromReflect, Clone, Debug)]
 pub enum Keyframes {
     /// Keyframes for rotation.
     Rotation(Vec<Quat>),
@@ -44,7 +44,7 @@ pub enum Keyframes {
 /// Describes how an attribute of a [`Transform`] should be animated.
 ///
 /// `keyframe_timestamps` and `keyframes` should have the same length.
-#[derive(Clone, Debug)]
+#[derive(Reflect, FromReflect, Clone, Debug)]
 pub struct VariableCurve {
     /// Timestamp for each of the keyframes.
     pub keyframe_timestamps: Vec<f32>,
@@ -53,14 +53,14 @@ pub struct VariableCurve {
 }
 
 /// Path to an entity, with [`Name`]s. Each entity in a path must have a name.
-#[derive(Clone, Debug, Hash, PartialEq, Eq, Default)]
+#[derive(Reflect, FromReflect, Clone, Debug, Hash, PartialEq, Eq, Default)]
 pub struct EntityPath {
     /// Parts of the path
     pub parts: Vec<Name>,
 }
 
 /// A list of [`VariableCurve`], and the [`EntityPath`] to which they apply.
-#[derive(Clone, TypeUuid, Debug, Default)]
+#[derive(Reflect, FromReflect, Clone, TypeUuid, Debug, Default)]
 #[uuid = "d81b7179-0448-4eb0-89fe-c067222725bf"]
 pub struct AnimationClip {
     curves: HashMap<EntityPath, Vec<VariableCurve>>,
@@ -301,6 +301,7 @@ pub struct AnimationPlugin {}
 impl Plugin for AnimationPlugin {
     fn build(&self, app: &mut App) {
         app.add_asset::<AnimationClip>()
+            .register_asset_reflect::<AnimationClip>()
             .register_type::<AnimationPlayer>()
             .add_system_to_stage(
                 CoreStage::PostUpdate,

crates/bevy_app/src/app.rs

@@ -1,6 +1,5 @@
 use crate::{CoreStage, Plugin, PluginGroup, StartupSchedule, StartupStage};
 pub use bevy_derive::AppLabel;
-use bevy_derive::{Deref, DerefMut};
 use bevy_ecs::{
     event::{Event, Events},
     prelude::FromWorld,
@@ -25,7 +24,7 @@ bevy_utils::define_label!(
 
 /// The [`Resource`] that stores the [`App`]'s [`TypeRegistry`](bevy_reflect::TypeRegistry).
 #[cfg(feature = "bevy_reflect")]
-#[derive(Resource, Clone, Deref, DerefMut, Default)]
+#[derive(Resource, Clone, bevy_derive::Deref, bevy_derive::DerefMut, Default)]
 pub struct AppTypeRegistry(pub bevy_reflect::TypeRegistryArc);
 
 #[allow(clippy::needless_doctest_main)]

crates/bevy_asset/src/assets.rs

@@ -1,13 +1,14 @@
 use crate::{
     update_asset_storage_system, Asset, AssetLoader, AssetServer, AssetStage, Handle, HandleId,
-    RefChange,
+    RefChange, ReflectAsset, ReflectHandle,
 };
-use bevy_app::App;
+use bevy_app::{App, AppTypeRegistry};
 use bevy_ecs::{
     event::{EventWriter, Events},
     system::{ResMut, Resource},
     world::FromWorld,
 };
+use bevy_reflect::{FromReflect, GetTypeRegistration, Reflect};
 use bevy_utils::HashMap;
 use crossbeam_channel::Sender;
 use std::fmt::Debug;
@@ -279,6 +280,14 @@ pub trait AddAsset {
     where
         T: Asset;
 
+    /// Registers the asset type `T` using `[App::register]`,
+    /// and adds [`ReflectAsset`] type data to `T` and [`ReflectHandle`] type data to [`Handle<T>`] in the type registry.
+    ///
+    /// This enables reflection code to access assets. For detailed information, see the docs on [`ReflectAsset`] and [`ReflectHandle`].
+    fn register_asset_reflect<T>(&mut self) -> &mut Self
+    where
+        T: Asset + Reflect + FromReflect + GetTypeRegistration;
+
     /// Registers `T` as a supported internal asset in the application.
     ///
     /// Internal assets (e.g. shaders) are bundled directly into the app and can't be hot reloaded
@@ -332,6 +341,23 @@ impl AddAsset for App {
             .add_event::<AssetEvent<T>>()
     }
 
+    fn register_asset_reflect<T>(&mut self) -> &mut Self
+    where
+        T: Asset + Reflect + FromReflect + GetTypeRegistration,
+    {
+        let type_registry = self.world.resource::<AppTypeRegistry>();
+        {
+            let mut type_registry = type_registry.write();
+
+            type_registry.register::<T>();
+            type_registry.register::<Handle<T>>();
+            type_registry.register_type_data::<T, ReflectAsset>();
+            type_registry.register_type_data::<Handle<T>, ReflectHandle>();
+        }
+
+        self
+    }
+
     fn add_debug_asset<T: Clone>(&mut self) -> &mut Self
     where
         T: Asset,

crates/bevy_asset/src/handle.rs

@@ -410,6 +410,16 @@ impl Drop for HandleUntyped {
     }
 }
 
+impl<A: Asset> From<Handle<A>> for HandleUntyped {
+    fn from(mut handle: Handle<A>) -> Self {
+        let handle_type = std::mem::replace(&mut handle.handle_type, HandleType::Weak);
+        HandleUntyped {
+            id: handle.id,
+            handle_type,
+        }
+    }
+}
+
 impl From<&HandleUntyped> for HandleId {
     fn from(value: &HandleUntyped) -> Self {
         value.id

crates/bevy_asset/src/lib.rs

@@ -25,6 +25,7 @@ mod info;
 mod io;
 mod loader;
 mod path;
+mod reflect;
 
 /// The `bevy_asset` prelude.
 pub mod prelude {
@@ -43,6 +44,7 @@ pub use info::*;
 pub use io::*;
 pub use loader::*;
 pub use path::*;
+pub use reflect::*;
 
 use bevy_app::{prelude::Plugin, App};
 use bevy_ecs::schedule::{StageLabel, SystemStage};