Introduce ReflectConvert, a generic reflection mechanism for type conversions, intended for asset BSN.#23742
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pcwalton wants to merge 6 commits intobevyengine:mainfrom
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Introduce ReflectConvert, a generic reflection mechanism for type conversions, intended for asset BSN.#23742pcwalton wants to merge 6 commits intobevyengine:mainfrom
ReflectConvert, a generic reflection mechanism for type conversions, intended for asset BSN.#23742pcwalton wants to merge 6 commits intobevyengine:mainfrom
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conversions, intended for asset BSN.
One of the important features of BSN is that supplying a value of type T
where a value of type U is expected is allowed if a `From` conversion
exists from T to U. This is what allows `HandleTemplate`s to be
automatically created from string literals. We need this feature for BSN
like this to be valid:
```rust
SceneRoot("models/FlightHelmet/FlightHelmet.gltf#Scene0")
```
And for this to be valid:
```rust
EnvironmentMapLight {
diffuse_map: "environment_maps/pisa_diffuse_rgb9e5_zstd.ktx2",
specular_map: "environment_maps/pisa_specular_rgb9e5_zstd.ktx2",
intensity: 250.0,
}
```
Unfortunately, the `From` trait isn't reflectable, and this is a problem
as reflection drives asset BSN. It's not immediately clear how to make
`From` reflectable either, as a syntax like `#[reflect(From)]` wouldn't
work as it's a generic trait.
Instead of making `From` directly reflectable, this patch introduces
`ReflectConvert`, a new `TypeData` that encodes a generic way to convert
a value of type T to type U. You register a type conversion like this:
```rust
App::new()
.register_type::<i32>()
.register_type::<String>()
.register_type_conversion::<i32, String>(|n| Ok(n.into()));
```
And you invoke the conversion like this:
```rust
let reflect_convert = registry
.get_type_data::<ReflectConvert>(TypeId::of::<String>())
.unwrap();
let converted = reflect_convert
.try_convert_from(Box::new(12345i32))
.unwrap()
.downcast::<String>()
.unwrap();
```
I tested `ReflectConvert` with asset BSN (PR bevyengine#23576). The
`ReflectConvert` trait as implemented in this patch successfully enables
`HandleTemplate`s to be created from strings, without hard-coding
`HandleTemplate` anywhere in the patch generation logic.
pcwalton
added
C-Feature
andriyDev
approved these changes
Apr 9, 2026
| /// | ||
| /// See [`bevy_reflect::TypeRegistry::register_type_conversion`]. | ||
| #[cfg(feature = "bevy_reflect")] | ||
| pub fn register_type_conversion<T, U>(&mut self, function: fn(T) -> Result<U, T>) -> &mut Self |
Contributor
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I wonder if we should have a register_into_type_conversion that just does the into for you? I suspect most calls to this will just be into.
Contributor
Author
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Yeah, I was considering that too. In an earlier version you were just able to write .register_type_conversion::<T,U>(Into::into) but I realized that was incompatible with fallible conversions, and I'm pretty sure we do want fallible conversions.
Contributor
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To clarify: I meant provide both: the into is just a convenience for calling the more general TryInto
Contributor
Author
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I added register_into_type_conversion as requested.
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One of the important features of BSN is that supplying a value of type T where a value of type U is expected is allowed if a
Fromconversion exists from T to U. This is what allowsHandleTemplates to be automatically created from string literals. We need this feature for BSN like this to be valid:And for this to be valid:
Unfortunately, the
Fromtrait isn't reflectable, and this is a problem as reflection drives asset BSN. It's not immediately clear how to makeFromreflectable either, as a syntax like#[reflect(From)]wouldn't work as it's a generic trait.Instead of making
Fromdirectly reflectable, this patch introducesReflectConvert, a newTypeDatathat encodes a generic way to convert a value of type T to type U. You register a type conversion like this:And you invoke the conversion like this:
I tested
ReflectConvertwith asset BSN (PR #23576). TheReflectConverttrait as implemented in this patch successfully enablesHandleTemplates to be created from strings, without hard-codingHandleTemplateanywhere in the patch generation logic.