Currently, our specialization API works through a series of wrapper
structs and traits, which make things confusing to follow and difficult
to generalize.
This pr takes a different approach, where "specializers" (types that
implement `Specialize`) are composable, but "flat" rather than composed
of a series of wrappers. The key is that specializers don't *produce*
pipeline descriptors, but instead *modify* existing ones:
```rs
pub trait Specialize<T: Specializable> {
type Key: SpecializeKey;
fn specialize(
&self,
key: Self::Key,
descriptor: &mut T::Descriptor
) -> Result<Canonical<Self::Key>, BevyError>;
}
```
This lets us use some derive magic to stick multiple specializers
together:
```rs
pub struct A;
pub struct B;
impl Specialize<RenderPipeline> for A { ... }
impl Specialize<RenderPipeline> for A { ... }
#[derive(Specialize)]
#[specialize(RenderPipeline)]
struct C {
// specialization is applied in struct field order
applied_first: A,
applied_second: B,
}
type C::Key = (A::Key, B::Key);
```
This approach is much easier to understand, IMO, and also lets us
separate concerns better. Specializers can be placed in fully separate
crates/modules, and key computation can be shared as well.
The only real breaking change here is that since specializers only
modify descriptors, we need a "base" descriptor to work off of. This can
either be manually supplied when constructing a `Specializer` (the new
collection replacing `Specialized[Render/Compute]Pipelines`), or
supplied by implementing `HasBaseDescriptor` on a specializer. See
`examples/shader/custom_phase_item.rs` for an example implementation.
## Testing
- Did some simple manual testing of the derive macro, it seems robust.
---
## Showcase
```rs
#[derive(Specialize, HasBaseDescriptor)]
#[specialize(RenderPipeline)]
pub struct SpecializeMeshMaterial<M: Material> {
// set mesh bind group layout and shader defs
mesh: SpecializeMesh,
// set view bind group layout and shader defs
view: SpecializeView,
// since type SpecializeMaterial::Key = (),
// we can hide it from the wrapper's external API
#[key(default)]
// defer to the GetBaseDescriptor impl of SpecializeMaterial,
// since it carries the vertex and fragment handles
#[base_descriptor]
// set material bind group layout, etc
material: SpecializeMaterial<M>,
}
// implementation generated by the derive macro
impl <M: Material> Specialize<RenderPipeline> for SpecializeMeshMaterial<M> {
type Key = (MeshKey, ViewKey);
fn specialize(
&self,
key: Self::Key,
descriptor: &mut RenderPipelineDescriptor
) -> Result<Canonical<Self::Key>, BevyError> {
let mesh_key = self.mesh.specialize(key.0, descriptor)?;
let view_key = self.view.specialize(key.1, descriptor)?;
let _ = self.material.specialize((), descriptor)?;
Ok((mesh_key, view_key));
}
}
impl <M: Material> HasBaseDescriptor<RenderPipeline> for SpecializeMeshMaterial<M> {
fn base_descriptor(&self) -> RenderPipelineDescriptor {
self.material.base_descriptor()
}
}
```
---------
Co-authored-by: Tim Overbeek <158390905+Bleachfuel@users.noreply.github.com>
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