AssetSaver and AssetTransformer split (#11260)

# Objective
One of a few Bevy Asset improvements I would like to make: #11216.

Currently asset processing and asset saving are handled by the same
trait, `AssetSaver`. This makes it difficult to reuse saving
implementations and impossible to have a single "universal" saver for a
given asset type.

## Solution
This PR splits off the processing portion of `AssetSaver` into
`AssetTransformer`, which is responsible for transforming assets. This
change involves adding the `LoadTransformAndSave` processor, which
utilizes the new API. The `LoadAndSave` still exists since it remains
useful in situations where no "transformation" of the asset is done,
such as when compressing assets.

## Notes:
As an aside, Bikeshedding is welcome on the names. I'm not entirely
convinced by `AssetTransformer`, which was chosen mostly because
`AssetProcessor` is taken. Additionally, `LoadTransformSave` may be
sufficient instead of `LoadTransformAndSave`.


---

## Changelog
### Added 
- `AssetTransformer` which is responsible for transforming Assets.
- `LoadTransformAndSave`, a `Process` implementation.
### Changed
- Changed `AssetSaver`'s responsibilities from processing and saving to
just saving.
- Updated `asset_processing` example to use new API.
- Old asset .meta files regenerated with new processor.

crates/bevy_asset/src/lib.rs

@@ -2,6 +2,7 @@ pub mod io;
 pub mod meta;
 pub mod processor;
 pub mod saver;
+pub mod transformer;
 
 pub mod prelude {
     #[doc(hidden)]

crates/bevy_asset/src/loader.rs

@@ -527,11 +527,11 @@ impl<'a> LoadContext<'a> {
     /// deriving a new asset from the referenced asset, or you are building a collection of assets. This will add the `path` as a
     /// "load dependency".
     ///
-    /// If the current loader is used in a [`Process`] "asset preprocessor", such as a [`LoadAndSave`] preprocessor,
+    /// If the current loader is used in a [`Process`] "asset preprocessor", such as a [`LoadTransformAndSave`] preprocessor,
     /// changing a "load dependency" will result in re-processing of the asset.
     ///
     /// [`Process`]: crate::processor::Process
-    /// [`LoadAndSave`]: crate::processor::LoadAndSave
+    /// [`LoadTransformAndSave`]: crate::processor::LoadTransformAndSave
     pub async fn load_direct<'b>(
         &mut self,
         path: impl Into<AssetPath<'b>>,
@@ -575,11 +575,11 @@ impl<'a> LoadContext<'a> {
     /// For example, if you are deriving a new asset from the referenced asset, or you are building a collection of assets. This will add the `path` as a
     /// "load dependency".
     ///
-    /// If the current loader is used in a [`Process`] "asset preprocessor", such as a [`LoadAndSave`] preprocessor,
+    /// If the current loader is used in a [`Process`] "asset preprocessor", such as a [`LoadTransformAndSave`] preprocessor,
     /// changing a "load dependency" will result in re-processing of the asset.
     ///
     /// [`Process`]: crate::processor::Process
-    /// [`LoadAndSave`]: crate::processor::LoadAndSave
+    /// [`LoadTransformAndSave`]: crate::processor::LoadTransformAndSave
     pub async fn load_direct_with_reader<'b>(
         &mut self,
         reader: &mut Reader<'_>,

crates/bevy_asset/src/processor/process.rs

@@ -6,7 +6,8 @@ use crate::{
     meta::{AssetAction, AssetMeta, AssetMetaDyn, ProcessDependencyInfo, ProcessedInfo, Settings},
     processor::AssetProcessor,
     saver::{AssetSaver, SavedAsset},
-    AssetLoadError, AssetLoader, AssetPath, DeserializeMetaError, ErasedLoadedAsset,
+    transformer::AssetTransformer,
+    AssetLoadError, AssetLoader, AssetPath, DeserializeMetaError, ErasedLoadedAsset, LoadedAsset,
     MissingAssetLoaderForExtensionError, MissingAssetLoaderForTypeNameError,
 };
 use bevy_utils::BoxedFuture;
@@ -17,7 +18,7 @@ use thiserror::Error;
 /// Asset "processor" logic that reads input asset bytes (stored on [`ProcessContext`]), processes the value in some way,
 /// and then writes the final processed bytes with [`Writer`]. The resulting bytes must be loadable with the given [`Process::OutputLoader`].
 ///
-/// This is a "low level", maximally flexible interface. Most use cases are better served by the [`LoadAndSave`] implementation
+/// This is a "low level", maximally flexible interface. Most use cases are better served by the [`LoadTransformAndSave`] implementation
 /// of [`Process`].
 pub trait Process: Send + Sync + Sized + 'static {
     /// The configuration / settings used to process the asset. This will be stored in the [`AssetMeta`] and is user-configurable per-asset.
@@ -34,13 +35,62 @@ pub trait Process: Send + Sync + Sized + 'static {
     ) -> BoxedFuture<'a, Result<<Self::OutputLoader as AssetLoader>::Settings, ProcessError>>;
 }
 
+/// A flexible [`Process`] implementation that loads the source [`Asset`] using the `L` [`AssetLoader`], then transforms
+/// the `L` asset into an `S` [`AssetSaver`] asset using the `T` [`AssetTransformer`], and lastly saves the asset using the `S` [`AssetSaver`].
+///
+/// When creating custom processors, it is generally recommended to use the [`LoadTransformAndSave`] [`Process`] implementation,
+/// as it encourages you to separate your code into an [`AssetLoader`] capable of loading assets without processing enabled,
+/// an [`AssetTransformer`] capable of converting from an `L` asset to an `S` asset, and
+/// an [`AssetSaver`] that allows you save any `S` asset. However you can
+/// also implement [`Process`] directly if [`LoadTransformAndSave`] feels limiting or unnecessary.
+///
+/// This uses [`LoadTransformAndSaveSettings`] to configure the processor.
+///
+/// [`Asset`]: crate::Asset
+pub struct LoadTransformAndSave<
+    L: AssetLoader,
+    T: AssetTransformer<AssetInput = L::Asset>,
+    S: AssetSaver<Asset = T::AssetOutput>,
+> {
+    transformer: T,
+    saver: S,
+    marker: PhantomData<fn() -> L>,
+}
+
+/// Settings for the [`LoadTransformAndSave`] [`Process::Settings`] implementation.
+///
+/// `LoaderSettings` corresponds to [`AssetLoader::Settings`], `TransformerSettings` corresponds to [`AssetTransformer::Settings`],
+/// and `SaverSettings` corresponds to [`AssetSaver::Settings`].
+#[derive(Serialize, Deserialize, Default)]
+pub struct LoadTransformAndSaveSettings<LoaderSettings, TransformerSettings, SaverSettings> {
+    /// The [`AssetLoader::Settings`] for [`LoadTransformAndSave`].
+    pub loader_settings: LoaderSettings,
+    /// The [`AssetTransformer::Settings`] for [`LoadTransformAndSave`].
+    pub transformer_settings: TransformerSettings,
+    /// The [`AssetSaver::Settings`] for [`LoadTransformAndSave`].
+    pub saver_settings: SaverSettings,
+}
+
+impl<
+        L: AssetLoader,
+        T: AssetTransformer<AssetInput = L::Asset>,
+        S: AssetSaver<Asset = T::AssetOutput>,
+    > LoadTransformAndSave<L, T, S>
+{
+    pub fn new(transformer: T, saver: S) -> Self {
+        LoadTransformAndSave {
+            transformer,
+            saver,
+            marker: PhantomData,
+        }
+    }
+}
+
 /// A flexible [`Process`] implementation that loads the source [`Asset`] using the `L` [`AssetLoader`], then
 /// saves that `L` asset using the `S` [`AssetSaver`].
 ///
-/// When creating custom processors, it is generally recommended to use the [`LoadAndSave`] [`Process`] implementation,
-/// as it encourages you to write both an [`AssetLoader`] capable of loading assets without processing enabled _and_
-/// an [`AssetSaver`] that allows you to efficiently process that asset type when that is desirable by users. However you can
-/// also implement [`Process`] directly if [`LoadAndSave`] feels limiting or unnecessary.
+/// This is a specialized use case of [`LoadTransformAndSave`] and is useful where there is no asset manipulation
+/// such as when compressing assets.
 ///
 /// This uses [`LoadAndSaveSettings`] to configure the processor.
 ///
@@ -112,6 +162,52 @@ pub enum ProcessError {
     ExtensionRequired,
 }
 
+impl<
+        Loader: AssetLoader,
+        T: AssetTransformer<AssetInput = Loader::Asset>,
+        Saver: AssetSaver<Asset = T::AssetOutput>,
+    > Process for LoadTransformAndSave<Loader, T, Saver>
+{
+    type Settings = LoadTransformAndSaveSettings<Loader::Settings, T::Settings, Saver::Settings>;
+    type OutputLoader = Saver::OutputLoader;
+
+    fn process<'a>(
+        &'a self,
+        context: &'a mut ProcessContext,
+        meta: AssetMeta<(), Self>,
+        writer: &'a mut Writer,
+    ) -> BoxedFuture<'a, Result<<Self::OutputLoader as AssetLoader>::Settings, ProcessError>> {
+        Box::pin(async move {
+            let AssetAction::Process { settings, .. } = meta.asset else {
+                return Err(ProcessError::WrongMetaType);
+            };
+            let loader_meta = AssetMeta::<Loader, ()>::new(AssetAction::Load {
+                loader: std::any::type_name::<Loader>().to_string(),
+                settings: settings.loader_settings,
+            });
+            let loaded_asset = context
+                .load_source_asset(loader_meta)
+                .await?
+                .take::<Loader::Asset>()
+                .expect("Asset type is known");
+            let transformed_asset = self
+                .transformer
+                .transform(loaded_asset, &settings.transformer_settings)?;
+            let loaded_transformed_asset =
+                ErasedLoadedAsset::from(LoadedAsset::from(transformed_asset));
+            let saved_asset =
+                SavedAsset::<T::AssetOutput>::from_loaded(&loaded_transformed_asset).unwrap();
+
+            let output_settings = self
+                .saver
+                .save(writer, saved_asset, &settings.saver_settings)
+                .await
+                .map_err(|error| ProcessError::AssetSaveError(error.into()))?;
+            Ok(output_settings)
+        })
+    }
+}
+
 impl<Loader: AssetLoader, Saver: AssetSaver<Asset = Loader::Asset>> Process
     for LoadAndSave<Loader, Saver>
 {

crates/bevy_asset/src/transformer.rs

@@ -0,0 +1,20 @@
+use crate::{meta::Settings, Asset};
+use serde::{Deserialize, Serialize};
+
+/// Transforms an [`Asset`] of a given [`AssetTransformer::AssetInput`] type to an [`Asset`] of [`AssetTransformer::AssetOutput`] type.
+pub trait AssetTransformer: Send + Sync + 'static {
+    /// The [`Asset`] type which this [`AssetTransformer`] takes as and input.
+    type AssetInput: Asset;
+    /// The [`Asset`] type which this [`AssetTransformer`] outputs.
+    type AssetOutput: Asset;
+    /// The settings type used by this [`AssetTransformer`].
+    type Settings: Settings + Default + Serialize + for<'a> Deserialize<'a>;
+    /// The type of [error](`std::error::Error`) which could be encountered by this saver.
+    type Error: Into<Box<dyn std::error::Error + Send + Sync + 'static>>;
+
+    fn transform<'a>(
+        &'a self,
+        asset: Self::AssetInput,
+        settings: &'a Self::Settings,
+    ) -> Result<Self::AssetOutput, Box<dyn std::error::Error + Send + Sync + 'static>>;
+}

examples/asset/processing/asset_processing.rs

@@ -1,12 +1,13 @@
-//! This example illustrates how to define custom `AssetLoader`s and `AssetSaver`s, how to configure them, and how to register asset processors.
+//! This example illustrates how to define custom `AssetLoader`s, `AssetTransfomers`, and `AssetSaver`s, how to configure them, and how to register asset processors.
 
 use bevy::{
     asset::{
         embedded_asset,
         io::{Reader, Writer},
-        processor::LoadAndSave,
+        processor::LoadTransformAndSave,
         ron,
         saver::{AssetSaver, SavedAsset},
+        transformer::AssetTransformer,
         AssetLoader, AsyncReadExt, AsyncWriteExt, LoadContext,
     },
     prelude::*,
@@ -14,6 +15,7 @@ use bevy::{
     utils::{thiserror, BoxedFuture},
 };
 use serde::{Deserialize, Serialize};
+use std::convert::Infallible;
 use thiserror::Error;
 
 fn main() {
@@ -59,10 +61,10 @@ impl Plugin for TextPlugin {
             .init_asset::<Text>()
             .register_asset_loader(CoolTextLoader)
             .register_asset_loader(TextLoader)
-            .register_asset_processor::<LoadAndSave<CoolTextLoader, CoolTextSaver>>(
-                LoadAndSave::from(CoolTextSaver),
+            .register_asset_processor::<LoadTransformAndSave<CoolTextLoader, CoolTextTransformer, CoolTextSaver>>(
+                LoadTransformAndSave::new(CoolTextTransformer, CoolTextSaver),
             )
-            .set_default_asset_processor::<LoadAndSave<CoolTextLoader, CoolTextSaver>>("cool.ron");
+            .set_default_asset_processor::<LoadTransformAndSave<CoolTextLoader, CoolTextTransformer, CoolTextSaver>>("cool.ron");
     }
 }
 
@@ -133,9 +135,7 @@ enum CoolTextLoaderError {
 
 impl AssetLoader for CoolTextLoader {
     type Asset = CoolText;
-
     type Settings = ();
-
     type Error = CoolTextLoaderError;
 
     fn load<'a>(
@@ -170,16 +170,37 @@ impl AssetLoader for CoolTextLoader {
     }
 }
 
-struct CoolTextSaver;
+#[derive(Default)]
+struct CoolTextTransformer;
 
 #[derive(Default, Serialize, Deserialize)]
-pub struct CoolTextSaverSettings {
+pub struct CoolTextTransformerSettings {
     appended: String,
 }
 
+impl AssetTransformer for CoolTextTransformer {
+    type AssetInput = CoolText;
+    type AssetOutput = CoolText;
+    type Settings = CoolTextTransformerSettings;
+    type Error = Infallible;
+
+    fn transform<'a>(
+        &'a self,
+        asset: Self::AssetInput,
+        settings: &'a Self::Settings,
+    ) -> Result<Self::AssetOutput, Box<dyn std::error::Error + Send + Sync + 'static>> {
+        Ok(CoolText {
+            text: format!("{}{}", asset.text, settings.appended),
+            dependencies: asset.dependencies.clone(),
+        })
+    }
+}
+
+struct CoolTextSaver;
+
 impl AssetSaver for CoolTextSaver {
     type Asset = CoolText;
-    type Settings = CoolTextSaverSettings;
+    type Settings = ();
     type OutputLoader = TextLoader;
     type Error = std::io::Error;
 
@@ -187,11 +208,10 @@ impl AssetSaver for CoolTextSaver {
         &'a self,
         writer: &'a mut Writer,
         asset: SavedAsset<'a, Self::Asset>,
-        settings: &'a Self::Settings,
+        _settings: &'a Self::Settings,
     ) -> BoxedFuture<'a, Result<TextSettings, Self::Error>> {
         Box::pin(async move {
-            let text = format!("{}{}", asset.text.clone(), settings.appended);
-            writer.write_all(text.as_bytes()).await?;
+            writer.write_all(asset.text.as_bytes()).await?;
             Ok(TextSettings::default())
         })
     }

examples/asset/processing/assets/a.cool.ron.meta

@@ -1,12 +1,13 @@
 (
     meta_format_version: "1.0",
     asset: Process(
-        processor: "bevy_asset::processor::process::LoadAndSave<asset_processing::CoolTextLoader, asset_processing::CoolTextSaver>",
+        processor: "bevy_asset::processor::process::LoadTransformAndSave<asset_processing::CoolTextLoader, asset_processing::CoolTextTransformer, asset_processing::CoolTextSaver>",
         settings: (
             loader_settings: (),
-            saver_settings: (
+            transformer_settings: (
                 appended: "X",
             ),
+            saver_settings: (),
         ),
     ),
 )

examples/asset/processing/assets/d.cool.ron

@@ -3,6 +3,7 @@
     dependencies: [
     ],
     embedded_dependencies: [
-        "foo/c.cool.ron"
+        "foo/c.cool.ron",
+        "embedded://asset_processing/e.txt"
     ],
 )

examples/asset/processing/assets/d.cool.ron.meta

@@ -1,12 +1,13 @@
 (
     meta_format_version: "1.0",
     asset: Process(
-        processor: "bevy_asset::processor::process::LoadAndSave<asset_processing::CoolTextLoader, asset_processing::CoolTextSaver>",
+        processor: "bevy_asset::processor::process::LoadTransformAndSave<asset_processing::CoolTextLoader, asset_processing::CoolTextTransformer, asset_processing::CoolTextSaver>",
         settings: (
             loader_settings: (),
-            saver_settings: (
+            transformer_settings: (
                 appended: "",
             ),
+            saver_settings: (),
         ),
     ),
 )

examples/asset/processing/assets/foo/b.cool.ron.meta

@@ -1,12 +1,13 @@
 (
     meta_format_version: "1.0",
     asset: Process(
-        processor: "bevy_asset::processor::process::LoadAndSave<asset_processing::CoolTextLoader, asset_processing::CoolTextSaver>",
+        processor: "bevy_asset::processor::process::LoadTransformAndSave<asset_processing::CoolTextLoader, asset_processing::CoolTextTransformer, asset_processing::CoolTextSaver>",
         settings: (
             loader_settings: (),
-            saver_settings: (
+            transformer_settings: (
                 appended: "",
             ),
+            saver_settings: (),
         ),
     ),
 )

examples/asset/processing/assets/foo/c.cool.ron.meta

@@ -1,12 +1,13 @@
 (
     meta_format_version: "1.0",
     asset: Process(
-        processor: "bevy_asset::processor::process::LoadAndSave<asset_processing::CoolTextLoader, asset_processing::CoolTextSaver>",
+        processor: "bevy_asset::processor::process::LoadTransformAndSave<asset_processing::CoolTextLoader, asset_processing::CoolTextTransformer, asset_processing::CoolTextSaver>",
         settings: (
             loader_settings: (),
-            saver_settings: (
+            transformer_settings: (
                 appended: "",
             ),
+            saver_settings: (),
         ),
     ),
 )