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# Objective - Fixes #10518 ## Solution I've added a method to `LoadContext`, `load_direct_with_reader`, which mirrors the behaviour of `load_direct` with a single key difference: it is provided with the `Reader` by the caller, rather than getting it from the contained `AssetServer`. This allows for an `AssetLoader` to process its `Reader` stream, and then directly hand the results off to the `LoadContext` to handle further loading. The outer `AssetLoader` can control how the `Reader` is interpreted by providing a relevant `AssetPath`. For example, a Gzip decompression loader could process the asset `images/my_image.png.gz` by decompressing the bytes, then handing the decompressed result to the `LoadContext` with the new path `images/my_image.png.gz/my_image.png`. This intuitively reflects the nature of contained assets, whilst avoiding unintended behaviour, since the generated path cannot be a real file path (a file and folder of the same name cannot coexist in most file-systems). ```rust #[derive(Asset, TypePath)] pub struct GzAsset { pub uncompressed: ErasedLoadedAsset, } #[derive(Default)] pub struct GzAssetLoader; impl AssetLoader for GzAssetLoader { type Asset = GzAsset; type Settings = (); type Error = GzAssetLoaderError; fn load<'a>( &'a self, reader: &'a mut Reader, _settings: &'a (), load_context: &'a mut LoadContext, ) -> BoxedFuture<'a, Result<Self::Asset, Self::Error>> { Box::pin(async move { let compressed_path = load_context.path(); let file_name = compressed_path .file_name() .ok_or(GzAssetLoaderError::IndeterminateFilePath)? .to_string_lossy(); let uncompressed_file_name = file_name .strip_suffix(".gz") .ok_or(GzAssetLoaderError::IndeterminateFilePath)?; let contained_path = compressed_path.join(uncompressed_file_name); let mut bytes_compressed = Vec::new(); reader.read_to_end(&mut bytes_compressed).await?; let mut decoder = GzDecoder::new(bytes_compressed.as_slice()); let mut bytes_uncompressed = Vec::new(); decoder.read_to_end(&mut bytes_uncompressed)?; // Now that we have decompressed the asset, let's pass it back to the // context to continue loading let mut reader = VecReader::new(bytes_uncompressed); let uncompressed = load_context .load_direct_with_reader(&mut reader, contained_path) .await?; Ok(GzAsset { uncompressed }) }) } fn extensions(&self) -> &[&str] { &["gz"] } } ``` Because this example is so prudent, I've included an `asset_decompression` example which implements this exact behaviour: ```rust fn main() { App::new() .add_plugins(DefaultPlugins) .init_asset::<GzAsset>() .init_asset_loader::<GzAssetLoader>() .add_systems(Startup, setup) .add_systems(Update, decompress::<Image>) .run(); } fn setup(mut commands: Commands, asset_server: Res<AssetServer>) { commands.spawn(Camera2dBundle::default()); commands.spawn(( Compressed::<Image> { compressed: asset_server.load("data/compressed_image.png.gz"), ..default() }, Sprite::default(), TransformBundle::default(), VisibilityBundle::default(), )); } fn decompress<A: Asset>( mut commands: Commands, asset_server: Res<AssetServer>, mut compressed_assets: ResMut<Assets<GzAsset>>, query: Query<(Entity, &Compressed<A>)>, ) { for (entity, Compressed { compressed, .. }) in query.iter() { let Some(GzAsset { uncompressed }) = compressed_assets.remove(compressed) else { continue; }; let uncompressed = uncompressed.take::<A>().unwrap(); commands .entity(entity) .remove::<Compressed<A>>() .insert(asset_server.add(uncompressed)); } } ``` A key limitation to this design is how to type the internally loaded asset, since the example `GzAssetLoader` is unaware of the internal asset type `A`. As such, in this example I store the contained asset as an `ErasedLoadedAsset`, and leave it up to the consumer of the `GzAsset` to handle typing the final result, which is the purpose of the `decompress` system. This limitation can be worked around by providing type information to the `GzAssetLoader`, such as `GzAssetLoader<Image, ImageAssetLoader>`, but this would require registering the asset loader for every possible decompression target. Aside from this limitation, nested asset containerisation works as an end user would expect; if the user registers a `TarAssetLoader`, and a `GzAssetLoader`, then they can load assets with compound containerisation, such as `images.tar.gz`. --- ## Changelog - Added `LoadContext::load_direct_with_reader` - Added `asset_decompression` example ## Notes - While I believe my implementation of a Gzip asset loader is reasonable, I haven't included it as a public feature of `bevy_asset` to keep the scope of this PR as focussed as possible. - I have included `flate2` as a `dev-dependency` for the example; it is not included in the main dependency graph. |
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