5876352206
# Objective
The `AssetReader` trait allows customizing the behavior of fetching
bytes for an `AssetPath`, and expects implementors to return `dyn
AsyncRead + AsyncSeek`. This gives implementors of `AssetLoader` great
flexibility to tightly integrate their asset loading behavior with the
asynchronous task system.
However, almost all implementors of `AssetLoader` don't use the async
functionality at all, and just call `AsyncReadExt::read_to_end(&mut
Vec<u8>)`. This is incredibly inefficient, as this method repeatedly
calls `poll_read` on the trait object, filling the vector 32 bytes at a
time. At my work we have assets that are hundreds of megabytes which
makes this a meaningful overhead.
## Solution
Turn the `Reader` type alias into an actual trait, with a provided
method `read_to_end`. This provided method should be more efficient than
the existing extension method, as the compiler will know the underlying
type of `Reader` when generating this function, which removes the
repeated dynamic dispatches and allows the compiler to make further
optimizations after inlining. Individual implementors are able to
override the provided implementation -- for simple asset readers that
just copy bytes from one buffer to another, this allows removing a large
amount of overhead from the provided implementation.
Now that `Reader` is an actual trait, I also improved the ergonomics for
implementing `AssetReader`. Currently, implementors are expected to box
their reader and return it as a trait object, which adds unnecessary
boilerplate to implementations. This PR changes that trait method to
return a pseudo trait alias, which allows implementors to return `impl
Reader` instead of `Box<dyn Reader>`. Now, the boilerplate for boxing
occurs in `ErasedAssetReader`.
## Testing
I made identical changes to my company's fork of bevy. Our app, which
makes heavy use of `read_to_end` for asset loading, still worked
properly after this. I am not aware if we have a more systematic way of
testing asset loading for correctness.
---
## Migration Guide
The trait method `bevy_asset::io::AssetReader::read` (and `read_meta`)
now return an opaque type instead of a boxed trait object. Implementors
of these methods should change the type signatures appropriately
```rust
impl AssetReader for MyReader {
// Before
async fn read<'a>(&'a self, path: &'a Path) -> Result<Box<Reader<'a>>, AssetReaderError> {
let reader = // construct a reader
Box::new(reader) as Box<Reader<'a>>
}
// After
async fn read<'a>(&'a self, path: &'a Path) -> Result<impl Reader + 'a, AssetReaderError> {
// create a reader
}
}
```
`bevy::asset::io::Reader` is now a trait, rather than a type alias for a
trait object. Implementors of `AssetLoader::load` will need to adjust
the method signature accordingly
```rust
impl AssetLoader for MyLoader {
async fn load<'a>(
&'a self,
// Before:
reader: &'a mut bevy::asset::io::Reader,
// After:
reader: &'a mut dyn bevy::asset::io::Reader,
_: &'a Self::Settings,
load_context: &'a mut LoadContext<'_>,
) -> Result<Self::Asset, Self::Error> {
}
```
Additionally, implementors of `AssetReader` that return a type
implementing `futures_io::AsyncRead` and `AsyncSeek` might need to
explicitly implement `bevy::asset::io::Reader` for that type.
```rust
impl bevy::asset::io::Reader for MyAsyncReadAndSeek {}
```
69 lines
2.1 KiB
Rust
69 lines
2.1 KiB
Rust
//! Implements a custom asset io loader.
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//! An [`AssetReader`] is what the asset server uses to read the raw bytes of assets.
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//! It does not know anything about the asset formats, only how to talk to the underlying storage.
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use bevy::{
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asset::io::{
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AssetReader, AssetReaderError, AssetSource, AssetSourceId, ErasedAssetReader, PathStream,
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Reader,
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},
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prelude::*,
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};
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use std::path::Path;
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/// A custom asset reader implementation that wraps a given asset reader implementation
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struct CustomAssetReader(Box<dyn ErasedAssetReader>);
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impl AssetReader for CustomAssetReader {
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async fn read<'a>(&'a self, path: &'a Path) -> Result<impl Reader + 'a, AssetReaderError> {
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info!("Reading {:?}", path);
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self.0.read(path).await
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}
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async fn read_meta<'a>(&'a self, path: &'a Path) -> Result<impl Reader + 'a, AssetReaderError> {
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self.0.read_meta(path).await
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}
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async fn read_directory<'a>(
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&'a self,
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path: &'a Path,
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) -> Result<Box<PathStream>, AssetReaderError> {
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self.0.read_directory(path).await
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}
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async fn is_directory<'a>(&'a self, path: &'a Path) -> Result<bool, AssetReaderError> {
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self.0.is_directory(path).await
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}
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}
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/// A plugins that registers our new asset reader
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struct CustomAssetReaderPlugin;
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impl Plugin for CustomAssetReaderPlugin {
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fn build(&self, app: &mut App) {
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app.register_asset_source(
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AssetSourceId::Default,
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AssetSource::build().with_reader(|| {
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Box::new(CustomAssetReader(
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// This is the default reader for the current platform
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AssetSource::get_default_reader("assets".to_string())(),
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))
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}),
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);
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}
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}
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fn main() {
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App::new()
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.add_plugins((CustomAssetReaderPlugin, DefaultPlugins))
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.add_systems(Startup, setup)
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.run();
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}
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fn setup(mut commands: Commands, asset_server: Res<AssetServer>) {
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commands.spawn(Camera2dBundle::default());
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commands.spawn(SpriteBundle {
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texture: asset_server.load("branding/icon.png"),
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..default()
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});
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}
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