mirror of
https://github.com/bevyengine/bevy
synced 2024-11-10 07:04:33 +00:00
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 {}
```
156 lines
4 KiB
Rust
156 lines
4 KiB
Rust
//! Implements loader for a custom asset type.
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use bevy::{
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asset::{io::Reader, AssetLoader, LoadContext},
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prelude::*,
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reflect::TypePath,
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};
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use serde::Deserialize;
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use thiserror::Error;
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#[derive(Asset, TypePath, Debug, Deserialize)]
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struct CustomAsset {
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#[allow(dead_code)]
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value: i32,
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}
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#[derive(Default)]
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struct CustomAssetLoader;
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/// Possible errors that can be produced by [`CustomAssetLoader`]
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#[non_exhaustive]
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#[derive(Debug, Error)]
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enum CustomAssetLoaderError {
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/// An [IO](std::io) Error
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#[error("Could not load asset: {0}")]
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Io(#[from] std::io::Error),
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/// A [RON](ron) Error
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#[error("Could not parse RON: {0}")]
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RonSpannedError(#[from] ron::error::SpannedError),
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}
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impl AssetLoader for CustomAssetLoader {
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type Asset = CustomAsset;
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type Settings = ();
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type Error = CustomAssetLoaderError;
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async fn load<'a>(
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&'a self,
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reader: &'a mut dyn Reader,
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_settings: &'a (),
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_load_context: &'a mut LoadContext<'_>,
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) -> Result<Self::Asset, Self::Error> {
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let mut bytes = Vec::new();
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reader.read_to_end(&mut bytes).await?;
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let custom_asset = ron::de::from_bytes::<CustomAsset>(&bytes)?;
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Ok(custom_asset)
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}
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fn extensions(&self) -> &[&str] {
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&["custom"]
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}
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}
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#[derive(Asset, TypePath, Debug)]
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struct Blob {
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bytes: Vec<u8>,
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}
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#[derive(Default)]
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struct BlobAssetLoader;
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/// Possible errors that can be produced by [`BlobAssetLoader`]
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#[non_exhaustive]
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#[derive(Debug, Error)]
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enum BlobAssetLoaderError {
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/// An [IO](std::io) Error
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#[error("Could not load file: {0}")]
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Io(#[from] std::io::Error),
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}
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impl AssetLoader for BlobAssetLoader {
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type Asset = Blob;
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type Settings = ();
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type Error = BlobAssetLoaderError;
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async fn load<'a>(
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&'a self,
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reader: &'a mut dyn Reader,
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_settings: &'a (),
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_load_context: &'a mut LoadContext<'_>,
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) -> Result<Self::Asset, Self::Error> {
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info!("Loading Blob...");
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let mut bytes = Vec::new();
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reader.read_to_end(&mut bytes).await?;
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Ok(Blob { bytes })
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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(DefaultPlugins)
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.init_resource::<State>()
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.init_asset::<CustomAsset>()
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.init_asset::<Blob>()
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.init_asset_loader::<CustomAssetLoader>()
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.init_asset_loader::<BlobAssetLoader>()
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.add_systems(Startup, setup)
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.add_systems(Update, print_on_load)
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.run();
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}
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#[derive(Resource, Default)]
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struct State {
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handle: Handle<CustomAsset>,
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other_handle: Handle<CustomAsset>,
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blob: Handle<Blob>,
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printed: bool,
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}
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fn setup(mut state: ResMut<State>, asset_server: Res<AssetServer>) {
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// Recommended way to load an asset
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state.handle = asset_server.load("data/asset.custom");
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// File extensions are optional, but are recommended for project management and last-resort inference
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state.other_handle = asset_server.load("data/asset_no_extension");
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// Will use BlobAssetLoader instead of CustomAssetLoader thanks to type inference
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state.blob = asset_server.load("data/asset.custom");
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}
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fn print_on_load(
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mut state: ResMut<State>,
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custom_assets: Res<Assets<CustomAsset>>,
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blob_assets: Res<Assets<Blob>>,
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) {
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let custom_asset = custom_assets.get(&state.handle);
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let other_custom_asset = custom_assets.get(&state.other_handle);
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let blob = blob_assets.get(&state.blob);
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// Can't print results if the assets aren't ready
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if state.printed {
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return;
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}
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if custom_asset.is_none() {
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info!("Custom Asset Not Ready");
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return;
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}
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if other_custom_asset.is_none() {
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info!("Other Custom Asset Not Ready");
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return;
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}
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if blob.is_none() {
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info!("Blob Not Ready");
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return;
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}
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info!("Custom asset loaded: {:?}", custom_asset.unwrap());
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info!("Custom asset loaded: {:?}", other_custom_asset.unwrap());
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info!("Blob Size: {:?} Bytes", blob.unwrap().bytes.len());
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// Once printed, we won't print again
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state.printed = true;
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}
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