bevy/crates/bevy_asset/src/processor/process.rs

use crate::{
    io::{
        AssetReaderError, AssetWriterError, MissingAssetWriterError,
        MissingProcessedAssetReaderError, MissingProcessedAssetWriterError, SliceReader, Writer,
    },
    meta::{AssetAction, AssetMeta, AssetMetaDyn, ProcessDependencyInfo, ProcessedInfo, Settings},
    processor::AssetProcessor,
    saver::{AssetSaver, SavedAsset},
    transformer::{AssetTransformer, IdentityAssetTransformer, TransformedAsset},
    AssetLoadError, AssetLoader, AssetPath, DeserializeMetaError, ErasedLoadedAsset,
    MissingAssetLoaderForExtensionError, MissingAssetLoaderForTypeNameError,
};
use alloc::{
    borrow::ToOwned,
    boxed::Box,
    string::{String, ToString},
};
use bevy_tasks::{BoxedFuture, ConditionalSendFuture};
use core::marker::PhantomData;
use serde::{Deserialize, Serialize};
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 [`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.
    type Settings: Settings + Default + Serialize + for<'a> Deserialize<'a>;
    /// The [`AssetLoader`] that will be used to load the final processed asset.
    type OutputLoader: AssetLoader;
    /// Processes the asset stored on `context` in some way using the settings stored on `meta`. The results are written to `writer`. The
    /// final written processed asset is loadable using [`Process::OutputLoader`]. This load will use the returned [`AssetLoader::Settings`].
    fn process(
        &self,
        context: &mut ProcessContext,
        meta: AssetMeta<(), Self>,
        writer: &mut Writer,
    ) -> impl ConditionalSendFuture<
        Output = 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.
///
/// If your [`Process`] does not need to transform the [`Asset`], you can use [`IdentityAssetTransformer`] as `T`.
/// This will directly return the input [`Asset`], allowing your [`Process`] to directly load and then save an [`Asset`].
/// However, this pattern should only be used for cases such as file format conversion.
/// Otherwise, consider refactoring your [`AssetLoader`] and [`AssetSaver`] to isolate the transformation step into an explicit [`AssetTransformer`].
///
/// 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>,
}

impl<L: AssetLoader, S: AssetSaver<Asset = L::Asset>> From<S>
    for LoadTransformAndSave<L, IdentityAssetTransformer<L::Asset>, S>
{
    fn from(value: S) -> Self {
        LoadTransformAndSave {
            transformer: IdentityAssetTransformer::new(),
            saver: value,
            marker: PhantomData,
        }
    }
}

/// 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,
        }
    }
}

/// An error that is encountered during [`Process::process`].
#[derive(Error, Debug)]
pub enum ProcessError {
    #[error(transparent)]
    MissingAssetLoaderForExtension(#[from] MissingAssetLoaderForExtensionError),
    #[error(transparent)]
    MissingAssetLoaderForTypeName(#[from] MissingAssetLoaderForTypeNameError),
    #[error("The processor '{0}' does not exist")]
    #[from(ignore)]
    MissingProcessor(String),
    #[error("Encountered an AssetReader error for '{path}': {err}")]
    #[from(ignore)]
    AssetReaderError {
        path: AssetPath<'static>,
        err: AssetReaderError,
    },
    #[error("Encountered an AssetWriter error for '{path}': {err}")]
    #[from(ignore)]
    AssetWriterError {
        path: AssetPath<'static>,
        err: AssetWriterError,
    },
    #[error(transparent)]
    MissingAssetWriterError(#[from] MissingAssetWriterError),
    #[error(transparent)]
    MissingProcessedAssetReaderError(#[from] MissingProcessedAssetReaderError),
    #[error(transparent)]
    MissingProcessedAssetWriterError(#[from] MissingProcessedAssetWriterError),
    #[error("Failed to read asset metadata for {path}: {err}")]
    #[from(ignore)]
    ReadAssetMetaError {
        path: AssetPath<'static>,
        err: AssetReaderError,
    },
    #[error(transparent)]
    DeserializeMetaError(#[from] DeserializeMetaError),
    #[error(transparent)]
    AssetLoadError(#[from] AssetLoadError),
    #[error("The wrong meta type was passed into a processor. This is probably an internal implementation error.")]
    WrongMetaType,
    #[error("Encountered an error while saving the asset: {0}")]
    #[from(ignore)]
    AssetSaveError(Box<dyn core::error::Error + Send + Sync + 'static>),
    #[error("Encountered an error while transforming the asset: {0}")]
    #[from(ignore)]
    AssetTransformError(Box<dyn core::error::Error + Send + Sync + 'static>),
    #[error("Assets without extensions are not supported.")]
    ExtensionRequired,
}

impl<Loader, Transformer, Saver> Process for LoadTransformAndSave<Loader, Transformer, Saver>
where
    Loader: AssetLoader,
    Transformer: AssetTransformer<AssetInput = Loader::Asset>,
    Saver: AssetSaver<Asset = Transformer::AssetOutput>,
{
    type Settings =
        LoadTransformAndSaveSettings<Loader::Settings, Transformer::Settings, Saver::Settings>;
    type OutputLoader = Saver::OutputLoader;

    async fn process(
        &self,
        context: &mut ProcessContext<'_>,
        meta: AssetMeta<(), Self>,
        writer: &mut Writer,
    ) -> Result<<Self::OutputLoader as AssetLoader>::Settings, ProcessError> {
        let AssetAction::Process { settings, .. } = meta.asset else {
            return Err(ProcessError::WrongMetaType);
        };
        let loader_meta = AssetMeta::<Loader, ()>::new(AssetAction::Load {
            loader: core::any::type_name::<Loader>().to_string(),
            settings: settings.loader_settings,
        });
        let pre_transformed_asset = TransformedAsset::<Loader::Asset>::from_loaded(
            context.load_source_asset(loader_meta).await?,
        )
        .unwrap();

        let post_transformed_asset = self
            .transformer
            .transform(pre_transformed_asset, &settings.transformer_settings)
            .await
            .map_err(|err| ProcessError::AssetTransformError(err.into()))?;

        let saved_asset =
            SavedAsset::<Transformer::AssetOutput>::from_transformed(&post_transformed_asset);

        let output_settings = self
            .saver
            .save(writer, saved_asset, &settings.saver_settings)
            .await
            .map_err(|error| ProcessError::AssetSaveError(error.into()))?;
        Ok(output_settings)
    }
}

/// A type-erased variant of [`Process`] that enables interacting with processor implementations without knowing
/// their type.
pub trait ErasedProcessor: Send + Sync {
    /// Type-erased variant of [`Process::process`].
    fn process<'a>(
        &'a self,
        context: &'a mut ProcessContext,
        meta: Box<dyn AssetMetaDyn>,
        writer: &'a mut Writer,
    ) -> BoxedFuture<'a, Result<Box<dyn AssetMetaDyn>, ProcessError>>;
    /// Deserialized `meta` as type-erased [`AssetMeta`], operating under the assumption that it matches the meta
    /// for the underlying [`Process`] impl.
    fn deserialize_meta(&self, meta: &[u8]) -> Result<Box<dyn AssetMetaDyn>, DeserializeMetaError>;
    /// Returns the default type-erased [`AssetMeta`] for the underlying [`Process`] impl.
    fn default_meta(&self) -> Box<dyn AssetMetaDyn>;
}

impl<P: Process> ErasedProcessor for P {
    fn process<'a>(
        &'a self,
        context: &'a mut ProcessContext,
        meta: Box<dyn AssetMetaDyn>,
        writer: &'a mut Writer,
    ) -> BoxedFuture<'a, Result<Box<dyn AssetMetaDyn>, ProcessError>> {
        Box::pin(async move {
            let meta = meta
                .downcast::<AssetMeta<(), P>>()
                .map_err(|_e| ProcessError::WrongMetaType)?;
            let loader_settings = <P as Process>::process(self, context, *meta, writer).await?;
            let output_meta: Box<dyn AssetMetaDyn> =
                Box::new(AssetMeta::<P::OutputLoader, ()>::new(AssetAction::Load {
                    loader: core::any::type_name::<P::OutputLoader>().to_string(),
                    settings: loader_settings,
                }));
            Ok(output_meta)
        })
    }

    fn deserialize_meta(&self, meta: &[u8]) -> Result<Box<dyn AssetMetaDyn>, DeserializeMetaError> {
        let meta: AssetMeta<(), P> = ron::de::from_bytes(meta)?;
        Ok(Box::new(meta))
    }

    fn default_meta(&self) -> Box<dyn AssetMetaDyn> {
        Box::new(AssetMeta::<(), P>::new(AssetAction::Process {
            processor: core::any::type_name::<P>().to_string(),
            settings: P::Settings::default(),
        }))
    }
}

/// Provides scoped data access to the [`AssetProcessor`].
/// This must only expose processor data that is represented in the asset's hash.
pub struct ProcessContext<'a> {
    /// The "new" processed info for the final processed asset. It is [`ProcessContext`]'s
    /// job to populate `process_dependencies` with any asset dependencies used to process
    /// this asset (ex: loading an asset value from the [`AssetServer`] of the [`AssetProcessor`])
    ///
    /// DO NOT CHANGE ANY VALUES HERE OTHER THAN APPENDING TO `process_dependencies`
    ///
    /// Do not expose this publicly as it would be too easily to invalidate state.
    ///
    /// [`AssetServer`]: crate::server::AssetServer
    pub(crate) new_processed_info: &'a mut ProcessedInfo,
    /// This exists to expose access to asset values (via the [`AssetServer`]).
    ///
    /// ANY ASSET VALUE THAT IS ACCESSED SHOULD BE ADDED TO `new_processed_info.process_dependencies`
    ///
    /// Do not expose this publicly as it would be too easily to invalidate state by forgetting to update
    /// `process_dependencies`.
    ///
    /// [`AssetServer`]: crate::server::AssetServer
    processor: &'a AssetProcessor,
    path: &'a AssetPath<'static>,
    asset_bytes: &'a [u8],
}

impl<'a> ProcessContext<'a> {
    pub(crate) fn new(
        processor: &'a AssetProcessor,
        path: &'a AssetPath<'static>,
        asset_bytes: &'a [u8],
        new_processed_info: &'a mut ProcessedInfo,
    ) -> Self {
        Self {
            processor,
            path,
            asset_bytes,
            new_processed_info,
        }
    }

    /// Load the source asset using the `L` [`AssetLoader`] and the passed in `meta` config.
    /// This will take the "load dependencies" (asset values used when loading with `L`]) and
    /// register them as "process dependencies" because they are asset values required to process the
    /// current asset.
    pub async fn load_source_asset<L: AssetLoader>(
        &mut self,
        meta: AssetMeta<L, ()>,
    ) -> Result<ErasedLoadedAsset, AssetLoadError> {
        let server = &self.processor.server;
        let loader_name = core::any::type_name::<L>();
        let loader = server.get_asset_loader_with_type_name(loader_name).await?;
        let mut reader = SliceReader::new(self.asset_bytes);
        let loaded_asset = server
            .load_with_meta_loader_and_reader(self.path, &meta, &*loader, &mut reader, false, true)
            .await?;
        for (path, full_hash) in &loaded_asset.loader_dependencies {
            self.new_processed_info
                .process_dependencies
                .push(ProcessDependencyInfo {
                    full_hash: *full_hash,
                    path: path.to_owned(),
                });
        }
        Ok(loaded_asset)
    }

    /// The path of the asset being processed.
    #[inline]
    pub fn path(&self) -> &AssetPath<'static> {
        self.path
    }

    /// The source bytes of the asset being processed.
    #[inline]
    pub fn asset_bytes(&self) -> &[u8] {
        self.asset_bytes
    }
}