bevy-bloxel-classic/common/src/assets/manifest.rs

use std::marker::PhantomData;

use bevy::prelude::*;

use bevy::asset::io::Reader;
use bevy::asset::{AssetLoader, AssetPath, LoadContext};
use bevy::ecs::system::SystemParam;
use derive_where::derive_where;

use crate::assets::asset_tracking::LoadResource;
use crate::identifier::{Identifier, IdentifierMap};

/// Trait implemented by [`Asset`]s which can appear in a [`Manifest`].
pub trait ManifestEntry: Asset {
    type Marker: TypePath;
}

#[derive(Resource, Asset, Reflect)]
#[derive_where(Clone)]
struct ManifestResource<T: ManifestEntry> {
    #[dependency]
    handle: Handle<ManifestAsset<T>>,
}

#[derive(Asset, TypePath)]
struct ManifestAsset<T: ManifestEntry> {
    #[dependency]
    map: IdentifierMap<T::Marker, Handle<T>>,
}

impl<T: ManifestEntry> ManifestAsset<T> {
    pub fn get(&self, id: &Identifier<T::Marker>) -> Option<&Handle<T>> {
        self.map.get(id)
    }
}

#[derive_where(Default)]
struct ManifestAssetLoader<T: ManifestEntry> {
    entry_type: PhantomData<T>,
}

impl<T: ManifestEntry> AssetLoader for ManifestAssetLoader<T> {
    type Asset = ManifestAsset<T>;
    type Settings = ();
    type Error = BevyError;

    async fn load(
        &self,
        reader: &mut dyn Reader,
        _settings: &Self::Settings,
        load_context: &mut LoadContext<'_>,
    ) -> Result<Self::Asset, Self::Error> {
        let mut bytes = Vec::new();
        reader.read_to_end(&mut bytes).await?;
        let raw = ron::de::from_bytes::<Vec<String>>(&bytes)?;

        let mut map = IdentifierMap::default();
        for id in raw {
            let id = Identifier::<T::Marker>::new(id)?;
            let handle = load_context.load(format!("blocks/{id}.ron"));
            map.try_insert(id, handle).map_err(|_| "duplicate entry")?;
        }
        Ok(ManifestAsset { map })
    }

    fn extensions(&self) -> &[&str] {
        &["manifest.ron"]
    }
}

#[derive(SystemParam)]
pub struct Manifest<'w, T: ManifestEntry> {
    resource: Option<Res<'w, ManifestResource<T>>>,
    manifest_assets: Res<'w, Assets<ManifestAsset<T>>>,
    entry_assets: Res<'w, Assets<T>>,
}

impl<T: ManifestEntry> Manifest<'_, T> {
    /// Looks up an [`Asset`] of type `T` from its associated manifest.
    ///
    /// Returns `None` if an asset with the given identifier is missing from
    /// the manifest, or if the manifest is not loaded, including if it has
    /// not been initialized with [`InitManifest::init_manifest`].
    pub fn get(&self, id: &Identifier<T::Marker>) -> Option<&T> {
        let manifest_handle = &self.resource.as_ref()?.handle;
        // SAFETY: Manifest loads this as a dependency, so it should exist.
        let manifest = self.manifest_assets.get(manifest_handle).unwrap();
        let entry_handle = manifest.get(id)?;
        // SAFETY: Entry loads this as a dependency, so it should exist.
        let entry = self.entry_assets.get(entry_handle).unwrap();
        Some(entry)
    }
}

pub trait InitManifest {
    fn init_manifest<'a, T: ManifestEntry>(&mut self, path: impl Into<AssetPath<'a>>);
}

impl InitManifest for App {
    fn init_manifest<'a, T: ManifestEntry>(&mut self, path: impl Into<AssetPath<'a>>) {
        self.init_asset::<T>();
        self.init_asset::<ManifestAsset<T>>();
        self.init_asset_loader::<ManifestAssetLoader<T>>();

        // Insert a `Resource` that holds a handle to the `ManifestAsset` once it's loaded.
        let handle = self.world_mut().load_asset(path);
        self.load_resource_with(ManifestResource::<T> { handle });
    }
}