Add more load_direct implementations (#13415)

# Objective
- Introduce variants of `LoadContext::load_direct` which allow picking
asset type & configuring settings.
- Fixes #12963.

## Solution
- Implements `ErasedLoadedAsset::downcast` and adds some accessors to
`LoadedAsset<A>`.
- Changes `load_direct`/`load_direct_with_reader` to be typed, and
introduces `load_direct_untyped`/`load_direct_untyped_with_reader`.
- Introduces `load_direct_with_settings` and
`load_direct_with_reader_and_settings`.

## Testing
- I've run cargo test and played with the examples which use
`load_direct`.
- I also extended the `asset_processing` example to use the new typed
version of `load_direct` and use `load_direct_with_settings`.

---

## Changelog
- Introduced new `load_direct` methods in `LoadContext` to allow
specifying type & settings

## Migration Guide
- `LoadContext::load_direct` has been renamed to
`LoadContext::load_direct_untyped`. You may find the new `load_direct`
is more appropriate for your use case (and the migration may only be
moving one type parameter).
- `LoadContext::load_direct_with_reader` has been renamed to
`LoadContext::load_direct_untyped_with_reader`.

---

This might not be an obvious win as a solution because it introduces
quite a few new `load_direct` alternatives - but it does follow the
existing pattern pretty well. I'm very open to alternatives.
:sweat_smile:

crates/bevy_asset/src/lib.rs

@@ -510,12 +510,13 @@ mod tests {
             let mut ron: CoolTextRon = ron::de::from_bytes(&bytes)?;
             let mut embedded = String::new();
             for dep in ron.embedded_dependencies {
-                let loaded = load_context.load_direct(&dep).await.map_err(|_| {
-                    Self::Error::CannotLoadDependency {
+                let loaded = load_context
+                    .load_direct::<CoolText>(&dep)
+                    .await
+                    .map_err(|_| Self::Error::CannotLoadDependency {
                         dependency: dep.into(),
-                    }
-                })?;
-                let cool = loaded.get::<CoolText>().unwrap();
+                    })?;
+                let cool = loaded.get();
                 embedded.push_str(&cool.text);
             }
             Ok(CoolText {

crates/bevy_asset/src/meta.rs

@@ -208,21 +208,26 @@ impl AssetLoader for () {
     }
 }
 
+pub(crate) fn meta_transform_settings<S: Settings>(
+    meta: &mut dyn AssetMetaDyn,
+    settings: &(impl Fn(&mut S) + Send + Sync + 'static),
+) {
+    if let Some(loader_settings) = meta.loader_settings_mut() {
+        if let Some(loader_settings) = loader_settings.downcast_mut::<S>() {
+            settings(loader_settings);
+        } else {
+            error!(
+                "Configured settings type {} does not match AssetLoader settings type",
+                std::any::type_name::<S>(),
+            );
+        }
+    }
+}
+
 pub(crate) fn loader_settings_meta_transform<S: Settings>(
     settings: impl Fn(&mut S) + Send + Sync + 'static,
 ) -> MetaTransform {
-    Box::new(move |meta| {
-        if let Some(loader_settings) = meta.loader_settings_mut() {
-            if let Some(loader_settings) = loader_settings.downcast_mut::<S>() {
-                settings(loader_settings);
-            } else {
-                error!(
-                    "Configured settings type {} does not match AssetLoader settings type",
-                    std::any::type_name::<S>(),
-                );
-            }
-        }
-    })
+    Box::new(move |meta| meta_transform_settings(meta, &settings))
 }
 
 pub type AssetHash = [u8; 32];

examples/asset/asset_decompression.rs

@@ -72,7 +72,7 @@ impl AssetLoader for GzAssetLoader {
         let mut reader = VecReader::new(bytes_uncompressed);
 
         let uncompressed = load_context
-            .load_direct_with_reader(&mut reader, contained_path)
+            .load_direct_untyped_with_reader(&mut reader, contained_path)
             .await?;
 
         Ok(GzAsset { uncompressed })

examples/asset/processing/asset_processing.rs

@@ -72,7 +72,7 @@ struct Text(String);
 #[derive(Default)]
 struct TextLoader;
 
-#[derive(Default, Serialize, Deserialize)]
+#[derive(Clone, Default, Serialize, Deserialize)]
 struct TextSettings {
     text_override: Option<String>,
 }
@@ -107,6 +107,7 @@ struct CoolTextRon {
     text: String,
     dependencies: Vec<String>,
     embedded_dependencies: Vec<String>,
+    dependencies_with_settings: Vec<(String, TextSettings)>,
 }
 
 #[derive(Asset, TypePath, Debug)]
@@ -145,9 +146,16 @@ impl AssetLoader for CoolTextLoader {
         let ron: CoolTextRon = ron::de::from_bytes(&bytes)?;
         let mut base_text = ron.text;
         for embedded in ron.embedded_dependencies {
-            let loaded = load_context.load_direct(&embedded).await?;
-            let text = loaded.get::<Text>().unwrap();
-            base_text.push_str(&text.0);
+            let loaded = load_context.load_direct::<Text>(&embedded).await?;
+            base_text.push_str(&loaded.get().0);
+        }
+        for (path, settings_override) in ron.dependencies_with_settings {
+            let loaded = load_context
+                .load_direct_with_settings::<Text, _>(&path, move |settings| {
+                    *settings = settings_override.clone();
+                })
+                .await?;
+            base_text.push_str(&loaded.get().0);
         }
         Ok(CoolText {
             text: base_text,

examples/asset/processing/assets/a.cool.ron

@@ -5,4 +5,5 @@
         "foo/c.cool.ron",
     ],
     embedded_dependencies: [],
-)
+    dependencies_with_settings: [],
+)

examples/asset/processing/assets/d.cool.ron

@@ -6,4 +6,7 @@
         "foo/c.cool.ron",
         "embedded://asset_processing/e.txt"
     ],
-)
+    dependencies_with_settings: [
+        ("embedded://asset_processing/e.txt", (text_override: Some("E"))),
+    ],
+)

examples/asset/processing/assets/foo/b.cool.ron

@@ -2,4 +2,5 @@
     text: "b",
     dependencies: [],
     embedded_dependencies: [],
-)
+    dependencies_with_settings: [],
+)

examples/asset/processing/assets/foo/c.cool.ron

@@ -2,4 +2,5 @@
     text: "c",
     dependencies: [],
     embedded_dependencies: ["a.cool.ron", "foo/b.cool.ron"],
-)
+    dependencies_with_settings: [],
+)