Enforce type safe usage of Handle::get (#4794)

# Objective

- Sometimes, people might load an asset as one type, then use it with an `Asset`s for a different type.
- See e.g. #4784. 
- This is especially likely with the Gltf types, since users may not have a clear conceptual model of what types the assets will be.
- We had an instance of this ourselves, in the `scene_viewer` example

## Solution

- Make `Assets::get` require a type safe handle.

---

## Changelog

### Changed

- `Assets::<T>::get` and `Assets::<T>::get_mut` now require that the passed handles are `Handle<T>`, improving the type safety of handles.

### Added
- `HandleUntyped::typed_weak`, a helper function for creating a weak typed version of an exisitng `HandleUntyped`.

## Migration Guide

`Assets::<T>::get` and `Assets::<T>::get_mut` now require that the passed handles are `Handle<T>`, improving the type safety of handles. If you were previously passing in:
   - a `HandleId`, use `&Handle::weak(id)` instead, to create a weak handle. You may have been able to store a type safe `Handle` instead.
   - a `HandleUntyped`, use `&handle_untyped.typed_weak()` to create a weak handle of the specified type. This is most likely to be the useful when using [load_folder](https://docs.rs/bevy_asset/latest/bevy_asset/struct.AssetServer.html#method.load_folder)
   - a `Handle<U>` of  of a different type, consider whether this is the correct handle type to store. If it is (i.e. the same handle id is used for multiple different Asset types) use `Handle::weak(handle.id)` to cast to a different type.

crates/bevy_asset/src/asset_server.rs

@@ -781,8 +781,11 @@ mod test {
             asset_server.get_handle_untyped(id)
         }
 
-        fn get_asset(id: impl Into<HandleId>, world: &World) -> Option<&PngAsset> {
-            world.resource::<Assets<PngAsset>>().get(id.into())
+        fn get_asset<'world>(
+            id: &Handle<PngAsset>,
+            world: &'world World,
+        ) -> Option<&'world PngAsset> {
+            world.resource::<Assets<PngAsset>>().get(id)
         }
 
         fn get_load_state(id: impl Into<HandleId>, world: &World) -> LoadState {
@@ -800,7 +803,7 @@ mod test {
         );
 
         // load the asset
-        let handle = load_asset(path.clone(), &app.world);
+        let handle = load_asset(path.clone(), &app.world).typed();
         let weak_handle = handle.clone_weak();
 
         // asset is loading
@@ -826,7 +829,7 @@ mod test {
         assert!(get_asset(&weak_handle, &app.world).is_none());
 
         // finally, reload the asset
-        let handle = load_asset(path.clone(), &app.world);
+        let handle = load_asset(path.clone(), &app.world).typed();
         assert_eq!(LoadState::Loading, get_load_state(&handle, &app.world));
         app.update();
         assert_eq!(LoadState::Loaded, get_load_state(&handle, &app.world));

crates/bevy_asset/src/assets.rs

@@ -129,20 +129,20 @@ impl<T: Asset> Assets<T> {
     ///
     /// This is the main method for accessing asset data from an [Assets] collection. If you need
     /// mutable access to the asset, use [`get_mut`](Assets::get_mut).
-    pub fn get<H: Into<HandleId>>(&self, handle: H) -> Option<&T> {
+    pub fn get(&self, handle: &Handle<T>) -> Option<&T> {
         self.assets.get(&handle.into())
     }
 
     /// Checks if an asset exists for the given handle
-    pub fn contains<H: Into<HandleId>>(&self, handle: H) -> bool {
+    pub fn contains(&self, handle: &Handle<T>) -> bool {
         self.assets.contains_key(&handle.into())
     }
 
     /// Get mutable access to the asset for the given handle.
     ///
     /// This is the main method for mutably accessing asset data from an [Assets] collection. If you
     /// do not need mutable access to the asset, you may also use [get](Assets::get).
-    pub fn get_mut<H: Into<HandleId>>(&mut self, handle: H) -> Option<&mut T> {
+    pub fn get_mut(&mut self, handle: &Handle<T>) -> Option<&mut T> {
         let id: HandleId = handle.into();
         self.events.send(AssetEvent::Modified {
             handle: Handle::weak(id),
@@ -398,6 +398,6 @@ mod tests {
         let handle = assets_before.add(MyAsset);
         app.add_asset::<MyAsset>(); // Ensure this doesn't overwrite the Asset
         let assets_after = app.world.resource_mut::<Assets<MyAsset>>();
-        assert!(assets_after.get(handle).is_some());
+        assert!(assets_after.get(&handle).is_some());
     }
 }

crates/bevy_asset/src/handle.rs

@@ -169,7 +169,7 @@ impl<T: Asset> Handle<T> {
     /// Makes this handle Strong if it wasn't already.
     ///
     /// This method requires the corresponding [Assets](crate::Assets) collection
-    pub fn make_strong(&mut self, assets: &mut Assets<T>) {
+    pub fn make_strong(&mut self, assets: &Assets<T>) {
         if self.is_strong() {
             return;
         }
@@ -341,6 +341,14 @@ impl HandleUntyped {
         matches!(self.handle_type, HandleType::Strong(_))
     }
 
+    /// Create a weak typed [`Handle`] from this handle.
+    ///
+    /// If this handle is strong and dropped, there is no guarantee that the asset
+    /// will still be available (if only the returned handle is kept)
+    pub fn typed_weak<T: Asset>(&self) -> Handle<T> {
+        self.clone_weak().typed()
+    }
+
     /// Convert this handle into a typed [Handle].
     ///
     /// The new handle will maintain the Strong or Weak status of the current handle.

crates/bevy_text/src/pipeline.rs

@@ -66,7 +66,7 @@ impl<ID: Hash + Eq> TextPipeline<ID> {
             .iter()
             .map(|section| {
                 let font = fonts
-                    .get(section.style.font.id)
+                    .get(&section.style.font)
                     .ok_or(TextError::NoSuchFont)?;
                 let font_id = self.get_or_insert_font_id(&section.style.font, font);
                 let font_size = scale_value(section.style.font_size, scale_factor);

crates/bevy_text/src/text2d.rs

@@ -98,7 +98,7 @@ pub fn extract_text2d_sprite(
                     .color
                     .as_rgba_linear();
                 let atlas = texture_atlases
-                    .get(text_glyph.atlas_info.texture_atlas.clone_weak())
+                    .get(&text_glyph.atlas_info.texture_atlas)
                     .unwrap();
                 let handle = atlas.texture.clone_weak();
                 let index = text_glyph.atlas_info.glyph_index as usize;

crates/bevy_ui/Cargo.toml

@@ -13,6 +13,7 @@ keywords = ["bevy"]
 bevy_app = { path = "../bevy_app", version = "0.8.0-dev" }
 bevy_asset = { path = "../bevy_asset", version = "0.8.0-dev" }
 bevy_core_pipeline = { path = "../bevy_core_pipeline", version = "0.8.0-dev" }
+bevy_derive = { path = "../bevy_derive", version = "0.8.0-dev" }
 bevy_ecs = { path = "../bevy_ecs", version = "0.8.0-dev" }
 bevy_hierarchy = { path = "../bevy_hierarchy", version = "0.8.0-dev" }
 bevy_input = { path = "../bevy_input", version = "0.8.0-dev" }

crates/bevy_ui/src/render/mod.rs

@@ -146,7 +146,7 @@ pub fn extract_uinodes(
         }
         let image = image.0.clone_weak();
         // Skip loading images
-        if !images.contains(image.clone_weak()) {
+        if !images.contains(&image) {
             continue;
         }
         extracted_uinodes.uinodes.push(ExtractedUiNode {
@@ -196,7 +196,7 @@ pub fn extract_text_uinodes(
             for text_glyph in text_glyphs {
                 let color = text.sections[text_glyph.section_index].style.color;
                 let atlas = texture_atlases
-                    .get(text_glyph.atlas_info.texture_atlas.clone_weak())
+                    .get(&text_glyph.atlas_info.texture_atlas)
                     .unwrap();
                 let texture = atlas.texture.clone_weak();
                 let index = text_glyph.atlas_info.glyph_index as usize;

crates/bevy_ui/src/ui_node.rs

@@ -1,5 +1,6 @@
 use crate::{Size, UiRect};
 use bevy_asset::Handle;
+use bevy_derive::{Deref, DerefMut};
 use bevy_ecs::{prelude::Component, reflect::ReflectComponent};
 use bevy_math::Vec2;
 use bevy_reflect::prelude::*;
@@ -369,7 +370,7 @@ impl From<Color> for UiColor {
 }
 
 /// The image of the node
-#[derive(Component, Clone, Debug, Reflect)]
+#[derive(Component, Clone, Debug, Reflect, Deref, DerefMut)]
 #[reflect(Component, Default)]
 pub struct UiImage(pub Handle<Image>);
 

crates/bevy_ui/src/widget/image.rs

@@ -31,7 +31,7 @@ pub fn image_node_system(
     mut query: Query<(&mut CalculatedSize, &UiImage), With<ImageMode>>,
 ) {
     for (mut calculated_size, image) in query.iter_mut() {
-        if let Some(texture) = textures.get(image.0.clone_weak()) {
+        if let Some(texture) = textures.get(image) {
             let size = Size {
                 width: texture.texture_descriptor.size.width as f32,
                 height: texture.texture_descriptor.size.height as f32,

examples/2d/texture_atlas.rs

@@ -50,8 +50,9 @@ fn setup(
 ) {
     let mut texture_atlas_builder = TextureAtlasBuilder::default();
     for handle in &rpg_sprite_handles.handles {
-        let texture = textures.get(handle).unwrap();
-        texture_atlas_builder.add_texture(handle.clone_weak().typed::<Image>(), texture);
+        let handle = handle.typed_weak();
+        let texture = textures.get(&handle).expect("Textures folder contained a file which way matched by a loader which did not create an `Image` asset");
+        texture_atlas_builder.add_texture(handle, texture);
     }
 
     let texture_atlas = texture_atlas_builder.finish(&mut textures).unwrap();

examples/tools/scene_viewer.rs

@@ -67,7 +67,7 @@ Controls:
 }
 
 struct SceneHandle {
-    handle: Handle<Scene>,
+    handle: Handle<Gltf>,
     animations: Vec<Handle<AnimationClip>>,
     instance_id: Option<InstanceId>,
     is_loaded: bool,