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bevy_reflect: Recursive registration (bevyengine#5781)
# Objective Resolves bevyengine#4154 Currently, registration must all be done manually: ```rust #[derive(Reflect)] struct Foo(Bar); #[derive(Reflect)] struct Bar(Baz); #[derive(Reflect)] struct Baz(usize); fn main() { // ... app .register_type::<Foo>() .register_type::<Bar>() .register_type::<Baz>() // .register_type::<usize>() <- This one is handled by Bevy, thankfully // ... } ``` This can grow really quickly and become very annoying to add, remove, and update as types change. It would be great if we could help reduce the number of types that a user must manually implement themselves. ## Solution As suggested in bevyengine#4154, this PR adds automatic recursive registration. Essentially, when a type is registered, it may now also choose to register additional types along with it using the new `GetTypeRegistration::register_type_dependencies` trait method. The `Reflect` derive macro now automatically does this for all fields in structs, tuple structs, struct variants, and tuple variants. This is also done for tuples, arrays, `Vec<T>`, `HashMap<K, V>`, and `Option<T>`. This allows us to simplify the code above like: ```rust #[derive(Reflect)] struct Foo(Bar); #[derive(Reflect)] struct Bar(Baz); #[derive(Reflect)] struct Baz(usize); fn main() { // ... app.register_type::<Foo>() // ... } ``` This automatic registration only occurs if the type has not yet been registered. If it has been registered, we simply skip it and move to the next one. This reduces the cost of registration and prevents overwriting customized registrations. ## Considerations While this does improve ergonomics on one front, it's important to look at some of the arguments against adopting a PR like this. #### Generic Bounds ~~Since we need to be able to register the fields individually, we need those fields to implement `GetTypeRegistration`. This forces users to then add this trait as a bound on their generic arguments. This annoyance could be relieved with something like bevyengine#5772.~~ This is no longer a major issue as the `Reflect` derive now adds the `GetTypeRegistration` bound by default. This should technically be okay, since we already add the `Reflect` bound. However, this can also be considered a breaking change for manual implementations that left out a `GetTypeRegistration` impl ~~or for items that contain dynamic types (e.g. `DynamicStruct`) since those also do not implement `GetTypeRegistration`~~. #### Registration Assumptions By automatically registering fields, users might inadvertently be relying on certain types to be automatically registered. If `Foo` auto-registers `Bar`, but `Foo` is later removed from the code, then anywhere that previously used or relied on `Bar`'s registration would now fail. --- ## Changelog - Added recursive type registration to structs, tuple structs, struct variants, tuple variants, tuples, arrays, `Vec<T>`, `HashMap<K, V>`, and `Option<T>` - Added a new trait in the hidden `bevy_reflect::__macro_exports` module called `RegisterForReflection` - Added `GetTypeRegistration` impl for `bevy_render::render_asset::RenderAssetUsages` ## Migration Guide All types that derive `Reflect` will now automatically add `GetTypeRegistration` as a bound on all (unignored) fields. This means that all reflected fields will need to also implement `GetTypeRegistration`. If all fields **derive** `Reflect` or are implemented in `bevy_reflect`, this should not cause any issues. However, manual implementations of `Reflect` that excluded a `GetTypeRegistration` impl for their type will need to add one. ```rust #[derive(Reflect)] struct Foo<T: FromReflect> { data: MyCustomType<T> } // OLD impl<T: FromReflect> Reflect for MyCustomType<T> {/* ... */} // NEW impl<T: FromReflect + GetTypeRegistration> Reflect for MyCustomType<T> {/* ... */} impl<T: FromReflect + GetTypeRegistration> GetTypeRegistration for MyCustomType<T> {/* ... */} ``` --------- Co-authored-by: James Liu <contact@jamessliu.com> Co-authored-by: radiish <cb.setho@gmail.com> Co-authored-by: Carter Anderson <mcanders1@gmail.com>
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