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bevy_reflect: Reflection-based cloning (#13432)
# Objective Using `Reflect::clone_value` can be somewhat confusing to those unfamiliar with how Bevy's reflection crate works. For example take the following code: ```rust let value: usize = 123; let clone: Box<dyn Reflect> = value.clone_value(); ``` What can we expect to be the underlying type of `clone`? If you guessed `usize`, then you're correct! Let's try another: ```rust #[derive(Reflect, Clone)] struct Foo(usize); let value: Foo = Foo(123); let clone: Box<dyn Reflect> = value.clone_value(); ``` What about this code? What is the underlying type of `clone`? If you guessed `Foo`, unfortunately you'd be wrong. It's actually `DynamicStruct`. It's not obvious that the generated `Reflect` impl actually calls `Struct::clone_dynamic` under the hood, which always returns `DynamicStruct`. There are already some efforts to make this a bit more apparent to the end-user: #7207 changes the signature of `Reflect::clone_value` to instead return `Box<dyn PartialReflect>`, signaling that we're potentially returning a dynamic type. But why _can't_ we return `Foo`? `Foo` can obviously be cloned— in fact, we already derived `Clone` on it. But even without the derive, this seems like something `Reflect` should be able to handle. Almost all types that implement `Reflect` either contain no data (trivially clonable), they contain a `#[reflect_value]` type (which, by definition, must implement `Clone`), or they contain another `Reflect` type (which recursively fall into one of these three categories). This PR aims to enable true reflection-based cloning where you get back exactly the type that you think you do. ## Solution Add a `Reflect::reflect_clone` method which returns `Result<Box<dyn Reflect>, ReflectCloneError>`, where the `Box<dyn Reflect>` is guaranteed to be the same type as `Self`. ```rust #[derive(Reflect)] struct Foo(usize); let value: Foo = Foo(123); let clone: Box<dyn Reflect> = value.reflect_clone().unwrap(); assert!(clone.is::<Foo>()); ``` Notice that we didn't even need to derive `Clone` for this to work: it's entirely powered via reflection! Under the hood, the macro generates something like this: ```rust fn reflect_clone(&self) -> Result<Box<dyn Reflect>, ReflectCloneError> { Ok(Box::new(Self { // The `reflect_clone` impl for `usize` just makes use of its `Clone` impl 0: Reflect::reflect_clone(&self.0)?.take().map_err(/* ... */)?, })) } ``` If we did derive `Clone`, we can tell `Reflect` to rely on that instead: ```rust #[derive(Reflect, Clone)] #[reflect(Clone)] struct Foo(usize); ``` <details> <summary>Generated Code</summary> ```rust fn reflect_clone(&self) -> Result<Box<dyn Reflect>, ReflectCloneError> { Ok(Box::new(Clone::clone(self))) } ``` </details> Or, we can specify our own cloning function: ```rust #[derive(Reflect)] #[reflect(Clone(incremental_clone))] struct Foo(usize); fn incremental_clone(value: &usize) -> usize { *value + 1 } ``` <details> <summary>Generated Code</summary> ```rust fn reflect_clone(&self) -> Result<Box<dyn Reflect>, ReflectCloneError> { Ok(Box::new(incremental_clone(self))) } ``` </details> Similarly, we can specify how fields should be cloned. This is important for fields that are `#[reflect(ignore)]`'d as we otherwise have no way to know how they should be cloned. ```rust #[derive(Reflect)] struct Foo { #[reflect(ignore, clone)] bar: usize, #[reflect(ignore, clone = "incremental_clone")] baz: usize, } fn incremental_clone(value: &usize) -> usize { *value + 1 } ``` <details> <summary>Generated Code</summary> ```rust fn reflect_clone(&self) -> Result<Box<dyn Reflect>, ReflectCloneError> { Ok(Box::new(Self { bar: Clone::clone(&self.bar), baz: incremental_clone(&self.baz), })) } ``` </details> If we don't supply a `clone` attribute for an ignored field, then the method will automatically return `Err(ReflectCloneError::FieldNotClonable {/* ... */})`. `Err` values "bubble up" to the caller. So if `Foo` contains `Bar` and the `reflect_clone` method for `Bar` returns `Err`, then the `reflect_clone` method for `Foo` also returns `Err`. ### Attribute Syntax You might have noticed the differing syntax between the container attribute and the field attribute. This was purely done for consistency with the current attributes. There are PRs aimed at improving this. #7317 aims at making the "special-cased" attributes more in line with the field attributes syntactically. And #9323 aims at moving away from the stringified paths in favor of just raw function paths. ### Compatibility with Unique Reflect This PR was designed with Unique Reflect (#7207) in mind. This method actually wouldn't change that much (if at all) under Unique Reflect. It would still exist on `Reflect` and it would still `Option<Box<dyn Reflect>>`. In fact, Unique Reflect would only _improve_ the user's understanding of what this method returns. We may consider moving what's currently `Reflect::clone_value` to `PartialReflect` and possibly renaming it to `partial_reflect_clone` or `clone_dynamic` to better indicate how it differs from `reflect_clone`. ## Testing You can test locally by running the following command: ``` cargo test --package bevy_reflect ``` --- ## Changelog - Added `Reflect::reflect_clone` method - Added `ReflectCloneError` error enum - Added `#[reflect(Clone)]` container attribute - Added `#[reflect(clone)]` field attribute
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