229d6c686f
# Objective
There are times where we want to simply take an owned `dyn Reflect` and cast it to a type `T`.
Currently, this involves doing:
```rust
let value = value.take::<T>().unwrap_or_else(|value| {
T::from_reflect(&*value).unwrap_or_else(|| {
panic!(
"expected value of type {} to convert to type {}.",
value.type_name(),
std::any::type_name::<T>()
)
})
});
```
This is a common operation that could be easily be simplified.
## Solution
Add the `FromReflect::take_from_reflect` method. This first tries to `take` the value, calling `from_reflect` iff that fails.
```rust
let value = T::take_from_reflect(value).unwrap_or_else(|value| {
panic!(
"expected value of type {} to convert to type {}.",
value.type_name(),
std::any::type_name::<T>()
)
});
```
Based on suggestion from @soqb on [Discord](https://discord.com/channels/691052431525675048/1002362493634629796/1041046880316043374).
---
## Changelog
- Add `FromReflect::take_from_reflect` method
113 lines
4.4 KiB
Rust
113 lines
4.4 KiB
Rust
use crate::{FromType, Reflect};
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/// A trait for types which can be constructed from a reflected type.
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///
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/// This trait can be derived on types which implement [`Reflect`]. Some complex
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/// types (such as `Vec<T>`) may only be reflected if their element types
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/// implement this trait.
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///
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/// For structs and tuple structs, fields marked with the `#[reflect(ignore)]`
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/// attribute will be constructed using the `Default` implementation of the
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/// field type, rather than the corresponding field value (if any) of the
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/// reflected value.
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pub trait FromReflect: Reflect + Sized {
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/// Constructs a concrete instance of `Self` from a reflected value.
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fn from_reflect(reflect: &dyn Reflect) -> Option<Self>;
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/// Attempts to downcast the given value to `Self` using,
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/// constructing the value using [`from_reflect`] if that fails.
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///
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/// This method is more efficient than using [`from_reflect`] for cases where
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/// the given value is likely a boxed instance of `Self` (i.e. `Box<Self>`)
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/// rather than a boxed dynamic type (e.g. [`DynamicStruct`], [`DynamicList`], etc.).
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///
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/// [`from_reflect`]: Self::from_reflect
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/// [`DynamicStruct`]: crate::DynamicStruct
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/// [`DynamicList`]: crate::DynamicList
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fn take_from_reflect(reflect: Box<dyn Reflect>) -> Result<Self, Box<dyn Reflect>> {
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match reflect.take::<Self>() {
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Ok(value) => Ok(value),
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Err(value) => match Self::from_reflect(value.as_ref()) {
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None => Err(value),
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Some(value) => Ok(value),
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},
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}
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}
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}
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/// Type data that represents the [`FromReflect`] trait and allows it to be used dynamically.
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///
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/// `FromReflect` allows dynamic types (e.g. [`DynamicStruct`], [`DynamicEnum`], etc.) to be converted
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/// to their full, concrete types. This is most important when it comes to deserialization where it isn't
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/// guaranteed that every field exists when trying to construct the final output.
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///
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/// However, to do this, you normally need to specify the exact concrete type:
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///
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/// ```
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/// # use bevy_reflect::{DynamicTupleStruct, FromReflect, Reflect};
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/// #[derive(Reflect, FromReflect, PartialEq, Eq, Debug)]
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/// struct Foo(#[reflect(default = "default_value")] usize);
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///
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/// fn default_value() -> usize { 123 }
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///
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/// let reflected = DynamicTupleStruct::default();
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///
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/// let concrete: Foo = <Foo as FromReflect>::from_reflect(&reflected).unwrap();
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///
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/// assert_eq!(Foo(123), concrete);
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/// ```
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///
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/// In a dynamic context where the type might not be known at compile-time, this is nearly impossible to do.
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/// That is why this type data struct exists— it allows us to construct the full type without knowing
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/// what the actual type is.
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///
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/// # Example
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///
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/// ```
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/// # use bevy_reflect::{DynamicTupleStruct, FromReflect, Reflect, ReflectFromReflect, TypeRegistry};
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/// # #[derive(Reflect, FromReflect, PartialEq, Eq, Debug)]
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/// # #[reflect(FromReflect)]
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/// # struct Foo(#[reflect(default = "default_value")] usize);
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/// # fn default_value() -> usize { 123 }
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/// # let mut registry = TypeRegistry::new();
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/// # registry.register::<Foo>();
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///
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/// let mut reflected = DynamicTupleStruct::default();
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/// reflected.set_name(std::any::type_name::<Foo>().to_string());
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///
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/// let registration = registry.get_with_name(reflected.type_name()).unwrap();
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/// let rfr = registration.data::<ReflectFromReflect>().unwrap();
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///
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/// let concrete: Box<dyn Reflect> = rfr.from_reflect(&reflected).unwrap();
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///
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/// assert_eq!(Foo(123), concrete.take::<Foo>().unwrap());
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/// ```
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///
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/// [`DynamicStruct`]: crate::DynamicStruct
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/// [`DynamicEnum`]: crate::DynamicEnum
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#[derive(Clone)]
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pub struct ReflectFromReflect {
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from_reflect: fn(&dyn Reflect) -> Option<Box<dyn Reflect>>,
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}
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impl ReflectFromReflect {
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/// Perform a [`FromReflect::from_reflect`] conversion on the given reflection object.
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///
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/// This will convert the object to a concrete type if it wasn't already, and return
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/// the value as `Box<dyn Reflect>`.
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#[allow(clippy::wrong_self_convention)]
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pub fn from_reflect(&self, reflect_value: &dyn Reflect) -> Option<Box<dyn Reflect>> {
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(self.from_reflect)(reflect_value)
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}
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}
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impl<T: FromReflect> FromType<T> for ReflectFromReflect {
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fn from_type() -> Self {
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Self {
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from_reflect: |reflect_value| {
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T::from_reflect(reflect_value).map(|value| Box::new(value) as Box<dyn Reflect>)
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},
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}
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}
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}
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