bevy/crates/bevy_reflect/src/std_traits.rs

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add `#[reflect(Default)]` to create default value for reflected types (#3733) ### Problem It currently isn't possible to construct the default value of a reflected type. Because of that, it isn't possible to use `add_component` of `ReflectComponent` to add a new component to an entity because you can't know what the initial value should be. ### Solution 1. add `ReflectDefault` type ```rust #[derive(Clone)] pub struct ReflectDefault { default: fn() -> Box<dyn Reflect>, } impl ReflectDefault { pub fn default(&self) -> Box<dyn Reflect> { (self.default)() } } impl<T: Reflect + Default> FromType<T> for ReflectDefault { fn from_type() -> Self { ReflectDefault { default: || Box::new(T::default()), } } } ``` 2. add `#[reflect(Default)]` to all component types that implement `Default` and are user facing (so not `ComputedSize`, `CubemapVisibleEntities` etc.) This makes it possible to add the default value of a component to an entity without any compile-time information: ```rust fn main() { let mut app = App::new(); app.register_type::<Camera>(); let type_registry = app.world.get_resource::<TypeRegistry>().unwrap(); let type_registry = type_registry.read(); let camera_registration = type_registry.get(std::any::TypeId::of::<Camera>()).unwrap(); let reflect_default = camera_registration.data::<ReflectDefault>().unwrap(); let reflect_component = camera_registration .data::<ReflectComponent>() .unwrap() .clone(); let default = reflect_default.default(); drop(type_registry); let entity = app.world.spawn().id(); reflect_component.add_component(&mut app.world, entity, &*default); let camera = app.world.entity(entity).get::<Camera>().unwrap(); dbg!(&camera); } ``` ### Open questions - should we have `ReflectDefault` or `ReflectFromWorld` or both?
2022-05-03 19:20:13 +00:00
use crate::{FromType, Reflect};
/// A struct used to provide the default value of a type.
///
/// A [`ReflectDefault`] for type `T` can be obtained via [`FromType::from_type`].
#[derive(Clone)]
pub struct ReflectDefault {
default: fn() -> Box<dyn Reflect>,
}
impl ReflectDefault {
pub fn default(&self) -> Box<dyn Reflect> {
(self.default)()
}
}
impl<T: Reflect + Default> FromType<T> for ReflectDefault {
fn from_type() -> Self {
ReflectDefault {
default: || Box::new(T::default()),
}
}
}