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reflection.rs
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reflection.rs
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//! Illustrates how "reflection" works in Bevy.
//!
//! Reflection provides a way to dynamically interact with Rust types, such as accessing fields
//! by their string name. Reflection is a core part of Bevy and enables a number of interesting
//! features (like scenes).
use bevy::{
prelude::*,
reflect::{
serde::{ReflectSerializer, UntypedReflectDeserializer},
DynamicStruct,
},
};
use serde::de::DeserializeSeed;
fn main() {
App::new()
.add_plugins(DefaultPlugins)
.register_type::<Foo>()
.register_type::<Bar>()
.add_systems(Startup, setup)
.run();
}
/// Deriving `Reflect` implements the relevant reflection traits. In this case, it implements the
/// `Reflect` trait and the `Struct` trait `derive(Reflect)` assumes that all fields also implement
/// Reflect.
///
/// All fields in a reflected item will need to be `Reflect` as well. You can opt a field out of
/// reflection by using the `#[reflect(ignore)]` attribute.
/// If you choose to ignore a field, you need to let the automatically-derived `FromReflect` implementation
/// how to handle the field.
/// To do this, you can either define a `#[reflect(default = "...")]` attribute on the ignored field, or
/// opt-out of `FromReflect`'s auto-derive using the `#[reflect(from_reflect = false)]` attribute.
#[derive(Reflect)]
#[reflect(from_reflect = false)]
pub struct Foo {
a: usize,
nested: Bar,
#[reflect(ignore)]
_ignored: NonReflectedValue,
}
/// This `Bar` type is used in the `nested` field on the `Test` type. We must derive `Reflect` here
/// too (or ignore it)
#[derive(Reflect)]
pub struct Bar {
b: usize,
}
#[derive(Default)]
struct NonReflectedValue {
_a: usize,
}
fn setup(type_registry: Res<AppTypeRegistry>) {
let mut value = Foo {
a: 1,
_ignored: NonReflectedValue { _a: 10 },
nested: Bar { b: 8 },
};
// You can set field values like this. The type must match exactly or this will fail.
*value.get_field_mut("a").unwrap() = 2usize;
assert_eq!(value.a, 2);
assert_eq!(*value.get_field::<usize>("a").unwrap(), 2);
// You can also get the &dyn Reflect value of a field like this
let field = value.field("a").unwrap();
// you can downcast Reflect values like this:
assert_eq!(*field.downcast_ref::<usize>().unwrap(), 2);
// DynamicStruct also implements the `Struct` and `Reflect` traits.
let mut patch = DynamicStruct::default();
patch.insert("a", 4usize);
// You can "apply" Reflect implementations on top of other Reflect implementations.
// This will only set fields with the same name, and it will fail if the types don't match.
// You can use this to "patch" your types with new values.
value.apply(&patch);
assert_eq!(value.a, 4);
let type_registry = type_registry.read();
// By default, all derived `Reflect` types can be Serialized using serde. No need to derive
// Serialize!
let serializer = ReflectSerializer::new(&value, &type_registry);
let ron_string =
ron::ser::to_string_pretty(&serializer, ron::ser::PrettyConfig::default()).unwrap();
info!("{}\n", ron_string);
// Dynamic properties can be deserialized
let reflect_deserializer = UntypedReflectDeserializer::new(&type_registry);
let mut deserializer = ron::de::Deserializer::from_str(&ron_string).unwrap();
let reflect_value = reflect_deserializer.deserialize(&mut deserializer).unwrap();
// Deserializing returns a Box<dyn Reflect> value. Generally, deserializing a value will return
// the "dynamic" variant of a type. For example, deserializing a struct will return the
// DynamicStruct type. "Value types" will be deserialized as themselves.
let _deserialized_struct = reflect_value.downcast_ref::<DynamicStruct>();
// Reflect has its own `partial_eq` implementation, named `reflect_partial_eq`. This behaves
// like normal `partial_eq`, but it treats "dynamic" and "non-dynamic" types the same. The
// `Foo` struct and deserialized `DynamicStruct` are considered equal for this reason:
assert!(reflect_value.reflect_partial_eq(&value).unwrap());
// By "patching" `Foo` with the deserialized DynamicStruct, we can "Deserialize" Foo.
// This means we can serialize and deserialize with a single `Reflect` derive!
value.apply(&*reflect_value);
}