Crate bevy_reflect

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Reflection in Rust.

Reflection is a powerful tool provided within many programming languages that allows for meta-programming: using information about the program to affect the program. In other words, reflection allows us to inspect the program itself, its syntax, and its type information at runtime.

This crate adds this missing reflection functionality to Rust. Though it was made with the Bevy game engine in mind, it’s a general-purpose solution that can be used in any Rust project.

At a very high level, this crate allows you to:

  • Dynamically interact with Rust values
  • Access type metadata at runtime
  • Serialize and deserialize (i.e. save and load) data

It’s important to note that because of missing features in Rust, there are some limitations with this crate.

§The Reflect and PartialReflect traits

At the root of bevy_reflect is the PartialReflect trait.

Its purpose is to allow dynamic introspection of values, following Rust’s type system through a system of subtraits.

Its primary purpose is to allow all implementors to be passed around as a dyn PartialReflect trait object in one of the following forms:

  • &dyn PartialReflect
  • &mut dyn PartialReflect
  • Box<dyn PartialReflect>

This allows values of types implementing PartialReflect to be operated upon completely dynamically (at a small runtime cost).

Building on PartialReflect is the Reflect trait.

PartialReflect is a supertrait of Reflect so any type implementing Reflect implements PartialReflect by definition. dyn Reflect trait objects can be used similarly to dyn PartialReflect, but Reflect is also often used in trait bounds (like T: Reflect).

The distinction between PartialReflect and Reflect is summarized in the following:

  • PartialReflect is a trait for interacting with values under bevy_reflect’s data model. This means values implementing PartialReflect can be dynamically constructed and introspected.
  • The Reflect trait, however, ensures that the interface exposed by PartialReflect on types which additionally implement Reflect mirrors the structure of a single Rust type.
  • This means dyn Reflect trait objects can be directly downcasted to concrete types, where dyn PartialReflect trait object cannot.
  • Reflect, since it provides a stronger type-correctness guarantee, is the trait used to interact with the type registry.

§Converting between PartialReflect and Reflect

Since T: Reflect implies T: PartialReflect, conversion from a dyn Reflect to a dyn PartialReflect trait object (upcasting) is infallible and can be performed with one of the following methods. Note that these are temporary while the language feature for dyn upcasting coercion is experimental:

For conversion in the other direction — downcasting dyn PartialReflect to dyn Reflect — there are fallible methods:

Additionally, FromReflect::from_reflect can be used to convert a dyn PartialReflect to a concrete type which implements Reflect.

§Implementing Reflect

Implementing Reflect (and PartialReflect) is easily done using the provided derive macro:

#[derive(Reflect)]
struct MyStruct {
  foo: i32
}

This will automatically generate the implementation of Reflect for any struct or enum.

It will also generate other very important trait implementations used for reflection:

§Requirements

We can implement Reflect on any type that satisfies both of the following conditions:

  • The type implements Any, Send, and Sync. For the Any requirement to be satisfied, the type itself must have a 'static lifetime.
  • All fields and sub-elements themselves implement Reflect (see the derive macro documentation for details on how to ignore certain fields when deriving).

Additionally, using the derive macro on enums requires a third condition to be met:

  • All fields and sub-elements must implement FromReflect— another important reflection trait discussed in a later section.

§The Reflection Subtraits

Since PartialReflect is meant to cover any and every type, this crate also comes with a few more traits to accompany PartialReflect and provide more specific interactions. We refer to these traits as the reflection subtraits since they all have PartialReflect as a supertrait. The current list of reflection subtraits include:

As mentioned previously, the last three are automatically implemented by the derive macro.

Each of these traits come with their own methods specific to their respective category. For example, we can access our struct’s fields by name using the Struct::field method.

let my_struct: Box<dyn Struct> = Box::new(MyStruct {
  foo: 123
});
let foo: &dyn PartialReflect = my_struct.field("foo").unwrap();
assert_eq!(Some(&123), foo.try_downcast_ref::<i32>());

Since most data is passed around as dyn PartialReflect or dyn Reflect trait objects, the PartialReflect trait has methods for going to and from these subtraits.

PartialReflect::reflect_kind, PartialReflect::reflect_ref, PartialReflect::reflect_mut, and PartialReflect::reflect_owned all return an enum that respectively contains zero-sized, immutable, mutable, and owned access to the type as a subtrait object.

For example, we can get out a dyn Tuple from our reflected tuple type using one of these methods.

let my_tuple: Box<dyn PartialReflect> = Box::new((1, 2, 3));
let my_tuple = my_tuple.reflect_ref().as_tuple().unwrap();
assert_eq!(3, my_tuple.field_len());

And to go back to a general-purpose dyn PartialReflect, we can just use the matching PartialReflect::as_partial_reflect, PartialReflect::as_partial_reflect_mut, or PartialReflect::into_partial_reflect methods.

§Opaque Types

Some types don’t fall under a particular subtrait.

These types hide their internal structure to reflection, either because it is not possible, difficult, or not useful to reflect its internals. Such types are known as opaque types.

This includes truly opaque types like String or Instant, but also includes all the primitive types (e.g. bool, usize, etc.) since they can’t be broken down any further.

§Dynamic Types

Each subtrait comes with a corresponding dynamic type.

The available dynamic types are:

These dynamic types may contain any arbitrary reflected data.

let mut data = DynamicStruct::default();
data.insert("foo", 123_i32);
assert_eq!(Some(&123), data.field("foo").unwrap().try_downcast_ref::<i32>())

They are most commonly used as “proxies” for other types, where they contain the same data as— and therefore, represent— a concrete type. The PartialReflect::clone_value method will return a dynamic type for all non-opaque types, allowing all types to essentially be “cloned”. And since dynamic types themselves implement PartialReflect, we may pass them around just like most other reflected types.

let original: Box<dyn Reflect> = Box::new(MyStruct {
  foo: 123
});

// `cloned` will be a `DynamicStruct` representing a `MyStruct`
let cloned: Box<dyn PartialReflect> = original.clone_value();
assert!(cloned.represents::<MyStruct>());

§Patching

These dynamic types come in handy when needing to apply multiple changes to another type. This is known as “patching” and is done using the PartialReflect::apply and PartialReflect::try_apply methods.

let mut value = Some(123_i32);
let patch = DynamicEnum::new("None", ());
value.apply(&patch);
assert_eq!(None, value);

§FromReflect

It’s important to remember that dynamic types are not the concrete type they may be representing. A common mistake is to treat them like such when trying to cast back to the original type or when trying to make use of a reflected trait which expects the actual type.

ⓘ
let original: Box<dyn Reflect> = Box::new(MyStruct {
  foo: 123
});

let cloned: Box<dyn PartialReflect> = original.clone_value();
let value = cloned.try_take::<MyStruct>().unwrap(); // PANIC!

To resolve this issue, we’ll need to convert the dynamic type to the concrete one. This is where FromReflect comes in.

FromReflect is a trait that allows an instance of a type to be generated from a dynamic representation— even partial ones. And since the FromReflect::from_reflect method takes the data by reference, this can be used to effectively clone data (to an extent).

It is automatically implemented when deriving Reflect on a type unless opted out of using #[reflect(from_reflect = false)] on the item.

#[derive(Reflect)]
struct MyStruct {
  foo: i32
}
let original: Box<dyn Reflect> = Box::new(MyStruct {
  foo: 123
});

let cloned: Box<dyn PartialReflect> = original.clone_value();
let value = <MyStruct as FromReflect>::from_reflect(&*cloned).unwrap(); // OK!

When deriving, all active fields and sub-elements must also implement FromReflect.

Fields can be given default values for when a field is missing in the passed value or even ignored. Ignored fields must either implement Default or have a default function specified using #[reflect(default = "path::to::function")].

See the derive macro documentation for details.

All primitives and simple types implement FromReflect by relying on their Default implementation.

§Path navigation

The GetPath trait allows accessing arbitrary nested fields of an PartialReflect type.

Using GetPath, it is possible to use a path string to access a specific field of a reflected type.

#[derive(Reflect)]
struct MyStruct {
  value: Vec<Option<u32>>
}

let my_struct = MyStruct {
  value: vec![None, None, Some(123)],
};
assert_eq!(
  my_struct.path::<u32>(".value[2].0").unwrap(),
  &123,
);

§Type Registration

This crate also comes with a TypeRegistry that can be used to store and retrieve additional type metadata at runtime, such as helper types and trait implementations.

The derive macro for Reflect also generates an implementation of the GetTypeRegistration trait, which is used by the registry to generate a TypeRegistration struct for that type. We can then register additional type data we want associated with that type.

For example, we can register ReflectDefault on our type so that its Default implementation may be used dynamically.

#[derive(Reflect, Default)]
struct MyStruct {
  foo: i32
}
let mut registry = TypeRegistry::empty();
registry.register::<MyStruct>();
registry.register_type_data::<MyStruct, ReflectDefault>();

let registration = registry.get(core::any::TypeId::of::<MyStruct>()).unwrap();
let reflect_default = registration.data::<ReflectDefault>().unwrap();

let new_value: Box<dyn Reflect> = reflect_default.default();
assert!(new_value.is::<MyStruct>());

Because this operation is so common, the derive macro actually has a shorthand for it. By using the #[reflect(Trait)] attribute, the derive macro will automatically register a matching, in-scope ReflectTrait type within the GetTypeRegistration implementation.

use bevy_reflect::prelude::{Reflect, ReflectDefault};

#[derive(Reflect, Default)]
#[reflect(Default)]
struct MyStruct {
  foo: i32
}

§Reflecting Traits

Type data doesn’t have to be tied to a trait, but it’s often extremely useful to create trait type data. These allow traits to be used directly on a dyn Reflect (and not a dyn PartialReflect) while utilizing the underlying type’s implementation.

For any object-safe trait, we can easily generate a corresponding ReflectTrait type for our trait using the #[reflect_trait] macro.

#[reflect_trait] // Generates a `ReflectMyTrait` type
pub trait MyTrait {}
impl<T: Reflect> MyTrait for T {}

let mut registry = TypeRegistry::new();
registry.register_type_data::<i32, ReflectMyTrait>();

The generated type data can be used to convert a valid dyn Reflect into a dyn MyTrait. See the trait reflection example for more information and usage details.

§Serialization

By using reflection, we are also able to get serialization capabilities for free. In fact, using bevy_reflect can result in faster compile times and reduced code generation over directly deriving the serde traits.

The way it works is by moving the serialization logic into common serializers and deserializers:

All of these structs require a reference to the registry so that type information can be retrieved, as well as registered type data, such as ReflectSerialize and ReflectDeserialize.

The general entry point are the “untyped” versions of these structs. These will automatically extract the type information and pass them into their respective “typed” version.

The output of the ReflectSerializer will be a map, where the key is the type path and the value is the serialized data. The TypedReflectSerializer will simply output the serialized data.

The ReflectDeserializer can be used to deserialize this map and return a Box<dyn Reflect>, where the underlying type will be a dynamic type representing some concrete type (except for opaque types).

Again, it’s important to remember that dynamic types may need to be converted to their concrete counterparts in order to be used in certain cases. This can be achieved using FromReflect.

#[derive(Reflect, PartialEq, Debug)]
struct MyStruct {
  foo: i32
}

let original_value = MyStruct {
  foo: 123
};

// Register
let mut registry = TypeRegistry::new();
registry.register::<MyStruct>();

// Serialize
let reflect_serializer = ReflectSerializer::new(original_value.as_partial_reflect(), &registry);
let serialized_value: String = ron::to_string(&reflect_serializer).unwrap();

// Deserialize
let reflect_deserializer = ReflectDeserializer::new(&registry);
let deserialized_value: Box<dyn PartialReflect> = reflect_deserializer.deserialize(
  &mut ron::Deserializer::from_str(&serialized_value).unwrap()
).unwrap();

// Convert
let converted_value = <MyStruct as FromReflect>::from_reflect(&*deserialized_value).unwrap();

assert_eq!(original_value, converted_value);

§Limitations

While this crate offers a lot in terms of adding reflection to Rust, it does come with some limitations that don’t make it as featureful as reflection in other programming languages.

§Non-Static Lifetimes

One of the most obvious limitations is the 'static requirement. Rust requires fields to define a lifetime for referenced data, but Reflect requires all types to have a 'static lifetime. This makes it impossible to reflect any type with non-static borrowed data.

§Generic Function Reflection

Another limitation is the inability to reflect over generic functions directly. It can be done, but will typically require manual monomorphization (i.e. manually specifying the types the generic method can take).

§Manual Registration

Since Rust doesn’t provide built-in support for running initialization code before main, there is no way for bevy_reflect to automatically register types into the type registry. This means types must manually be registered, including their desired monomorphized representations if generic.

§Features

§bevy

DefaultDependencies
❌bevy_math, glam, smallvec

This feature makes it so that the appropriate reflection traits are implemented on all the types necessary for the Bevy game engine. enables the optional dependencies: bevy_math, glam, and smallvec. These dependencies are used by the Bevy game engine and must define their reflection implementations within this crate due to Rust’s orphan rule.

§functions

DefaultDependencies
❌bevy_reflect_derive/functions

This feature allows creating a DynamicFunction or DynamicFunctionMut from Rust functions. Dynamic functions can then be called with valid ArgLists.

For more information, read the func module docs.

§documentation

DefaultDependencies
❌bevy_reflect_derive/documentation

This feature enables capturing doc comments as strings for items that derive Reflect. Documentation information can then be accessed at runtime on the TypeInfo of that item.

This can be useful for generating documentation for scripting language interop or for displaying tooltips in an editor.

§debug

DefaultDependencies
✅debug_stack

This feature enables useful debug features for reflection.

This includes the debug_stack feature, which enables capturing the type stack when serializing or deserializing a type and displaying it in error messages.

Re-exports§

Modules§

Macros§

Structs§

Enums§

Traits§

Functions§

Attribute Macros§

  • reflect_remote
    Generates a wrapper type that can be used to “derive Reflect” for remote types.
  • reflect_trait
    A macro that automatically generates type data for traits, which their implementors can then register.

Derive Macros§

  • FromReflect
    Derives the FromReflect trait.
  • Reflect
    The main derive macro used by bevy_reflect for deriving its Reflect trait.
  • TypePath
    Derives the TypePath trait, providing a stable alternative to [std::any::type_name].