bevy_scene: Add ReflectBundle (bevyengine#9165)

# Objective

Similar to bevyengine#6344, but contains only `ReflectBundle` changes. Useful for
scripting. The implementation has also been updated to look exactly like
`ReflectComponent`.

---

## Changelog

### Added

- Reflection for bundles.

---------

Co-authored-by: Gino Valente <49806985+MrGVSV@users.noreply.github.com>

crates/bevy_ecs/src/reflect/bundle.rs

@@ -0,0 +1,218 @@
+//! Definitions for [`Bundle`] reflection.
+//!
+//! This module exports two types: [`ReflectBundleFns`] and [`ReflectBundle`].
+//!
+//! Same as [`super::component`], but for bundles.
+use std::any::TypeId;
+
+use crate::{
+    prelude::Bundle,
+    world::{EntityMut, FromWorld, World},
+};
+use bevy_reflect::{FromType, Reflect, ReflectRef, TypeRegistry};
+
+use super::ReflectComponent;
+
+/// A struct used to operate on reflected [`Bundle`] of a type.
+///
+/// A [`ReflectBundle`] for type `T` can be obtained via
+/// [`bevy_reflect::TypeRegistration::data`].
+#[derive(Clone)]
+pub struct ReflectBundle(ReflectBundleFns);
+
+/// The raw function pointers needed to make up a [`ReflectBundle`].
+///
+/// The also [`super::component::ReflectComponentFns`].
+#[derive(Clone)]
+pub struct ReflectBundleFns {
+    /// Function pointer implementing [`ReflectBundle::from_world()`].
+    pub from_world: fn(&mut World) -> Box<dyn Reflect>,
+    /// Function pointer implementing [`ReflectBundle::insert()`].
+    pub insert: fn(&mut EntityMut, &dyn Reflect),
+    /// Function pointer implementing [`ReflectBundle::apply()`].
+    pub apply: fn(&mut EntityMut, &dyn Reflect, &TypeRegistry),
+    /// Function pointer implementing [`ReflectBundle::apply_or_insert()`].
+    pub apply_or_insert: fn(&mut EntityMut, &dyn Reflect, &TypeRegistry),
+    /// Function pointer implementing [`ReflectBundle::remove()`].
+    pub remove: fn(&mut EntityMut),
+}
+
+impl ReflectBundleFns {
+    /// Get the default set of [`ReflectBundleFns`] for a specific bundle type using its
+    /// [`FromType`] implementation.
+    ///
+    /// This is useful if you want to start with the default implementation before overriding some
+    /// of the functions to create a custom implementation.
+    pub fn new<T: Bundle + Reflect + FromWorld>() -> Self {
+        <ReflectBundle as FromType<T>>::from_type().0
+    }
+}
+
+impl ReflectBundle {
+    /// Constructs default reflected [`Bundle`] from world using [`from_world()`](FromWorld::from_world).
+    pub fn from_world(&self, world: &mut World) -> Box<dyn Reflect> {
+        (self.0.from_world)(world)
+    }
+
+    /// Insert a reflected [`Bundle`] into the entity like [`insert()`](crate::world::EntityMut::insert).
+    pub fn insert(&self, entity: &mut EntityMut, bundle: &dyn Reflect) {
+        (self.0.insert)(entity, bundle);
+    }
+
+    /// Uses reflection to set the value of this [`Bundle`] type in the entity to the given value.
+    ///
+    /// # Panics
+    ///
+    /// Panics if there is no [`Bundle`] of the given type.
+    pub fn apply(&self, entity: &mut EntityMut, bundle: &dyn Reflect, registry: &TypeRegistry) {
+        (self.0.apply)(entity, bundle, registry);
+    }
+
+    /// Uses reflection to set the value of this [`Bundle`] type in the entity to the given value or insert a new one if it does not exist.
+    pub fn apply_or_insert(
+        &self,
+        entity: &mut EntityMut,
+        bundle: &dyn Reflect,
+        registry: &TypeRegistry,
+    ) {
+        (self.0.apply_or_insert)(entity, bundle, registry);
+    }
+
+    /// Removes this [`Bundle`] type from the entity. Does nothing if it doesn't exist.
+    pub fn remove(&self, entity: &mut EntityMut) {
+        (self.0.remove)(entity);
+    }
+
+    /// Create a custom implementation of [`ReflectBundle`].
+    ///
+    /// This is an advanced feature,
+    /// useful for scripting implementations,
+    /// that should not be used by most users
+    /// unless you know what you are doing.
+    ///
+    /// Usually you should derive [`Reflect`] and add the `#[reflect(Bundle)]` bundle
+    /// to generate a [`ReflectBundle`] implementation automatically.
+    ///
+    /// See [`ReflectBundleFns`] for more information.
+    pub fn new(fns: ReflectBundleFns) -> Self {
+        Self(fns)
+    }
+
+    /// The underlying function pointers implementing methods on `ReflectBundle`.
+    ///
+    /// This is useful when you want to keep track locally of an individual
+    /// function pointer.
+    ///
+    /// Calling [`TypeRegistry::get`] followed by
+    /// [`TypeRegistration::data::<ReflectBundle>`] can be costly if done several
+    /// times per frame. Consider cloning [`ReflectBundle`] and keeping it
+    /// between frames, cloning a `ReflectBundle` is very cheap.
+    ///
+    /// If you only need a subset of the methods on `ReflectBundle`,
+    /// use `fn_pointers` to get the underlying [`ReflectBundleFns`]
+    /// and copy the subset of function pointers you care about.
+    ///
+    /// [`TypeRegistration::data::<ReflectBundle>`]: bevy_reflect::TypeRegistration::data
+    /// [`TypeRegistry::get`]: bevy_reflect::TypeRegistry::get
+    pub fn fn_pointers(&self) -> &ReflectBundleFns {
+        &self.0
+    }
+}
+
+impl<B: Bundle + Reflect + FromWorld> FromType<B> for ReflectBundle {
+    fn from_type() -> Self {
+        ReflectBundle(ReflectBundleFns {
+            from_world: |world| Box::new(B::from_world(world)),
+            insert: |entity, reflected_bundle| {
+                let mut bundle = entity.world_scope(|world| B::from_world(world));
+                bundle.apply(reflected_bundle);
+                entity.insert(bundle);
+            },
+            apply: |entity, reflected_bundle, registry| {
+                let mut bundle = entity.world_scope(|world| B::from_world(world));
+                bundle.apply(reflected_bundle);
+
+                match bundle.reflect_ref() {
+                    ReflectRef::Struct(bundle) => bundle
+                        .iter_fields()
+                        .for_each(|field| insert_field::<B>(entity, field, registry)),
+                    ReflectRef::Tuple(bundle) => bundle
+                        .iter_fields()
+                        .for_each(|field| insert_field::<B>(entity, field, registry)),
+                    _ => panic!(
+                        "expected bundle `{}` to be named struct or tuple",
+                        std::any::type_name::<B>()
+                    ),
+                }
+            },
+            apply_or_insert: |entity, reflected_bundle, registry| {
+                let mut bundle = entity.world_scope(|world| B::from_world(world));
+                bundle.apply(reflected_bundle);
+
+                match bundle.reflect_ref() {
+                    ReflectRef::Struct(bundle) => bundle
+                        .iter_fields()
+                        .for_each(|field| apply_or_insert_field::<B>(entity, field, registry)),
+                    ReflectRef::Tuple(bundle) => bundle
+                        .iter_fields()
+                        .for_each(|field| apply_or_insert_field::<B>(entity, field, registry)),
+                    _ => panic!(
+                        "expected bundle `{}` to be named struct or tuple",
+                        std::any::type_name::<B>()
+                    ),
+                }
+            },
+            remove: |entity| {
+                entity.remove::<B>();
+            },
+        })
+    }
+}
+
+fn insert_field<B: 'static>(entity: &mut EntityMut, field: &dyn Reflect, registry: &TypeRegistry) {
+    if let Some(reflect_component) = registry.get_type_data::<ReflectComponent>(field.type_id()) {
+        reflect_component.apply(entity, field);
+    } else if let Some(reflect_bundle) = registry.get_type_data::<ReflectBundle>(field.type_id()) {
+        reflect_bundle.apply(entity, field, registry);
+    } else {
+        entity.world_scope(|world| {
+            if world.components().get_id(TypeId::of::<B>()).is_some() {
+                panic!(
+                    "no `ReflectComponent` registration found for `{}`",
+                    field.type_name()
+                );
+            };
+        });
+
+        panic!(
+            "no `ReflectBundle` registration found for `{}`",
+            field.type_name()
+        )
+    }
+}
+
+fn apply_or_insert_field<B: 'static>(
+    entity: &mut EntityMut,
+    field: &dyn Reflect,
+    registry: &TypeRegistry,
+) {
+    if let Some(reflect_component) = registry.get_type_data::<ReflectComponent>(field.type_id()) {
+        reflect_component.apply_or_insert(entity, field);
+    } else if let Some(reflect_bundle) = registry.get_type_data::<ReflectBundle>(field.type_id()) {
+        reflect_bundle.apply_or_insert(entity, field, registry);
+    } else {
+        entity.world_scope(|world| {
+            if world.components().get_id(TypeId::of::<B>()).is_some() {
+                panic!(
+                    "no `ReflectComponent` registration found for `{}`",
+                    field.type_name()
+                );
+            };
+        });
+
+        panic!(
+            "no `ReflectBundle` registration found for `{}`",
+            field.type_name()
+        )
+    }
+}

crates/bevy_ecs/src/reflect/mod.rs

@@ -6,10 +6,12 @@ use crate as bevy_ecs;
 use crate::{entity::Entity, system::Resource};
 use bevy_reflect::{impl_reflect_value, ReflectDeserialize, ReflectSerialize, TypeRegistryArc};
 
+mod bundle;
 mod component;
 mod map_entities;
 mod resource;
 
+pub use bundle::{ReflectBundle, ReflectBundleFns};
 pub use component::{ReflectComponent, ReflectComponentFns};
 pub use map_entities::ReflectMapEntities;
 pub use resource::{ReflectResource, ReflectResourceFns};