Merge 9acf43fa51 into 877d278785

benches/benches/bevy_ecs/world/world_get.rs

@@ -51,9 +51,10 @@ pub fn world_entity(criterion: &mut Criterion) {
     for entity_count in RANGE.map(|i| i * 10_000) {
         group.bench_function(format!("{entity_count}_entities"), |bencher| {
             let world = setup::<Table>(entity_count);
+            let offset = world.resource_count();
 
             bencher.iter(|| {
-                for i in 0..entity_count {
+                for i in offset..(entity_count + offset) {
                     let entity =
                         // SAFETY: Range is exclusive.
                         Entity::from_raw(EntityRow::new(unsafe { NonMaxU32::new_unchecked(i) }));
@@ -74,9 +75,10 @@ pub fn world_get(criterion: &mut Criterion) {
     for entity_count in RANGE.map(|i| i * 10_000) {
         group.bench_function(format!("{entity_count}_entities_table"), |bencher| {
             let world = setup::<Table>(entity_count);
+            let offset = world.resource_count();
 
             bencher.iter(|| {
-                for i in 0..entity_count {
+                for i in offset..(entity_count + offset) {
                     let entity =
                         // SAFETY: Range is exclusive.
                         Entity::from_raw(EntityRow::new(unsafe { NonMaxU32::new_unchecked(i) }));
@@ -86,9 +88,10 @@ pub fn world_get(criterion: &mut Criterion) {
         });
         group.bench_function(format!("{entity_count}_entities_sparse"), |bencher| {
             let world = setup::<Sparse>(entity_count);
+            let offset = world.resource_count();
 
             bencher.iter(|| {
-                for i in 0..entity_count {
+                for i in offset..(entity_count + offset) {
                     let entity =
                         // SAFETY: Range is exclusive.
                         Entity::from_raw(EntityRow::new(unsafe { NonMaxU32::new_unchecked(i) }));
@@ -109,10 +112,11 @@ pub fn world_query_get(criterion: &mut Criterion) {
     for entity_count in RANGE.map(|i| i * 10_000) {
         group.bench_function(format!("{entity_count}_entities_table"), |bencher| {
             let mut world = setup::<Table>(entity_count);
+            let offset = world.resource_count();
             let mut query = world.query::<&Table>();
 
             bencher.iter(|| {
-                for i in 0..entity_count {
+                for i in offset..(entity_count + offset) {
                     let entity =
                         // SAFETY: Range is exclusive.
                         Entity::from_raw(EntityRow::new(unsafe { NonMaxU32::new_unchecked(i) }));
@@ -137,9 +141,10 @@ pub fn world_query_get(criterion: &mut Criterion) {
                 &WideTable<4>,
                 &WideTable<5>,
             )>();
+            let offset = world.resource_count();
 
             bencher.iter(|| {
-                for i in 0..entity_count {
+                for i in offset..(entity_count + offset) {
                     let entity =
                         // SAFETY: Range is exclusive.
                         Entity::from_raw(EntityRow::new(unsafe { NonMaxU32::new_unchecked(i) }));
@@ -149,10 +154,11 @@ pub fn world_query_get(criterion: &mut Criterion) {
         });
         group.bench_function(format!("{entity_count}_entities_sparse"), |bencher| {
             let mut world = setup::<Sparse>(entity_count);
+            let offset = world.resource_count();
             let mut query = world.query::<&Sparse>();
 
             bencher.iter(|| {
-                for i in 0..entity_count {
+                for i in offset..(entity_count + offset) {
                     let entity =
                         // SAFETY: Range is exclusive.
                         Entity::from_raw(EntityRow::new(unsafe { NonMaxU32::new_unchecked(i) }));
@@ -177,9 +183,10 @@ pub fn world_query_get(criterion: &mut Criterion) {
                 &WideSparse<4>,
                 &WideSparse<5>,
             )>();
+            let offset = world.resource_count();
 
             bencher.iter(|| {
-                for i in 0..entity_count {
+                for i in offset..(entity_count + offset) {
                     // SAFETY: Range is exclusive.
                     let entity =
                         Entity::from_raw(EntityRow::new(unsafe { NonMaxU32::new_unchecked(i) }));

crates/bevy_ecs/src/component/info.rs

@@ -1,5 +1,5 @@
 use alloc::{borrow::Cow, vec::Vec};
-use bevy_platform::{collections::HashSet, sync::PoisonError};
+use bevy_platform::{collections::HashMap, collections::HashSet, sync::PoisonError};
 use bevy_ptr::OwningPtr;
 #[cfg(feature = "bevy_reflect")]
 use bevy_reflect::Reflect;
@@ -18,6 +18,7 @@ use crate::{
         RequiredComponents, StorageType,
     },
     lifecycle::ComponentHooks,
+    prelude::Entity,
     query::DebugCheckedUnwrap as _,
     resource::Resource,
     storage::SparseSetIndex,
@@ -346,6 +347,9 @@ pub struct Components {
     pub(super) components: Vec<Option<ComponentInfo>>,
     pub(super) indices: TypeIdMap<ComponentId>,
     pub(super) resource_indices: TypeIdMap<ComponentId>,
+    /// A lookup for the entities on which resources are stored.
+    /// It uses `ComponentId`s instead of `TypeId`s for untyped APIs
+    pub(crate) resource_entities: HashMap<ComponentId, Entity>,
     // This is kept internal and local to verify that no deadlocks can occor.
     pub(super) queued: bevy_platform::sync::RwLock<QueuedComponents>,
 }

crates/bevy_ecs/src/entity_disabling.rs

@@ -70,6 +70,7 @@
 use crate::{
     component::{ComponentId, Components, StorageType},
     query::FilteredAccess,
+    resource::IsResource,
     world::{FromWorld, World},
 };
 use bevy_ecs_macros::{Component, Resource};
@@ -143,6 +144,8 @@ impl FromWorld for DefaultQueryFilters {
         let mut filters = DefaultQueryFilters::empty();
         let disabled_component_id = world.register_component::<Disabled>();
         filters.register_disabling_component(disabled_component_id);
+        let is_resource_component_id = world.register_component::<IsResource>();
+        filters.register_disabling_component(is_resource_component_id);
         filters
     }
 }

crates/bevy_ecs/src/lib.rs

@@ -378,9 +378,9 @@ mod tests {
         let mut world = World::new();
         let e = world.spawn((TableStored("abc"), A(123))).id();
         let f = world.spawn((TableStored("def"), A(456))).id();
-        assert_eq!(world.entities.len(), 2);
+        assert_eq!(world.entity_count(), 2);
         assert!(world.despawn(e));
-        assert_eq!(world.entities.len(), 1);
+        assert_eq!(world.entity_count(), 1);
         assert!(world.get::<TableStored>(e).is_none());
         assert!(world.get::<A>(e).is_none());
         assert_eq!(world.get::<TableStored>(f).unwrap().0, "def");
@@ -393,9 +393,9 @@ mod tests {
 
         let e = world.spawn((TableStored("abc"), SparseStored(123))).id();
         let f = world.spawn((TableStored("def"), SparseStored(456))).id();
-        assert_eq!(world.entities.len(), 2);
+        assert_eq!(world.entity_count(), 2);
         assert!(world.despawn(e));
-        assert_eq!(world.entities.len(), 1);
+        assert_eq!(world.entity_count(), 1);
         assert!(world.get::<TableStored>(e).is_none());
         assert!(world.get::<SparseStored>(e).is_none());
         assert_eq!(world.get::<TableStored>(f).unwrap().0, "def");
@@ -1786,7 +1786,7 @@ mod tests {
     fn try_insert_batch() {
         let mut world = World::default();
         let e0 = world.spawn(A(0)).id();
-        let e1 = Entity::from_raw_u32(1).unwrap();
+        let e1 = Entity::from_raw_u32(2).unwrap();
 
         let values = vec![(e0, (A(1), B(0))), (e1, (A(0), B(1)))];
 
@@ -1810,7 +1810,7 @@ mod tests {
     fn try_insert_batch_if_new() {
         let mut world = World::default();
         let e0 = world.spawn(A(0)).id();
-        let e1 = Entity::from_raw_u32(1).unwrap();
+        let e1 = Entity::from_raw_u32(2).unwrap();
 
         let values = vec![(e0, (A(1), B(0))), (e1, (A(0), B(1)))];
 

crates/bevy_ecs/src/name.rs

@@ -278,7 +278,7 @@ mod tests {
         let mut query = world.query::<NameOrEntity>();
         let d1 = query.get(&world, e1).unwrap();
         // NameOrEntity Display for entities without a Name should be {index}v{generation}
-        assert_eq!(d1.to_string(), "0v0");
+        assert_eq!(d1.to_string(), "1v0");
         let d2 = query.get(&world, e2).unwrap();
         // NameOrEntity Display for entities with a Name should be the Name
         assert_eq!(d2.to_string(), "MyName");

crates/bevy_ecs/src/query/builder.rs

@@ -275,6 +275,7 @@ impl<'w, D: QueryData, F: QueryFilter> QueryBuilder<'w, D, F> {
 mod tests {
     use crate::{
         prelude::*,
+        resource::IsResource,
         world::{EntityMutExcept, EntityRefExcept, FilteredEntityMut, FilteredEntityRef},
     };
     use std::dbg;
@@ -332,6 +333,7 @@ mod tests {
     #[test]
     fn builder_or() {
         let mut world = World::new();
+
         world.spawn((A(0), B(0)));
         world.spawn(B(0));
         world.spawn(C(0));
@@ -485,6 +487,7 @@ mod tests {
 
         let mut query = QueryBuilder::<(FilteredEntityMut, EntityMutExcept<A>)>::new(&mut world)
             .data::<EntityMut>()
+            .filter::<Without<IsResource>>()
             .build();
 
         // Removing `EntityMutExcept<A>` just leaves A
@@ -496,6 +499,7 @@ mod tests {
 
         let mut query = QueryBuilder::<(FilteredEntityMut, EntityRefExcept<A>)>::new(&mut world)
             .data::<EntityMut>()
+            .filter::<Without<IsResource>>()
             .build();
 
         // Removing `EntityRefExcept<A>` just leaves A, plus read access

crates/bevy_ecs/src/query/iter.rs

@@ -2669,6 +2669,7 @@ mod tests {
     #[test]
     fn query_iter_sorts() {
         let mut world = World::new();
+
         for i in 0..100 {
             world.spawn(A(i as f32));
             world.spawn((A(i as f32), Sparse(i)));

crates/bevy_ecs/src/query/state.rs

@@ -1850,6 +1850,7 @@ mod tests {
     #[test]
     fn can_transmute_empty_tuple() {
         let mut world = World::new();
+
         world.register_component::<A>();
         let entity = world.spawn(A(10)).id();
 
@@ -2172,9 +2173,7 @@ mod tests {
         world.spawn((B(0), C(0)));
         world.spawn(C(0));
 
-        let mut df = DefaultQueryFilters::empty();
+        world.register_disabling_component::<C>();
-        df.register_disabling_component(world.register_component::<C>());
-        world.insert_resource(df);
 
         // Without<C> only matches the first entity
         let mut query = QueryState::<()>::new(&mut world);
@@ -2216,9 +2215,9 @@ mod tests {
         assert!(query.is_dense);
         assert_eq!(3, query.iter(&world).count());
 
-        let mut df = DefaultQueryFilters::empty();
+        let df = DefaultQueryFilters::from_world(&mut world);
-        df.register_disabling_component(world.register_component::<Sparse>());
         world.insert_resource(df);
+        world.register_disabling_component::<Sparse>();
 
         let mut query = QueryState::<()>::new(&mut world);
         // The query doesn't ask for sparse components, but the default filters adds
@@ -2226,9 +2225,9 @@ mod tests {
         assert!(!query.is_dense);
         assert_eq!(1, query.iter(&world).count());
 
-        let mut df = DefaultQueryFilters::empty();
+        let df = DefaultQueryFilters::from_world(&mut world);
-        df.register_disabling_component(world.register_component::<Table>());
         world.insert_resource(df);
+        world.register_disabling_component::<Table>();
 
         let mut query = QueryState::<()>::new(&mut world);
         // If the filter is instead a table components, the query can still be dense

crates/bevy_ecs/src/resource.rs

@@ -1,5 +1,10 @@
 //! Resources are unique, singleton-like data types that can be accessed from systems and stored in the [`World`](crate::world::World).
 
+use crate::prelude::Component;
+use crate::prelude::ReflectComponent;
+use bevy_reflect::prelude::ReflectDefault;
+use bevy_reflect::Reflect;
+use core::marker::PhantomData;
 // The derive macro for the `Resource` trait
 pub use bevy_ecs_macros::Resource;
 
@@ -73,3 +78,89 @@ pub use bevy_ecs_macros::Resource;
     note = "consider annotating `{Self}` with `#[derive(Resource)]`"
 )]
 pub trait Resource: Send + Sync + 'static {}
+
+/// A marker component for the entity that stores the resource of type `T`.
+///
+/// This component is automatically inserted when a resource of type `T` is inserted into the world,
+/// and can be used to find the entity that stores a particular resource.
+///
+/// By contrast, the [`IsResource`] component is used to find all entities that store resources,
+/// regardless of the type of resource they store.
+///
+/// This component comes with a hook that ensures that at most one entity has this component for any given `R`:
+/// adding this component to an entity (or spawning an entity with this component) will despawn any other entity with this component.
+#[derive(Component, Debug)]
+#[require(IsResource)]
+#[cfg_attr(feature = "bevy_reflect", derive(Reflect), reflect(Component, Default))]
+pub struct ResourceEntity<R: Resource>(#[reflect(ignore)] PhantomData<R>);
+
+impl<R: Resource> Default for ResourceEntity<R> {
+    fn default() -> Self {
+        ResourceEntity(PhantomData)
+    }
+}
+
+/// A marker component for entities which store resources.
+///
+/// By contrast, the [`ResourceEntity<R>`] component is used to find the entity that stores a particular resource.
+/// This component is required by the [`ResourceEntity<R>`] component, and will automatically be added.
+#[cfg_attr(
+    feature = "bevy_reflect",
+    derive(Reflect),
+    reflect(Component, Default, Debug)
+)]
+#[derive(Component, Default, Debug)]
+pub struct IsResource;
+
+/// Used in conjunction with [`ResourceEntity<R>`], when no type information is available.
+/// This is used by [`World::insert_resource_by_id`](crate::world::World).
+#[derive(Resource)]
+pub(crate) struct TypeErasedResource;
+
+#[cfg(test)]
+mod tests {
+    use crate::change_detection::MaybeLocation;
+    use crate::ptr::OwningPtr;
+    use crate::resource::Resource;
+    use crate::world::World;
+    use bevy_platform::prelude::String;
+
+    #[test]
+    fn unique_resource_entities() {
+        #[derive(Default, Resource)]
+        struct TestResource1;
+
+        #[derive(Resource)]
+        #[expect(dead_code, reason = "field needed for testing")]
+        struct TestResource2(String);
+
+        #[derive(Resource)]
+        #[expect(dead_code, reason = "field needed for testing")]
+        struct TestResource3(u8);
+
+        let mut world = World::new();
+        let start = world.entities().len();
+        world.init_resource::<TestResource1>();
+        assert_eq!(world.entities().len(), start + 1);
+        world.insert_resource(TestResource2(String::from("Foo")));
+        assert_eq!(world.entities().len(), start + 2);
+        // like component registration, which just makes it known to the world that a component exists,
+        // registering a resource should not spawn an entity.
+        let id = world.register_resource::<TestResource3>();
+        assert_eq!(world.entities().len(), start + 2);
+        OwningPtr::make(20_u8, |ptr| {
+            // SAFETY: id was just initialized and corresponds to a resource.
+            unsafe {
+                world.insert_resource_by_id(id, ptr, MaybeLocation::caller());
+            }
+        });
+        assert_eq!(world.entities().len(), start + 3);
+        assert!(world.remove_resource_by_id(id).is_some());
+        assert_eq!(world.entities().len(), start + 2);
+        world.remove_resource::<TestResource1>();
+        assert_eq!(world.entities().len(), start + 1);
+        // make sure that trying to add a resource twice results, doesn't change the entity count
+        world.insert_resource(TestResource2(String::from("Bar")));
+        assert_eq!(world.entities().len(), start + 1);
+    }
+}

crates/bevy_ecs/src/system/system.rs

@@ -501,9 +501,9 @@ mod tests {
     #[test]
     fn command_processing() {
         let mut world = World::new();
-        assert_eq!(world.entities.len(), 0);
+        assert_eq!(world.entity_count(), 0);
         world.run_system_once(spawn_entity).unwrap();
-        assert_eq!(world.entities.len(), 1);
+        assert_eq!(world.entity_count(), 1);
     }
 
     #[test]

crates/bevy_ecs/src/world/entity_ref.rs

@@ -4926,6 +4926,7 @@ mod tests {
         change_detection::{MaybeLocation, MutUntyped},
         component::ComponentId,
         prelude::*,
+        resource::IsResource,
         system::{assert_is_system, RunSystemOnce as _},
         world::{error::EntityComponentError, DeferredWorld, FilteredEntityMut, FilteredEntityRef},
     };
@@ -5374,7 +5375,7 @@ mod tests {
 
         world.spawn(TestComponent(0)).insert(TestComponent2(0));
 
-        let mut query = world.query::<EntityRefExcept<TestComponent>>();
+        let mut query = world.query::<EntityRefExcept<(TestComponent, IsResource)>>();
 
         let mut found = false;
         for entity_ref in query.iter_mut(&mut world) {
@@ -5432,7 +5433,10 @@ mod tests {
 
         world.run_system_once(system).unwrap();
 
-        fn system(_: Query<&mut TestComponent>, query: Query<EntityRefExcept<TestComponent>>) {
+        fn system(
+            _: Query<&mut TestComponent>,
+            query: Query<EntityRefExcept<(TestComponent, IsResource)>>,
+        ) {
             for entity_ref in query.iter() {
                 assert!(matches!(
                     entity_ref.get::<TestComponent2>(),
@@ -5449,7 +5453,7 @@ mod tests {
         let mut world = World::new();
         world.spawn(TestComponent(0)).insert(TestComponent2(0));
 
-        let mut query = world.query::<EntityMutExcept<TestComponent>>();
+        let mut query = world.query::<EntityMutExcept<(TestComponent, IsResource)>>();
 
         let mut found = false;
         for mut entity_mut in query.iter_mut(&mut world) {
@@ -5514,7 +5518,10 @@ mod tests {
 
         world.run_system_once(system).unwrap();
 
-        fn system(_: Query<&mut TestComponent>, mut query: Query<EntityMutExcept<TestComponent>>) {
+        fn system(
+            _: Query<&mut TestComponent>,
+            mut query: Query<EntityMutExcept<(TestComponent, IsResource)>>,
+        ) {
             for mut entity_mut in query.iter_mut() {
                 assert!(entity_mut
                     .get_mut::<TestComponent2>()

crates/bevy_ecs/src/world/mod.rs

@@ -22,6 +22,7 @@ use crate::{
     event::BufferedEvent,
     lifecycle::{ComponentHooks, ADD, DESPAWN, INSERT, REMOVE, REPLACE},
     prelude::{Add, Despawn, Insert, Remove, Replace},
+    resource::{ResourceEntity, TypeErasedResource},
 };
 pub use bevy_ecs_macros::FromWorld;
 use bevy_utils::prelude::DebugName;
@@ -216,12 +217,20 @@ impl World {
         &mut self.entities
     }
 
-    /// Retrieves the number of [`Entities`] in the world.
+    /// Retrieves the number of [`Entities`] in the world. This count does not include resource entities.
     ///
     /// This is helpful as a diagnostic, but it can also be used effectively in tests.
     #[inline]
     pub fn entity_count(&self) -> u32 {
-        self.entities.len()
+        self.entities
+            .len()
+            .saturating_sub(self.components.resource_entities.len() as u32)
+    }
+
+    /// Retrieves the number of [`Resource`]s in the world.
+    #[inline]
+    pub fn resource_count(&self) -> u32 {
+        self.components.resource_entities.len() as u32
     }
 
     /// Retrieves this world's [`Archetypes`] collection.
@@ -1701,6 +1710,18 @@ impl World {
     pub fn init_resource<R: Resource + FromWorld>(&mut self) -> ComponentId {
         let caller = MaybeLocation::caller();
         let component_id = self.components_registrator().register_resource::<R>();
+
+        if !self
+            .components
+            .resource_entities
+            .contains_key(&component_id)
+        {
+            let entity = self.spawn(ResourceEntity::<R>::default()).id();
+            self.components
+                .resource_entities
+                .insert(component_id, entity);
+        }
+
         if self
             .storages
             .resources
@@ -1738,6 +1759,17 @@ impl World {
         caller: MaybeLocation,
     ) {
         let component_id = self.components_registrator().register_resource::<R>();
+        if !self
+            .components
+            .resource_entities
+            .contains_key(&component_id)
+        {
+            let entity = self.spawn(ResourceEntity::<R>::default()).id();
+            self.components
+                .resource_entities
+                .insert(component_id, entity);
+        }
+
         OwningPtr::make(value, |ptr| {
             // SAFETY: component_id was just initialized and corresponds to resource of type R.
             unsafe {
@@ -1805,6 +1837,10 @@ impl World {
     #[inline]
     pub fn remove_resource<R: Resource>(&mut self) -> Option<R> {
         let component_id = self.components.get_valid_resource_id(TypeId::of::<R>())?;
+        if let Some(entity) = self.components.resource_entities.remove(&component_id) {
+            self.despawn(entity);
+        }
+
         let (ptr, _, _) = self.storages.resources.get_mut(component_id)?.remove()?;
         // SAFETY: `component_id` was gotten via looking up the `R` type
         unsafe { Some(ptr.read::<R>()) }
@@ -2711,6 +2747,20 @@ impl World {
     ) {
         let change_tick = self.change_tick();
 
+        if !self
+            .components
+            .resource_entities
+            .contains_key(&component_id)
+        {
+            // Since we don't know the type, we use a placeholder type.
+            let entity = self
+                .spawn(ResourceEntity::<TypeErasedResource>::default())
+                .id();
+            self.components
+                .resource_entities
+                .insert(component_id, entity);
+        }
+
         let resource = self.initialize_resource_internal(component_id);
         // SAFETY: `value` is valid for `component_id`, ensured by caller
         unsafe {
@@ -3403,6 +3453,10 @@ impl World {
     /// **You should prefer to use the typed API [`World::remove_resource`] where possible and only
     /// use this in cases where the actual types are not known at compile time.**
     pub fn remove_resource_by_id(&mut self, component_id: ComponentId) -> Option<()> {
+        if let Some(entity) = self.components.resource_entities.remove(&component_id) {
+            self.despawn(entity);
+        }
+
         self.storages
             .resources
             .get_mut(component_id)?
@@ -3676,7 +3730,7 @@ mod tests {
         entity::EntityHashSet,
         entity_disabling::{DefaultQueryFilters, Disabled},
         ptr::OwningPtr,
-        resource::Resource,
+        resource::{IsResource, Resource},
         world::{error::EntityMutableFetchError, DeferredWorld},
     };
     use alloc::{
@@ -4108,9 +4162,11 @@ mod tests {
         let iterate_and_count_entities = |world: &World, entity_counters: &mut HashMap<_, _>| {
             entity_counters.clear();
             for entity in world.iter_entities() {
+                if !entity.contains::<IsResource>() {
                     let counter = entity_counters.entry(entity.id()).or_insert(0);
                     *counter += 1;
                 }
+            }
         };
 
         // Adding one entity and validating iteration
@@ -4206,9 +4262,9 @@ mod tests {
 
         let mut entities = world.iter_entities_mut().collect::<Vec<_>>();
         entities.sort_by_key(|e| e.get::<A>().map(|a| a.0).or(e.get::<B>().map(|b| b.0)));
-        let (a, b) = entities.split_at_mut(2);
+        let (a, b) = entities.split_at_mut(3);
         core::mem::swap(
-            &mut a[1].get_mut::<A>().unwrap().0,
+            &mut a[2].get_mut::<A>().unwrap().0,
             &mut b[0].get_mut::<B>().unwrap().0,
         );
         assert_eq!(world.entity(a1).get(), Some(&A(0)));

crates/bevy_scene/src/lib.rs

@@ -49,7 +49,13 @@ pub mod prelude {
 use bevy_app::prelude::*;
 
 #[cfg(feature = "serialize")]
-use {bevy_asset::AssetApp, bevy_ecs::schedule::IntoScheduleConfigs};
+use {
+    bevy_asset::AssetApp,
+    bevy_ecs::schedule::IntoScheduleConfigs,
+    bevy_ecs::{
+        entity_disabling::DefaultQueryFilters, resource::IsResource, resource::ResourceEntity,
+    },
+};
 
 /// Plugin that provides scene functionality to an [`App`].
 #[derive(Default)]
@@ -64,6 +70,8 @@ impl Plugin for ScenePlugin {
             .init_resource::<SceneSpawner>()
             .register_type::<SceneRoot>()
             .register_type::<DynamicSceneRoot>()
+            .register_type::<IsResource>()
+            .register_type::<ResourceEntity<DefaultQueryFilters>>()
             .add_systems(SpawnScene, (scene_spawner, scene_spawner_system).chain());
 
         // Register component hooks for DynamicSceneRoot

crates/bevy_scene/src/scene_spawner.rs

@@ -610,7 +610,7 @@ mod tests {
         assert_eq!(scene_component_a.y, 4.0);
         assert_eq!(
             app.world().entity(entity).get::<Children>().unwrap().len(),
-            1
+            3 // two resources-as-entities are also counted
         );
 
         // let's try to delete the scene

crates/bevy_scene/src/serde.rs

@@ -516,9 +516,11 @@ mod tests {
     };
     use bevy_ecs::{
         entity::{Entity, EntityHashMap},
+        entity_disabling::DefaultQueryFilters,
         prelude::{Component, ReflectComponent, ReflectResource, Resource, World},
         query::{With, Without},
         reflect::AppTypeRegistry,
+        resource::{IsResource, ResourceEntity},
         world::FromWorld,
     };
     use bevy_reflect::{Reflect, ReflectDeserialize, ReflectSerialize};
@@ -611,6 +613,8 @@ mod tests {
             registry.register::<MyEntityRef>();
             registry.register::<Entity>();
             registry.register::<MyResource>();
+            registry.register::<IsResource>();
+            registry.register::<ResourceEntity<DefaultQueryFilters>>();
         }
         world.insert_resource(registry);
         world
@@ -638,20 +642,20 @@ mod tests {
     ),
   },
   entities: {
-    4294967293: (
+    4294967291: (
       components: {
         "bevy_scene::serde::tests::Bar": (345),
         "bevy_scene::serde::tests::Baz": (789),
         "bevy_scene::serde::tests::Foo": (123),
       },
     ),
-    4294967294: (
+    4294967292: (
       components: {
         "bevy_scene::serde::tests::Bar": (345),
         "bevy_scene::serde::tests::Foo": (123),
       },
     ),
-    4294967295: (
+    4294967293: (
       components: {
         "bevy_scene::serde::tests::Foo": (123),
       },
@@ -757,7 +761,7 @@ mod tests {
             .write_to_world(&mut dst_world, &mut map)
             .unwrap();
 
-        assert_eq!(2, deserialized_scene.entities.len());
+        assert_eq!(4, deserialized_scene.entities.len());
         assert_scene_eq(&scene, &deserialized_scene);
 
         let bar_to_foo = dst_world
@@ -785,7 +789,7 @@ mod tests {
 
         let (scene, deserialized_scene) = roundtrip_ron(&world);
 
-        assert_eq!(1, deserialized_scene.entities.len());
+        assert_eq!(3, deserialized_scene.entities.len());
         assert_scene_eq(&scene, &deserialized_scene);
 
         let mut world = create_world();
@@ -815,10 +819,19 @@ mod tests {
 
         assert_eq!(
             vec![
-                0, 1, 255, 255, 255, 255, 15, 1, 37, 98, 101, 118, 121, 95, 115, 99, 101, 110, 101,
+                0, 3, 253, 255, 255, 255, 15, 1, 37, 98, 101, 118, 121, 95, 115, 99, 101, 110, 101,
                 58, 58, 115, 101, 114, 100, 101, 58, 58, 116, 101, 115, 116, 115, 58, 58, 77, 121,
                 67, 111, 109, 112, 111, 110, 101, 110, 116, 1, 2, 3, 102, 102, 166, 63, 205, 204,
-                108, 64, 1, 12, 72, 101, 108, 108, 111, 32, 87, 111, 114, 108, 100, 33
+                108, 64, 1, 12, 72, 101, 108, 108, 111, 32, 87, 111, 114, 108, 100, 33, 254, 255,
+                255, 255, 15, 1, 30, 98, 101, 118, 121, 95, 101, 99, 115, 58, 58, 114, 101, 115,
+                111, 117, 114, 99, 101, 58, 58, 73, 115, 82, 101, 115, 111, 117, 114, 99, 101, 255,
+                255, 255, 255, 15, 2, 30, 98, 101, 118, 121, 95, 101, 99, 115, 58, 58, 114, 101,
+                115, 111, 117, 114, 99, 101, 58, 58, 73, 115, 82, 101, 115, 111, 117, 114, 99, 101,
+                83, 98, 101, 118, 121, 95, 101, 99, 115, 58, 58, 114, 101, 115, 111, 117, 114, 99,
+                101, 58, 58, 82, 101, 115, 111, 117, 114, 99, 101, 69, 110, 116, 105, 116, 121, 60,
+                98, 101, 118, 121, 95, 101, 99, 115, 58, 58, 101, 110, 116, 105, 116, 121, 95, 100,
+                105, 115, 97, 98, 108, 105, 110, 103, 58, 58, 68, 101, 102, 97, 117, 108, 116, 81,
+                117, 101, 114, 121, 70, 105, 108, 116, 101, 114, 115, 62
             ],
             serialized_scene
         );
@@ -830,7 +843,7 @@ mod tests {
             .deserialize(&mut postcard::Deserializer::from_bytes(&serialized_scene))
             .unwrap();
 
-        assert_eq!(1, deserialized_scene.entities.len());
+        assert_eq!(3, deserialized_scene.entities.len());
         assert_scene_eq(&scene, &deserialized_scene);
     }
 
@@ -856,11 +869,21 @@ mod tests {
 
         assert_eq!(
             vec![
-                146, 128, 129, 206, 255, 255, 255, 255, 145, 129, 217, 37, 98, 101, 118, 121, 95,
+                146, 128, 131, 206, 255, 255, 255, 253, 145, 129, 217, 37, 98, 101, 118, 121, 95,
                 115, 99, 101, 110, 101, 58, 58, 115, 101, 114, 100, 101, 58, 58, 116, 101, 115,
                 116, 115, 58, 58, 77, 121, 67, 111, 109, 112, 111, 110, 101, 110, 116, 147, 147, 1,
                 2, 3, 146, 202, 63, 166, 102, 102, 202, 64, 108, 204, 205, 129, 165, 84, 117, 112,
-                108, 101, 172, 72, 101, 108, 108, 111, 32, 87, 111, 114, 108, 100, 33
+                108, 101, 172, 72, 101, 108, 108, 111, 32, 87, 111, 114, 108, 100, 33, 206, 255,
+                255, 255, 254, 145, 129, 190, 98, 101, 118, 121, 95, 101, 99, 115, 58, 58, 114,
+                101, 115, 111, 117, 114, 99, 101, 58, 58, 73, 115, 82, 101, 115, 111, 117, 114, 99,
+                101, 144, 206, 255, 255, 255, 255, 145, 130, 190, 98, 101, 118, 121, 95, 101, 99,
+                115, 58, 58, 114, 101, 115, 111, 117, 114, 99, 101, 58, 58, 73, 115, 82, 101, 115,
+                111, 117, 114, 99, 101, 144, 217, 83, 98, 101, 118, 121, 95, 101, 99, 115, 58, 58,
+                114, 101, 115, 111, 117, 114, 99, 101, 58, 58, 82, 101, 115, 111, 117, 114, 99,
+                101, 69, 110, 116, 105, 116, 121, 60, 98, 101, 118, 121, 95, 101, 99, 115, 58, 58,
+                101, 110, 116, 105, 116, 121, 95, 100, 105, 115, 97, 98, 108, 105, 110, 103, 58,
+                58, 68, 101, 102, 97, 117, 108, 116, 81, 117, 101, 114, 121, 70, 105, 108, 116,
+                101, 114, 115, 62, 144
             ],
             buf
         );
@@ -874,7 +897,7 @@ mod tests {
             .deserialize(&mut rmp_serde::Deserializer::new(&mut reader))
             .unwrap();
 
-        assert_eq!(1, deserialized_scene.entities.len());
+        assert_eq!(3, deserialized_scene.entities.len());
         assert_scene_eq(&scene, &deserialized_scene);
     }
 
@@ -899,13 +922,23 @@ mod tests {
 
         assert_eq!(
             vec![
-                0, 0, 0, 0, 0, 0, 0, 0, 1, 0, 0, 0, 0, 0, 0, 0, 255, 255, 255, 255, 0, 0, 0, 0, 1,
+                0, 0, 0, 0, 0, 0, 0, 0, 3, 0, 0, 0, 0, 0, 0, 0, 253, 255, 255, 255, 0, 0, 0, 0, 1,
                 0, 0, 0, 0, 0, 0, 0, 37, 0, 0, 0, 0, 0, 0, 0, 98, 101, 118, 121, 95, 115, 99, 101,
                 110, 101, 58, 58, 115, 101, 114, 100, 101, 58, 58, 116, 101, 115, 116, 115, 58, 58,
                 77, 121, 67, 111, 109, 112, 111, 110, 101, 110, 116, 1, 0, 0, 0, 0, 0, 0, 0, 2, 0,
                 0, 0, 0, 0, 0, 0, 3, 0, 0, 0, 0, 0, 0, 0, 102, 102, 166, 63, 205, 204, 108, 64, 1,
                 0, 0, 0, 12, 0, 0, 0, 0, 0, 0, 0, 72, 101, 108, 108, 111, 32, 87, 111, 114, 108,
-                100, 33
+                100, 33, 254, 255, 255, 255, 0, 0, 0, 0, 1, 0, 0, 0, 0, 0, 0, 0, 30, 0, 0, 0, 0, 0,
+                0, 0, 98, 101, 118, 121, 95, 101, 99, 115, 58, 58, 114, 101, 115, 111, 117, 114,
+                99, 101, 58, 58, 73, 115, 82, 101, 115, 111, 117, 114, 99, 101, 255, 255, 255, 255,
+                0, 0, 0, 0, 2, 0, 0, 0, 0, 0, 0, 0, 30, 0, 0, 0, 0, 0, 0, 0, 98, 101, 118, 121, 95,
+                101, 99, 115, 58, 58, 114, 101, 115, 111, 117, 114, 99, 101, 58, 58, 73, 115, 82,
+                101, 115, 111, 117, 114, 99, 101, 83, 0, 0, 0, 0, 0, 0, 0, 98, 101, 118, 121, 95,
+                101, 99, 115, 58, 58, 114, 101, 115, 111, 117, 114, 99, 101, 58, 58, 82, 101, 115,
+                111, 117, 114, 99, 101, 69, 110, 116, 105, 116, 121, 60, 98, 101, 118, 121, 95,
+                101, 99, 115, 58, 58, 101, 110, 116, 105, 116, 121, 95, 100, 105, 115, 97, 98, 108,
+                105, 110, 103, 58, 58, 68, 101, 102, 97, 117, 108, 116, 81, 117, 101, 114, 121, 70,
+                105, 108, 116, 101, 114, 115, 62
             ],
             serialized_scene
         );
@@ -918,7 +951,7 @@ mod tests {
             bincode::serde::seed_decode_from_slice(scene_deserializer, &serialized_scene, config)
                 .unwrap();
 
-        assert_eq!(1, deserialized_scene.entities.len());
+        assert_eq!(3, deserialized_scene.entities.len());
         assert_scene_eq(&scene, &deserialized_scene);
     }
 

release-content/migration-guides/resources_as_components.md

@@ -0,0 +1,25 @@
+---
+title: Resources as Components
+pull_requests: [19711]
+---
+
+Resources are very similar to Components: they are both data that can be stored in the ECS and queried.
+The only real difference between them is that querying a resource will return either one or zero resources, whereas querying for a component can return any number of entities that match it.
+
+Even so, resources and components have always been separate concepts within the ECS.
+This leads to some annoying restrictions.
+While components have [`ComponentHooks`](https://docs.rs/bevy/latest/bevy/ecs/component/struct.ComponentHooks.html), it's not possible to add lifecycle hooks to resources.
+Moreover, the engine internals contain a lot of duplication because of it.
+
+This motivates us to transition resources to components, and while most of the public API will stay the same, some breaking changes are inevitable.
+
+This PR adds a dummy entity alongside every resource. This entity is inserted and removed alongside resources and doesn't do anything (yet).
+
+This changes `World::entities().len()` as there are more entities than you might expect there to be. For example, a new world, no longer contains zero entities. This is mostly important for unit tests.
+
+Two methods have been added `World::entity_count()` and `World::resource_count()`. The former returns the number of entities without the resource entities, while the latter returns the number of resources in the world.
+
+While the marker component `IsResource` is added to [default query filters](https://docs.rs/bevy/latest/bevy/ecs/entity_disabling/struct.DefaultQueryFilters.html), during world creation, resource entities might still show up in broad queries with [`EntityMutExcept`](https://docs.rs/bevy/latest/bevy/ecs/world/struct.EntityMutExcept.html) and [`EntityRefExcept`](https://docs.rs/bevy/latest/bevy/ecs/world/struct.EntityRefExcept.html).
+They also show up in `World::iter_entities()`, `World::iter_entities_mut()` and [`QueryBuilder`](https://docs.rs/bevy/latest/bevy/ecs/prelude/struct.QueryBuilder.html).
+
+Lastly, because of the entity bump, the input and output of the `bevy_scene` crate is not equivalent to the previous version, meaning that it's unadvisable to read in scenes from the previous version into the current one.