dynamic scene builder (#6227)

# Objective

- make it easier to build dynamic scenes

## Solution

- add a builder to create a dynamic scene from a world. it can extract an entity or an iterator of entities
- alternative to #6013, leaving the "hierarchy iteration" part to #6185 which does it better
- alternative to #6004 
- using a builder makes it easier to chain several extractions

crates/bevy_scene/src/dynamic_scene.rs

@@ -1,4 +1,4 @@
-use crate::{serde::SceneSerializer, Scene, SceneSpawnError};
+use crate::{serde::SceneSerializer, DynamicSceneBuilder, Scene, SceneSpawnError};
 use anyhow::Result;
 use bevy_app::AppTypeRegistry;
 use bevy_ecs::{
@@ -33,47 +33,23 @@ pub struct DynamicEntity {
 
 impl DynamicScene {
     /// Create a new dynamic scene from a given scene.
-    pub fn from_scene(scene: &Scene, type_registry: &TypeRegistryArc) -> Self {
+    pub fn from_scene(scene: &Scene, type_registry: &AppTypeRegistry) -> Self {
         Self::from_world(&scene.world, type_registry)
     }
 
     /// Create a new dynamic scene from a given world.
-    pub fn from_world(world: &World, type_registry: &TypeRegistryArc) -> Self {
-        let mut scene = DynamicScene::default();
-        let type_registry = type_registry.read();
+    pub fn from_world(world: &World, type_registry: &AppTypeRegistry) -> Self {
+        let mut builder =
+            DynamicSceneBuilder::from_world_with_type_registry(world, type_registry.clone());
 
-        for archetype in world.archetypes().iter() {
-            let entities_offset = scene.entities.len();
+        builder.extract_entities(
+            world
+                .archetypes()
+                .iter()
+                .flat_map(|archetype| archetype.entities().iter().copied()),
+        );
 
-            // Create a new dynamic entity for each entity of the given archetype
-            // and insert it into the dynamic scene.
-            for entity in archetype.entities() {
-                scene.entities.push(DynamicEntity {
-                    entity: entity.id(),
-                    components: Vec::new(),
-                });
-            }
-
-            // Add each reflection-powered component to the entity it belongs to.
-            for component_id in archetype.components() {
-                let reflect_component = world
-                    .components()
-                    .get_info(component_id)
-                    .and_then(|info| type_registry.get(info.type_id().unwrap()))
-                    .and_then(|registration| registration.data::<ReflectComponent>());
-                if let Some(reflect_component) = reflect_component {
-                    for (i, entity) in archetype.entities().iter().enumerate() {
-                        if let Some(component) = reflect_component.reflect(world, *entity) {
-                            scene.entities[entities_offset + i]
-                                .components
-                                .push(component.clone_value());
-                        }
-                    }
-                }
-            }
-        }
-
-        scene
+        builder.build()
     }
 
     /// Write the dynamic entities and their corresponding components to the given world.

crates/bevy_scene/src/dynamic_scene_builder.rs

@@ -0,0 +1,237 @@
+use crate::{DynamicEntity, DynamicScene};
+use bevy_app::AppTypeRegistry;
+use bevy_ecs::{prelude::Entity, reflect::ReflectComponent, world::World};
+use bevy_utils::{default, HashMap};
+
+/// A [`DynamicScene`] builder, used to build a scene from a [`World`] by extracting some entities.
+///
+/// ```
+/// # use bevy_scene::DynamicSceneBuilder;
+/// # use bevy_app::AppTypeRegistry;
+/// # use bevy_ecs::{
+/// #     component::Component, prelude::Entity, query::With, reflect::ReflectComponent, world::World,
+/// # };
+/// # use bevy_reflect::Reflect;
+/// # #[derive(Component, Reflect, Default, Eq, PartialEq, Debug)]
+/// # #[reflect(Component)]
+/// # struct ComponentA;
+/// # let mut world = World::default();
+/// # world.init_resource::<AppTypeRegistry>();
+/// # let entity = world.spawn(ComponentA).id();
+/// let mut builder = DynamicSceneBuilder::from_world(&world);
+/// builder.extract_entity(entity);
+/// let dynamic_scene = builder.build();
+/// ```
+pub struct DynamicSceneBuilder<'w> {
+    scene: HashMap<u32, DynamicEntity>,
+    type_registry: AppTypeRegistry,
+    world: &'w World,
+}
+
+impl<'w> DynamicSceneBuilder<'w> {
+    /// Prepare a builder that will extract entities and their component from the given [`World`].
+    /// All components registered in that world's [`AppTypeRegistry`] resource will be extracted.
+    pub fn from_world(world: &'w World) -> Self {
+        Self {
+            scene: default(),
+            type_registry: world.resource::<AppTypeRegistry>().clone(),
+            world,
+        }
+    }
+
+    /// Prepare a builder that will extract entities and their component from the given [`World`].
+    /// Only components registered in the given [`AppTypeRegistry`] will be extracted.
+    pub fn from_world_with_type_registry(world: &'w World, type_registry: AppTypeRegistry) -> Self {
+        Self {
+            scene: default(),
+            type_registry,
+            world,
+        }
+    }
+
+    /// Consume the builder, producing a [`DynamicScene`].
+    pub fn build(self) -> DynamicScene {
+        DynamicScene {
+            entities: self.scene.into_values().collect(),
+        }
+    }
+
+    /// Extract one entity from the builder's [`World`].
+    ///
+    /// Re-extracting an entity that was already extracted will have no effect.
+    pub fn extract_entity(&mut self, entity: Entity) -> &mut Self {
+        self.extract_entities(std::iter::once(entity))
+    }
+
+    /// Extract entities from the builder's [`World`].
+    ///
+    /// Re-extracting an entity that was already extracted will have no effect.
+    ///
+    /// Extracting entities can be used to extract entities from a query:
+    /// ```
+    /// # use bevy_scene::DynamicSceneBuilder;
+    /// # use bevy_app::AppTypeRegistry;
+    /// # use bevy_ecs::{
+    /// #     component::Component, prelude::Entity, query::With, reflect::ReflectComponent, world::World,
+    /// # };
+    /// # use bevy_reflect::Reflect;
+    /// #[derive(Component, Default, Reflect)]
+    /// #[reflect(Component)]
+    /// struct MyComponent;
+    ///
+    /// # let mut world = World::default();
+    /// # world.init_resource::<AppTypeRegistry>();
+    /// # let _entity = world.spawn(MyComponent).id();
+    /// let mut query = world.query_filtered::<Entity, With<MyComponent>>();
+    ///
+    /// let mut builder = DynamicSceneBuilder::from_world(&world);
+    /// builder.extract_entities(query.iter(&world));
+    /// let scene = builder.build();
+    /// ```
+    pub fn extract_entities(&mut self, entities: impl Iterator<Item = Entity>) -> &mut Self {
+        let type_registry = self.type_registry.read();
+
+        for entity in entities {
+            if self.scene.contains_key(&entity.id()) {
+                continue;
+            }
+
+            let mut entry = DynamicEntity {
+                entity: entity.id(),
+                components: Vec::new(),
+            };
+
+            for component_id in self.world.entity(entity).archetype().components() {
+                let reflect_component = self
+                    .world
+                    .components()
+                    .get_info(component_id)
+                    .and_then(|info| type_registry.get(info.type_id().unwrap()))
+                    .and_then(|registration| registration.data::<ReflectComponent>());
+
+                if let Some(reflect_component) = reflect_component {
+                    if let Some(component) = reflect_component.reflect(self.world, entity) {
+                        entry.components.push(component.clone_value());
+                    }
+                }
+            }
+
+            self.scene.insert(entity.id(), entry);
+        }
+
+        drop(type_registry);
+        self
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use bevy_app::AppTypeRegistry;
+    use bevy_ecs::{
+        component::Component, prelude::Entity, query::With, reflect::ReflectComponent, world::World,
+    };
+
+    use bevy_reflect::Reflect;
+
+    use super::DynamicSceneBuilder;
+
+    #[derive(Component, Reflect, Default, Eq, PartialEq, Debug)]
+    #[reflect(Component)]
+    struct ComponentA;
+    #[derive(Component, Reflect, Default, Eq, PartialEq, Debug)]
+    #[reflect(Component)]
+    struct ComponentB;
+
+    #[test]
+    fn extract_one_entity() {
+        let mut world = World::default();
+
+        let atr = AppTypeRegistry::default();
+        atr.write().register::<ComponentA>();
+        world.insert_resource(atr);
+
+        let entity = world.spawn((ComponentA, ComponentB)).id();
+
+        let mut builder = DynamicSceneBuilder::from_world(&world);
+        builder.extract_entity(entity);
+        let scene = builder.build();
+
+        assert_eq!(scene.entities.len(), 1);
+        assert_eq!(scene.entities[0].entity, entity.id());
+        assert_eq!(scene.entities[0].components.len(), 1);
+        assert!(scene.entities[0].components[0].represents::<ComponentA>());
+    }
+
+    #[test]
+    fn extract_one_entity_twice() {
+        let mut world = World::default();
+
+        let atr = AppTypeRegistry::default();
+        atr.write().register::<ComponentA>();
+        world.insert_resource(atr);
+
+        let entity = world.spawn((ComponentA, ComponentB)).id();
+
+        let mut builder = DynamicSceneBuilder::from_world(&world);
+        builder.extract_entity(entity);
+        builder.extract_entity(entity);
+        let scene = builder.build();
+
+        assert_eq!(scene.entities.len(), 1);
+        assert_eq!(scene.entities[0].entity, entity.id());
+        assert_eq!(scene.entities[0].components.len(), 1);
+        assert!(scene.entities[0].components[0].represents::<ComponentA>());
+    }
+
+    #[test]
+    fn extract_one_entity_two_components() {
+        let mut world = World::default();
+
+        let atr = AppTypeRegistry::default();
+        {
+            let mut register = atr.write();
+            register.register::<ComponentA>();
+            register.register::<ComponentB>();
+        }
+        world.insert_resource(atr);
+
+        let entity = world.spawn((ComponentA, ComponentB)).id();
+
+        let mut builder = DynamicSceneBuilder::from_world(&world);
+        builder.extract_entity(entity);
+        let scene = builder.build();
+
+        assert_eq!(scene.entities.len(), 1);
+        assert_eq!(scene.entities[0].entity, entity.id());
+        assert_eq!(scene.entities[0].components.len(), 2);
+        assert!(scene.entities[0].components[0].represents::<ComponentA>());
+        assert!(scene.entities[0].components[1].represents::<ComponentB>());
+    }
+
+    #[test]
+    fn extract_query() {
+        let mut world = World::default();
+
+        let atr = AppTypeRegistry::default();
+        {
+            let mut register = atr.write();
+            register.register::<ComponentA>();
+            register.register::<ComponentB>();
+        }
+        world.insert_resource(atr);
+
+        let entity_a_b = world.spawn((ComponentA, ComponentB)).id();
+        let entity_a = world.spawn(ComponentA).id();
+        let _entity_b = world.spawn(ComponentB).id();
+
+        let mut query = world.query_filtered::<Entity, With<ComponentA>>();
+        let mut builder = DynamicSceneBuilder::from_world(&world);
+        builder.extract_entities(query.iter(&world));
+        let scene = builder.build();
+
+        assert_eq!(scene.entities.len(), 2);
+        let mut scene_entities = vec![scene.entities[0].entity, scene.entities[1].entity];
+        scene_entities.sort();
+        assert_eq!(scene_entities, [entity_a_b.id(), entity_a.id()]);
+    }
+}

crates/bevy_scene/src/lib.rs

@@ -1,5 +1,6 @@
 mod bundle;
 mod dynamic_scene;
+mod dynamic_scene_builder;
 mod scene;
 mod scene_loader;
 mod scene_spawner;
@@ -7,13 +8,16 @@ pub mod serde;
 
 pub use bundle::*;
 pub use dynamic_scene::*;
+pub use dynamic_scene_builder::*;
 pub use scene::*;
 pub use scene_loader::*;
 pub use scene_spawner::*;
 
 pub mod prelude {
     #[doc(hidden)]
-    pub use crate::{DynamicScene, DynamicSceneBundle, Scene, SceneBundle, SceneSpawner};
+    pub use crate::{
+        DynamicScene, DynamicSceneBuilder, DynamicSceneBundle, Scene, SceneBundle, SceneSpawner,
+    };
 }
 
 use bevy_app::prelude::*;