add a SceneBundle to spawn a scene (bevyengine#2424)

# Objective

- Spawning a scene is handled as a special case with a command `spawn_scene` that takes an handle but doesn't let you specify anything else. This is the only handle that works that way.
- Workaround for this have been to add the `spawn_scene` on `ChildBuilder` to be able to specify transform of parent, or to make the `SceneSpawner` available to be able to select entities from a scene by their instance id

## Solution

Add a bundle
```rust
pub struct SceneBundle {
    pub scene: Handle<Scene>,
    pub transform: Transform,
    pub global_transform: GlobalTransform,
    pub instance_id: Option<InstanceId>,
}
```

and instead of 
```rust
commands.spawn_scene(asset_server.load("models/FlightHelmet/FlightHelmet.gltf#Scene0"));
```
you can do
```rust
commands.spawn_bundle(SceneBundle {
    scene: asset_server.load("models/FlightHelmet/FlightHelmet.gltf#Scene0"),
    ..Default::default()
});
```

The scene will be spawned as a child of the entity with the `SceneBundle`

~I would like to remove the command `spawn_scene` in favor of this bundle but didn't do it yet to get feedback first~

Co-authored-by: François <[email protected]>
Co-authored-by: Carter Anderson <[email protected]>

Author: mockersf

crates/bevy_hierarchy/src/components/parent.rs

@@ -25,7 +25,11 @@ impl FromWorld for Parent {
 
 impl MapEntities for Parent {
     fn map_entities(&mut self, entity_map: &EntityMap) -> Result<(), MapEntitiesError> {
-        self.0 = entity_map.get(self.0)?;
+        // Parent of an entity in the new world can be in outside world, in which case it
+        // should not be mapped.
+        if let Ok(mapped_entity) = entity_map.get(self.0) {
+            self.0 = mapped_entity;
+        }
         Ok(())
     }
 }
@@ -51,7 +55,11 @@ pub struct PreviousParent(pub(crate) Entity);
 
 impl MapEntities for PreviousParent {
     fn map_entities(&mut self, entity_map: &EntityMap) -> Result<(), MapEntitiesError> {
-        self.0 = entity_map.get(self.0)?;
+        // PreviousParent of an entity in the new world can be in outside world, in which
+        // case it should not be mapped.
+        if let Ok(mapped_entity) = entity_map.get(self.0) {
+            self.0 = mapped_entity;
+        }
         Ok(())
     }
 }

crates/bevy_scene/Cargo.toml

@@ -12,9 +12,11 @@ keywords = ["bevy"]
 # bevy
 bevy_app = { path = "../bevy_app", version = "0.8.0-dev" }
 bevy_asset = { path = "../bevy_asset", version = "0.8.0-dev" }
+bevy_derive = { path = "../bevy_derive", version = "0.8.0-dev" }
 bevy_ecs = { path = "../bevy_ecs", version = "0.8.0-dev" }
 bevy_reflect = { path = "../bevy_reflect", version = "0.8.0-dev", features = ["bevy"] }
 bevy_hierarchy = { path = "../bevy_hierarchy", version = "0.8.0-dev" }
+bevy_transform = { path = "../bevy_transform", version = "0.8.0-dev" }
 bevy_utils = { path = "../bevy_utils", version = "0.8.0-dev" }
 
 # other

crates/bevy_scene/src/bundle.rs

@@ -0,0 +1,74 @@
+use bevy_asset::Handle;
+use bevy_derive::{Deref, DerefMut};
+use bevy_ecs::{
+    bundle::Bundle,
+    change_detection::ResMut,
+    entity::Entity,
+    prelude::{Changed, Component, Without},
+    system::{Commands, Query},
+};
+use bevy_transform::components::{GlobalTransform, Transform};
+
+use crate::{DynamicScene, InstanceId, Scene, SceneSpawner};
+
+/// [`InstanceId`] of a spawned scene. It can be used with the [`SceneSpawner`] to
+/// interact with the spawned scene.
+#[derive(Component, Deref, DerefMut)]
+pub struct SceneInstance(InstanceId);
+
+/// A component bundle for a [`Scene`] root.
+///
+/// The scene from `scene` will be spawn as a child of the entity with this component.
+/// Once it's spawned, the entity will have a [`SceneInstance`] component.
+#[derive(Default, Bundle)]
+pub struct SceneBundle {
+    /// Handle to the scene to spawn
+    pub scene: Handle<Scene>,
+    pub transform: Transform,
+    pub global_transform: GlobalTransform,
+}
+
+/// A component bundle for a [`DynamicScene`] root.
+///
+/// The dynamic scene from `scene` will be spawn as a child of the entity with this component.
+/// Once it's spawned, the entity will have a [`SceneInstance`] component.
+#[derive(Default, Bundle)]
+pub struct DynamicSceneBundle {
+    /// Handle to the scene to spawn
+    pub scene: Handle<DynamicScene>,
+    pub transform: Transform,
+    pub global_transform: GlobalTransform,
+}
+
+/// System that will spawn scenes from [`SceneBundle`].
+pub fn scene_spawner(
+    mut commands: Commands,
+    mut scene_to_spawn: Query<
+        (Entity, &Handle<Scene>, Option<&mut SceneInstance>),
+        (Changed<Handle<Scene>>, Without<Handle<DynamicScene>>),
+    >,
+    mut dynamic_scene_to_spawn: Query<
+        (Entity, &Handle<DynamicScene>, Option<&mut SceneInstance>),
+        (Changed<Handle<DynamicScene>>, Without<Handle<Scene>>),
+    >,
+    mut scene_spawner: ResMut<SceneSpawner>,
+) {
+    for (entity, scene, instance) in scene_to_spawn.iter_mut() {
+        let new_instance = scene_spawner.spawn_as_child(scene.clone(), entity);
+        if let Some(mut old_instance) = instance {
+            scene_spawner.despawn_instance(**old_instance);
+            *old_instance = SceneInstance(new_instance);
+        } else {
+            commands.entity(entity).insert(SceneInstance(new_instance));
+        }
+    }
+    for (entity, dynamic_scene, instance) in dynamic_scene_to_spawn.iter_mut() {
+        let new_instance = scene_spawner.spawn_dynamic_as_child(dynamic_scene.clone(), entity);
+        if let Some(mut old_instance) = instance {
+            scene_spawner.despawn_instance(**old_instance);
+            *old_instance = SceneInstance(new_instance);
+        } else {
+            commands.entity(entity).insert(SceneInstance(new_instance));
+        }
+    }
+}

crates/bevy_scene/src/command.rs

@@ -9,6 +9,12 @@ use bevy_reflect::{Reflect, TypeRegistryArc, TypeUuid};
 use serde::Serialize;
 
 /// A collection of serializable dynamic entities, each with its own run-time defined set of components.
+/// To spawn a dynamic scene, you can use either:
+/// * [`SceneSpawner::spawn_dynamic`](crate::SceneSpawner::spawn_dynamic)
+/// * adding the [`DynamicSceneBundle`](crate::DynamicSceneBundle) to an entity
+/// * adding the [`Handle<DynamicScene>`](bevy_asset::Handle) to an entity (the scene will only be
+/// visible if the entity already has [`Transform`](bevy_transform::components::Transform) and
+/// [`GlobalTransform`](bevy_transform::components::GlobalTransform) components)
 #[derive(Default, TypeUuid)]
 #[uuid = "749479b1-fb8c-4ff8-a775-623aa76014f5"]
 pub struct DynamicScene {

crates/bevy_scene/src/dynamic_scene.rs

@@ -1,21 +1,19 @@
-mod command;
+mod bundle;
 mod dynamic_scene;
 mod scene;
 mod scene_loader;
 mod scene_spawner;
 pub mod serde;
 
-pub use command::*;
+pub use bundle::*;
 pub use dynamic_scene::*;
 pub use scene::*;
 pub use scene_loader::*;
 pub use scene_spawner::*;
 
 pub mod prelude {
     #[doc(hidden)]
-    pub use crate::{
-        DynamicScene, Scene, SceneSpawner, SpawnSceneAsChildCommands, SpawnSceneCommands,
-    };
+    pub use crate::{DynamicScene, DynamicSceneBundle, Scene, SceneBundle, SceneSpawner};
 }
 
 use bevy_app::prelude::*;
@@ -34,6 +32,8 @@ impl Plugin for ScenePlugin {
             .add_system_to_stage(
                 CoreStage::PreUpdate,
                 scene_spawner_system.exclusive_system().at_end(),
-            );
+            )
+            // Systems `*_bundle_spawner` must run before `scene_spawner_system`
+            .add_system_to_stage(CoreStage::PreUpdate, scene_spawner);
     }
 }

crates/bevy_scene/src/lib.rs

@@ -1,6 +1,12 @@
 use bevy_ecs::world::World;
 use bevy_reflect::TypeUuid;
 
+/// To spawn a scene, you can use either:
+/// * [`SceneSpawner::spawn`](crate::SceneSpawner::spawn)
+/// * adding the [`SceneBundle`](crate::SceneBundle) to an entity
+/// * adding the [`Handle<Scene>`](bevy_asset::Handle) to an entity (the scene will only be
+/// visible if the entity already has [`Transform`](bevy_transform::components::Transform) and
+/// [`GlobalTransform`](bevy_transform::components::GlobalTransform) components)
 #[derive(Debug, TypeUuid)]
 #[uuid = "c156503c-edd9-4ec7-8d33-dab392df03cd"]
 pub struct Scene {

crates/bevy_scene/src/scene.rs

@@ -33,9 +33,10 @@ pub struct SceneSpawner {
     spawned_dynamic_scenes: HashMap<Handle<DynamicScene>, Vec<InstanceId>>,
     spawned_instances: HashMap<InstanceId, InstanceInfo>,
     scene_asset_event_reader: ManualEventReader<AssetEvent<DynamicScene>>,
-    dynamic_scenes_to_spawn: Vec<Handle<DynamicScene>>,
+    dynamic_scenes_to_spawn: Vec<(Handle<DynamicScene>, InstanceId)>,
     scenes_to_spawn: Vec<(Handle<Scene>, InstanceId)>,
     scenes_to_despawn: Vec<Handle<DynamicScene>>,
+    instances_to_despawn: Vec<InstanceId>,
     scenes_with_parent: Vec<(InstanceId, Entity)>,
 }
 
@@ -53,7 +54,21 @@ pub enum SceneSpawnError {
 
 impl SceneSpawner {
     pub fn spawn_dynamic(&mut self, scene_handle: Handle<DynamicScene>) {
-        self.dynamic_scenes_to_spawn.push(scene_handle);
+        let instance_id = InstanceId::new();
+        self.dynamic_scenes_to_spawn
+            .push((scene_handle, instance_id));
+    }
+
+    pub fn spawn_dynamic_as_child(
+        &mut self,
+        scene_handle: Handle<DynamicScene>,
+        parent: Entity,
+    ) -> InstanceId {
+        let instance_id = InstanceId::new();
+        self.dynamic_scenes_to_spawn
+            .push((scene_handle, instance_id));
+        self.scenes_with_parent.push((instance_id, parent));
+        instance_id
     }
 
     pub fn spawn(&mut self, scene_handle: Handle<Scene>) -> InstanceId {
@@ -73,26 +88,31 @@ impl SceneSpawner {
         self.scenes_to_despawn.push(scene_handle);
     }
 
+    pub fn despawn_instance(&mut self, instance_id: InstanceId) {
+        self.instances_to_despawn.push(instance_id);
+    }
+
     pub fn despawn_sync(
         &mut self,
         world: &mut World,
         scene_handle: Handle<DynamicScene>,
     ) -> Result<(), SceneSpawnError> {
-        if let Some(instance_ids) = self.spawned_dynamic_scenes.get(&scene_handle) {
+        if let Some(instance_ids) = self.spawned_dynamic_scenes.remove(&scene_handle) {
             for instance_id in instance_ids {
-                if let Some(instance) = self.spawned_instances.get(instance_id) {
-                    for entity in instance.entity_map.values() {
-                        let _ = world.despawn(entity); // Ignore the result, despawn only cares if
-                                                       // it exists.
-                    }
-                }
+                self.despawn_instance_sync(world, &instance_id);
             }
-
-            self.spawned_dynamic_scenes.remove(&scene_handle);
         }
         Ok(())
     }
 
+    pub fn despawn_instance_sync(&mut self, world: &mut World, instance_id: &InstanceId) {
+        if let Some(instance) = self.spawned_instances.remove(instance_id) {
+            for entity in instance.entity_map.values() {
+                let _ = world.despawn(entity);
+            }
+        }
+    }
+
     pub fn spawn_dynamic_sync(
         &mut self,
         world: &mut World,
@@ -235,14 +255,33 @@ impl SceneSpawner {
         Ok(())
     }
 
+    pub fn despawn_queued_instances(&mut self, world: &mut World) {
+        let instances_to_despawn = std::mem::take(&mut self.instances_to_despawn);
+
+        for instance_id in instances_to_despawn {
+            self.despawn_instance_sync(world, &instance_id);
+        }
+    }
+
     pub fn spawn_queued_scenes(&mut self, world: &mut World) -> Result<(), SceneSpawnError> {
         let scenes_to_spawn = std::mem::take(&mut self.dynamic_scenes_to_spawn);
 
-        for scene_handle in scenes_to_spawn {
-            match self.spawn_dynamic_sync(world, &scene_handle) {
-                Ok(_) => {}
+        for (scene_handle, instance_id) in scenes_to_spawn {
+            let mut entity_map = EntityMap::default();
+
+            match Self::spawn_dynamic_internal(world, &scene_handle, &mut entity_map) {
+                Ok(_) => {
+                    self.spawned_instances
+                        .insert(instance_id, InstanceInfo { entity_map });
+                    let spawned = self
+                        .spawned_dynamic_scenes
+                        .entry(scene_handle.clone())
+                        .or_insert_with(Vec::new);
+                    spawned.push(instance_id);
+                }
                 Err(SceneSpawnError::NonExistentScene { .. }) => {
-                    self.dynamic_scenes_to_spawn.push(scene_handle);
+                    self.dynamic_scenes_to_spawn
+                        .push((scene_handle, instance_id));
                 }
                 Err(err) => return Err(err),
             }
@@ -327,6 +366,7 @@ pub fn scene_spawner_system(world: &mut World) {
         }
 
         scene_spawner.despawn_queued_scenes(world).unwrap();
+        scene_spawner.despawn_queued_instances(world);
         scene_spawner
             .spawn_queued_scenes(world)
             .unwrap_or_else(|err| panic!("{}", err));

crates/bevy_scene/src/scene_spawner.rs

@@ -15,7 +15,6 @@ fn main() {
 }
 
 fn setup(mut commands: Commands, asset_server: Res<AssetServer>) {
-    commands.spawn_scene(asset_server.load("models/FlightHelmet/FlightHelmet.gltf#Scene0"));
     commands.spawn_bundle(Camera3dBundle {
         transform: Transform::from_xyz(0.7, 0.7, 1.0).looking_at(Vec3::new(0.0, 0.3, 0.0), Vec3::Y),
         ..default()
@@ -37,6 +36,10 @@ fn setup(mut commands: Commands, asset_server: Res<AssetServer>) {
         },
         ..default()
     });
+    commands.spawn_bundle(SceneBundle {
+        scene: asset_server.load("models/FlightHelmet/FlightHelmet.gltf#Scene0"),
+        ..default()
+    });
 }
 
 fn animate_light_direction(

examples/3d/load_gltf.rs

@@ -29,7 +29,10 @@ fn setup(
         ..default()
     });
 
-    commands.spawn_scene(asset_server.load("models/animated/Fox.glb#Scene0"));
+    commands.spawn_bundle(SceneBundle {
+        scene: asset_server.load("models/animated/Fox.glb#Scene0"),
+        ..default()
+    });
 
     // Light
     commands.spawn_bundle(DirectionalLightBundle {