Send SceneInstanceReady when spawning any kind of scene (#11741)

# Objective

- Emit an event regardless of scene type (`Scene` and `DynamicScene`).
- Also send the `InstanceId` along.

Follow-up to #11002.
Fixes #2218.

## Solution

- Send `SceneInstanceReady` regardless of scene type.
- Make `SceneInstanceReady::parent` `Option`al.
- Add `SceneInstanceReady::id`.

---

## Changelog

### Changed

- `SceneInstanceReady` is now sent for `Scene` as well.
`SceneInstanceReady::parent` is an `Option` and
`SceneInstanceReady::id`, an `InstanceId`, is added to identify the
corresponding `Scene`.

## Migration Guide

- `SceneInstanceReady { parent: Entity }` is now `SceneInstanceReady {
id: InstanceId, parent: Option<Entity> }`.

crates/bevy_scene/src/scene_spawner.rs

@@ -18,8 +18,10 @@ use uuid::Uuid;
 /// See also [`SceneSpawner::instance_is_ready`].
 #[derive(Clone, Copy, Debug, Eq, PartialEq, Event)]
 pub struct SceneInstanceReady {
+    /// ID of the spawned instance.
+    pub id: InstanceId,
     /// Entity to which the scene was spawned as a child.
-    pub parent: Entity,
+    pub parent: Option<Entity>,
 }
 
 /// Information about a scene instance.
@@ -63,8 +65,8 @@ pub struct SceneSpawner {
     pub(crate) spawned_dynamic_scenes: HashMap<AssetId<DynamicScene>, HashSet<InstanceId>>,
     pub(crate) spawned_instances: HashMap<InstanceId, InstanceInfo>,
     scene_asset_event_reader: ManualEventReader<AssetEvent<DynamicScene>>,
-    dynamic_scenes_to_spawn: Vec<(Handle<DynamicScene>, InstanceId)>,
-    scenes_to_spawn: Vec<(Handle<Scene>, InstanceId)>,
+    dynamic_scenes_to_spawn: Vec<(Handle<DynamicScene>, InstanceId, Option<Entity>)>,
+    scenes_to_spawn: Vec<(Handle<Scene>, InstanceId, Option<Entity>)>,
     scenes_to_despawn: Vec<AssetId<DynamicScene>>,
     instances_to_despawn: Vec<InstanceId>,
     scenes_with_parent: Vec<(InstanceId, Entity)>,
@@ -128,7 +130,8 @@ impl SceneSpawner {
     /// Schedule the spawn of a new instance of the provided dynamic scene.
     pub fn spawn_dynamic(&mut self, id: impl Into<Handle<DynamicScene>>) -> InstanceId {
         let instance_id = InstanceId::new();
-        self.dynamic_scenes_to_spawn.push((id.into(), instance_id));
+        self.dynamic_scenes_to_spawn
+            .push((id.into(), instance_id, None));
         instance_id
     }
 
@@ -139,22 +142,24 @@ impl SceneSpawner {
         parent: Entity,
     ) -> InstanceId {
         let instance_id = InstanceId::new();
-        self.dynamic_scenes_to_spawn.push((id.into(), instance_id));
+        self.dynamic_scenes_to_spawn
+            .push((id.into(), instance_id, Some(parent)));
         self.scenes_with_parent.push((instance_id, parent));
         instance_id
     }
 
     /// Schedule the spawn of a new instance of the provided scene.
     pub fn spawn(&mut self, id: impl Into<Handle<Scene>>) -> InstanceId {
         let instance_id = InstanceId::new();
-        self.scenes_to_spawn.push((id.into(), instance_id));
+        self.scenes_to_spawn.push((id.into(), instance_id, None));
         instance_id
     }
 
     /// Schedule the spawn of a new instance of the provided scene as a child of `parent`.
     pub fn spawn_as_child(&mut self, id: impl Into<Handle<Scene>>, parent: Entity) -> InstanceId {
         let instance_id = InstanceId::new();
-        self.scenes_to_spawn.push((id.into(), instance_id));
+        self.scenes_to_spawn
+            .push((id.into(), instance_id, Some(parent)));
         self.scenes_with_parent.push((instance_id, parent));
         instance_id
     }
@@ -296,7 +301,7 @@ impl SceneSpawner {
     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 (handle, instance_id) in scenes_to_spawn {
+        for (handle, instance_id, parent) in scenes_to_spawn {
             let mut entity_map = EntityHashMap::default();
 
             match Self::spawn_dynamic_internal(world, handle.id(), &mut entity_map) {
@@ -308,21 +313,41 @@ impl SceneSpawner {
                         .entry(handle.id())
                         .or_insert_with(HashSet::new);
                     spawned.insert(instance_id);
+
+                    // Scenes with parents need more setup before they are ready.
+                    // See `set_scene_instance_parent_sync()`.
+                    if parent.is_none() {
+                        world.send_event(SceneInstanceReady {
+                            id: instance_id,
+                            parent: None,
+                        });
+                    }
                 }
                 Err(SceneSpawnError::NonExistentScene { .. }) => {
-                    self.dynamic_scenes_to_spawn.push((handle, instance_id));
+                    self.dynamic_scenes_to_spawn
+                        .push((handle, instance_id, parent));
                 }
                 Err(err) => return Err(err),
             }
         }
 
         let scenes_to_spawn = std::mem::take(&mut self.scenes_to_spawn);
 
-        for (scene_handle, instance_id) in scenes_to_spawn {
+        for (scene_handle, instance_id, parent) in scenes_to_spawn {
             match self.spawn_sync_internal(world, scene_handle.id(), instance_id) {
-                Ok(_) => {}
+                Ok(_) => {
+                    // Scenes with parents need more setup before they are ready.
+                    // See `set_scene_instance_parent_sync()`.
+                    if parent.is_none() {
+                        world.send_event(SceneInstanceReady {
+                            id: instance_id,
+                            parent: None,
+                        });
+                    }
+                }
                 Err(SceneSpawnError::NonExistentRealScene { .. }) => {
-                    self.scenes_to_spawn.push((scene_handle, instance_id));
+                    self.scenes_to_spawn
+                        .push((scene_handle, instance_id, parent));
                 }
                 Err(err) => return Err(err),
             }
@@ -356,7 +381,10 @@ impl SceneSpawner {
                     }
                 }
 
-                world.send_event(SceneInstanceReady { parent });
+                world.send_event(SceneInstanceReady {
+                    id: instance_id,
+                    parent: Some(parent),
+                });
             } else {
                 self.scenes_with_parent.push((instance_id, parent));
             }
@@ -403,10 +431,10 @@ pub fn scene_spawner_system(world: &mut World) {
             });
         scene_spawner
             .dynamic_scenes_to_spawn
-            .retain(|(_, instance)| !dead_instances.contains(instance));
+            .retain(|(_, instance, _)| !dead_instances.contains(instance));
         scene_spawner
             .scenes_to_spawn
-            .retain(|(_, instance)| !dead_instances.contains(instance));
+            .retain(|(_, instance, _)| !dead_instances.contains(instance));
 
         let scene_asset_events = world.resource::<Events<AssetEvent<DynamicScene>>>();
 
@@ -438,6 +466,7 @@ pub fn scene_spawner_system(world: &mut World) {
 #[cfg(test)]
 mod tests {
     use bevy_app::App;
+    use bevy_asset::Handle;
     use bevy_asset::{AssetPlugin, AssetServer};
     use bevy_ecs::event::EventReader;
     use bevy_ecs::prelude::ReflectComponent;
@@ -505,28 +534,150 @@ mod tests {
     #[reflect(Component)]
     struct ComponentA;
 
-    #[test]
-    fn event() {
+    fn setup() -> App {
         let mut app = App::new();
         app.add_plugins((AssetPlugin::default(), ScenePlugin));
 
         app.register_type::<ComponentA>();
         app.world_mut().spawn(ComponentA);
         app.world_mut().spawn(ComponentA);
 
+        app
+    }
+
+    fn build_scene(app: &mut App) -> Handle<Scene> {
+        app.world_mut().run_system_once(
+            |world: &World,
+             type_registry: Res<'_, AppTypeRegistry>,
+             asset_server: Res<'_, AssetServer>| {
+                asset_server.add(
+                    Scene::from_dynamic_scene(&DynamicScene::from_world(world), &type_registry)
+                        .unwrap(),
+                )
+            },
+        )
+    }
+
+    #[test]
+    fn event_scene() {
+        let mut app = setup();
+
         // Build scene.
-        let scene =
+        let scene = build_scene(&mut app);
+
+        // Spawn scene.
+        let scene_id =
             app.world_mut()
-                .run_system_once(|world: &World, asset_server: Res<'_, AssetServer>| {
-                    asset_server.add(DynamicScene::from_world(world))
+                .run_system_once(move |mut scene_spawner: ResMut<'_, SceneSpawner>| {
+                    scene_spawner.spawn(scene.clone())
                 });
 
+        // Check for event arrival.
+        app.update();
+        app.world_mut().run_system_once(
+            move |mut ev_scene: EventReader<'_, '_, SceneInstanceReady>| {
+                let mut events = ev_scene.read();
+
+                let event = events.next().expect("found no `SceneInstanceReady` event");
+                assert_eq!(
+                    event.id, scene_id,
+                    "`SceneInstanceReady` contains the wrong `InstanceId`"
+                );
+
+                assert!(events.next().is_none(), "found more than one event");
+            },
+        );
+    }
+
+    #[test]
+    fn event_scene_as_child() {
+        let mut app = setup();
+
+        // Build scene.
+        let scene = build_scene(&mut app);
+
+        // Spawn scene as child.
+        let (scene_id, scene_entity) = app.world_mut().run_system_once(
+            move |mut commands: Commands<'_, '_>, mut scene_spawner: ResMut<'_, SceneSpawner>| {
+                let entity = commands.spawn_empty().id();
+                let id = scene_spawner.spawn_as_child(scene.clone(), entity);
+                (id, entity)
+            },
+        );
+
+        // Check for event arrival.
+        app.update();
+        app.world_mut().run_system_once(
+            move |mut ev_scene: EventReader<'_, '_, SceneInstanceReady>| {
+                let mut events = ev_scene.read();
+
+                let event = events.next().expect("found no `SceneInstanceReady` event");
+                assert_eq!(
+                    event.id, scene_id,
+                    "`SceneInstanceReady` contains the wrong `InstanceId`"
+                );
+                assert_eq!(
+                    event.parent,
+                    Some(scene_entity),
+                    "`SceneInstanceReady` contains the wrong parent entity"
+                );
+
+                assert!(events.next().is_none(), "found more than one event");
+            },
+        );
+    }
+
+    fn build_dynamic_scene(app: &mut App) -> Handle<DynamicScene> {
+        app.world_mut()
+            .run_system_once(|world: &World, asset_server: Res<'_, AssetServer>| {
+                asset_server.add(DynamicScene::from_world(world))
+            })
+    }
+
+    #[test]
+    fn event_dynamic_scene() {
+        let mut app = setup();
+
+        // Build scene.
+        let scene = build_dynamic_scene(&mut app);
+
         // Spawn scene.
-        let scene_entity = app.world_mut().run_system_once(
+        let scene_id =
+            app.world_mut()
+                .run_system_once(move |mut scene_spawner: ResMut<'_, SceneSpawner>| {
+                    scene_spawner.spawn_dynamic(scene.clone())
+                });
+
+        // Check for event arrival.
+        app.update();
+        app.world_mut().run_system_once(
+            move |mut ev_scene: EventReader<'_, '_, SceneInstanceReady>| {
+                let mut events = ev_scene.read();
+
+                let event = events.next().expect("found no `SceneInstanceReady` event");
+                assert_eq!(
+                    event.id, scene_id,
+                    "`SceneInstanceReady` contains the wrong `InstanceId`"
+                );
+
+                assert!(events.next().is_none(), "found more than one event");
+            },
+        );
+    }
+
+    #[test]
+    fn event_dynamic_scene_as_child() {
+        let mut app = setup();
+
+        // Build scene.
+        let scene = build_dynamic_scene(&mut app);
+
+        // Spawn scene as child.
+        let (scene_id, scene_entity) = app.world_mut().run_system_once(
             move |mut commands: Commands<'_, '_>, mut scene_spawner: ResMut<'_, SceneSpawner>| {
-                let scene_entity = commands.spawn_empty().id();
-                scene_spawner.spawn_dynamic_as_child(scene.clone(), scene_entity);
-                scene_entity
+                let entity = commands.spawn_empty().id();
+                let id = scene_spawner.spawn_dynamic_as_child(scene.clone(), entity);
+                (id, entity)
             },
         );
 
@@ -536,13 +687,17 @@ mod tests {
             move |mut ev_scene: EventReader<'_, '_, SceneInstanceReady>| {
                 let mut events = ev_scene.read();
 
+                let event = events.next().expect("found no `SceneInstanceReady` event");
                 assert_eq!(
-                    events.next().expect("found no `SceneInstanceReady` event"),
-                    &SceneInstanceReady {
-                        parent: scene_entity
-                    },
+                    event.id, scene_id,
+                    "`SceneInstanceReady` contains the wrong `InstanceId`"
+                );
+                assert_eq!(
+                    event.parent,
+                    Some(scene_entity),
                     "`SceneInstanceReady` contains the wrong parent entity"
                 );
+
                 assert!(events.next().is_none(), "found more than one event");
             },
         );