From f630a11a6fbd43954ebd17d2b7930836f035a6f4 Mon Sep 17 00:00:00 2001
From: andriyDev <andriydzikh@gmail.com>
Date: Tue, 15 Jul 2025 17:02:52 -0700
Subject: [PATCH] Despawn scenes before hot reloading them.

---
 crates/bevy_scene/src/scene_spawner.rs | 26 ++++++++++++++++++++------
 1 file changed, 20 insertions(+), 6 deletions(-)

diff --git a/crates/bevy_scene/src/scene_spawner.rs b/crates/bevy_scene/src/scene_spawner.rs
index 56db96f4f5..a319470af7 100644
--- a/crates/bevy_scene/src/scene_spawner.rs
+++ b/crates/bevy_scene/src/scene_spawner.rs
@@ -236,15 +236,21 @@ impl SceneSpawner {
 
     /// Immediately despawns a scene instance, removing all its entities from the world.
     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() {
-                if let Ok(entity_mut) = world.get_entity_mut(entity) {
-                    entity_mut.despawn();
-                };
-            }
+        if let Some(mut instance) = self.spawned_instances.remove(instance_id) {
+            Self::despawn_instance_internal(world, &mut instance);
         }
     }
 
+    fn despawn_instance_internal(world: &mut World, instance: &mut InstanceInfo) {
+        for &entity in instance.entity_map.values() {
+            if let Ok(entity_mut) = world.get_entity_mut(entity) {
+                entity_mut.despawn();
+            };
+        }
+        // Just make sure if we reuse `InstanceInfo` for something, we don't reuse the despawned entities.
+        instance.entity_map.clear();
+    }
+
     /// Immediately spawns a new instance of the provided dynamic scene.
     pub fn spawn_dynamic_sync(
         &mut self,
@@ -324,6 +330,10 @@ impl SceneSpawner {
             if let Some(spawned_instances) = self.spawned_scenes.get(id) {
                 for instance_id in spawned_instances {
                     if let Some(instance_info) = self.spawned_instances.get_mut(instance_id) {
+                        // Despawn the scene before respawning it. This is a very heavy operation,
+                        // but otherwise, entities may be left behind, or be left in an otherwise
+                        // invalid state (e.g., invalid relationships).
+                        Self::despawn_instance_internal(world, instance_info);
                         Self::spawn_sync_internal(world, *id, &mut instance_info.entity_map)?;
                     }
                 }
@@ -345,6 +355,10 @@ impl SceneSpawner {
             if let Some(spawned_instances) = self.spawned_dynamic_scenes.get(id) {
                 for instance_id in spawned_instances {
                     if let Some(instance_info) = self.spawned_instances.get_mut(instance_id) {
+                        // Despawn the scene before respawning it. This is a very heavy operation,
+                        // but otherwise, entities may be left behind, or be left in an otherwise
+                        // invalid state (e.g., invalid relationships).
+                        Self::despawn_instance_internal(world, instance_info);
                         Self::spawn_dynamic_internal(world, *id, &mut instance_info.entity_map)?;
                     }
                 }