Also fix for entities without children

crates/bevy_transform/src/lib.rs

@@ -86,6 +86,7 @@ pub enum TransformSystem {
 pub fn transform_propagate_system_set() -> SystemSet {
     SystemSet::new()
         .with_system(systems::sync_simple_transforms)
+        .with_system(systems::sync_simple_transforms_orphaned)
         .with_system(systems::propagate_transforms)
 }
 
@@ -103,6 +104,10 @@ impl Plugin for TransformPlugin {
                 StartupStage::PostStartup,
                 systems::sync_simple_transforms.label(TransformSystem::TransformPropagate),
             )
+            .add_startup_system_to_stage(
+                StartupStage::PostStartup,
+                systems::sync_simple_transforms_orphaned.label(TransformSystem::TransformPropagate),
+            )
             .add_startup_system_to_stage(
                 StartupStage::PostStartup,
                 systems::propagate_transforms.label(TransformSystem::TransformPropagate),
@@ -111,6 +116,10 @@ impl Plugin for TransformPlugin {
                 CoreStage::PostUpdate,
                 systems::sync_simple_transforms.label(TransformSystem::TransformPropagate),
             )
+            .add_system_to_stage(
+                CoreStage::PostUpdate,
+                systems::sync_simple_transforms_orphaned.label(TransformSystem::TransformPropagate),
+            )
             .add_system_to_stage(
                 CoreStage::PostUpdate,
                 systems::propagate_transforms.label(TransformSystem::TransformPropagate),

crates/bevy_transform/src/systems.rs

@@ -20,6 +20,21 @@ pub fn sync_simple_transforms(
         *global_transform = GlobalTransform::from(*transform);
     });
 }
+/// Update [`GlobalTransform`] component of entities that aren't in the hierarchy
+/// and lost their parent last frame.
+///
+/// Third party plugins should use [`transform_propagate_system_set`](crate::transform_propagate_system_set)
+/// to propagate transforms correctly.
+pub fn sync_simple_transforms_orphaned(
+    mut query: Query<(&Transform, &mut GlobalTransform), Without<Children>>,
+    changed: Query<(), Changed<Transform>>,
+    orphaned: RemovedComponents<Parent>,
+) {
+    let mut iter = query.iter_many_mut(orphaned.iter().filter(|e| !changed.contains(*e)));
+    while let Some((transform, mut global_transform)) = iter.fetch_next() {
+        *global_transform = GlobalTransform::from(*transform);
+    }
+}
 
 /// Update [`GlobalTransform`] component of entities based on entity hierarchy and
 /// [`Transform`] component.
@@ -181,6 +196,10 @@ mod test {
     fn correct_parent_removed() {
         ComputeTaskPool::init(TaskPool::default);
         let mut world = World::default();
+        let offset_global_transform =
+            |offset| GlobalTransform::from(Transform::from_xyz(offset, offset, offset));
+        let offset_transform =
+            |offset| TransformBundle::from_transform(Transform::from_xyz(offset, offset, offset));
 
         let mut update_stage = SystemStage::parallel();
         update_stage.add_system_set(transform_propagate_system_set());
@@ -190,13 +209,9 @@ mod test {
 
         let mut command_queue = CommandQueue::default();
         let mut commands = Commands::new(&mut command_queue, &world);
-        let root = commands
-            .spawn(TransformBundle::from_transform(Transform::from_xyz(
-                9.9, 9.9, 9.9,
-            )))
-            .id();
-        let parent = commands.spawn(TransformBundle::IDENTITY).id();
-        let child = commands.spawn(TransformBundle::IDENTITY).id();
+        let root = commands.spawn(offset_transform(3.3)).id();
+        let parent = commands.spawn(offset_transform(4.4)).id();
+        let child = commands.spawn(offset_transform(5.5)).id();
         commands.entity(parent).set_parent(root);
         commands.entity(child).set_parent(parent);
         command_queue.apply(&mut world);
@@ -216,7 +231,19 @@ mod test {
 
         assert_eq!(
             world.get::<GlobalTransform>(parent).unwrap(),
-            &GlobalTransform::from(Transform::IDENTITY)
+            &offset_global_transform(4.4)
+        );
+
+        // Remove parent of `child`
+        let mut command_queue = CommandQueue::default();
+        let mut commands = Commands::new(&mut command_queue, &world);
+        commands.entity(child).remove_parent();
+        command_queue.apply(&mut world);
+        schedule.run(&mut world);
+
+        assert_eq!(
+            world.get::<GlobalTransform>(child).unwrap(),
+            &offset_global_transform(5.5)
         );
     }