Split bevy_hierarchy out from bevy_transform (#4168)

# Objective

- Hierarchy tools are not just used for `Transform`: they are also used for scenes.
- In the future there's interest in using them for other features, such as visiibility inheritance.
- The fact that these tools are found in `bevy_transform` causes a great deal of user and developer confusion
- Fixes #2758.

## Solution

- Split `bevy_transform` into two!
- Make everything work again.

Note that this is a very tightly scoped PR: I *know* there are code quality and docs issues that existed in bevy_transform that I've just moved around. We should fix those in a seperate PR and try to merge this ASAP to reduce the bitrot involved in splitting an entire crate.

## Frustrations

The API around `GlobalTransform` is a mess: we have massive code and docs duplication, no link between the two types and no clear way to extend this to other forms of inheritance.

In the medium-term, I feel pretty strongly that `GlobalTransform` should be replaced by something like `Inherited<Transform>`, which lives in `bevy_hierarchy`:

- avoids code duplication
- makes the inheritance pattern extensible
- links the types at the type-level
- allows us to remove all references to inheritance from `bevy_transform`, making it more useful as a standalone crate and cleaning up its docs

## Additional context

- double-blessed by @cart in https://github.com/bevyengine/bevy/issues/4141#issuecomment-1063592414 and https://github.com/bevyengine/bevy/issues/2758#issuecomment-913810963
- preparation for more advanced / cleaner hierarchy tools: go read https://github.com/bevyengine/rfcs/pull/53 !
- originally attempted by @finegeometer in #2789. It was a great idea, just needed more discussion!

Co-authored-by: Carter Anderson <mcanders1@gmail.com>

crates/bevy_gltf/Cargo.toml

@@ -14,6 +14,7 @@ bevy_app = { path = "../bevy_app", version = "0.6.0" }
 bevy_asset = { path = "../bevy_asset", version = "0.6.0" }
 bevy_core = { path = "../bevy_core", version = "0.6.0" }
 bevy_ecs = { path = "../bevy_ecs", version = "0.6.0" }
+bevy_hierarchy = { path = "../bevy_hierarchy", version = "0.6.0" }
 bevy_pbr = { path = "../bevy_pbr", version = "0.6.0" }
 bevy_reflect = { path = "../bevy_reflect", version = "0.6.0", features = ["bevy"] }
 bevy_render = { path = "../bevy_render", version = "0.6.0" }

crates/bevy_gltf/src/loader.rs

@@ -4,6 +4,7 @@ use bevy_asset::{
 };
 use bevy_core::Name;
 use bevy_ecs::world::World;
+use bevy_hierarchy::{BuildWorldChildren, WorldChildBuilder};
 use bevy_log::warn;
 use bevy_math::{Mat4, Vec3};
 use bevy_pbr::{
@@ -24,11 +25,8 @@ use bevy_render::{
     view::VisibleEntities,
 };
 use bevy_scene::Scene;
-use bevy_transform::{
+use bevy_transform::{components::Transform, TransformBundle};
-    hierarchy::{BuildWorldChildren, WorldChildBuilder},
+
-    prelude::Transform,
-    TransformBundle,
-};
 use bevy_utils::{HashMap, HashSet};
 use gltf::{
     mesh::Mode,

crates/bevy_hierarchy/Cargo.toml

@@ -0,0 +1,19 @@
+[package]
+name = "bevy_hierarchy"
+version = "0.6.0"
+edition = "2021"
+description = "Provides hierarchy functionality for Bevy Engine"
+homepage = "https://bevyengine.org"
+repository = "https://github.com/bevyengine/bevy"
+license = "MIT OR Apache-2.0"
+keywords = ["bevy"]
+
+[dependencies]
+# bevy
+bevy_app = { path = "../bevy_app", version = "0.6.0" }
+bevy_ecs = { path = "../bevy_ecs", version = "0.6.0", features = ["bevy_reflect"] }
+bevy_reflect = { path = "../bevy_reflect", version = "0.6.0", features = ["bevy"] }
+bevy_utils = { path = "../bevy_utils", version = "0.6.0" }
+
+# other
+smallvec = { version = "1.6", features = ["serde", "union", "const_generics"] }

crates/bevy_hierarchy/src/components/mod.rs

@@ -0,0 +1,5 @@
+mod children;
+mod parent;
+
+pub use children::Children;
+pub use parent::{Parent, PreviousParent};

crates/bevy_hierarchy/src/hierarchy.rs

@@ -116,7 +116,7 @@ mod tests {
     };
 
     use super::DespawnRecursiveExt;
-    use crate::{components::Children, hierarchy::BuildChildren};
+    use crate::{child_builder::BuildChildren, components::Children};
 
     #[derive(Component, Clone, Copy, PartialEq, Eq, Ord, PartialOrd, Debug)]
     struct Idx(u32);

crates/bevy_hierarchy/src/lib.rs

@@ -0,0 +1,53 @@
+#![warn(missing_docs)]
+//! `bevy_hierarchy` can be used to define hierarchies of entities.
+//!
+//! Most commonly, these hierarchies are used for inheriting `Transform` values
+//! from the [`Parent`] to its [`Children`].
+
+mod components;
+pub use components::*;
+
+mod hierarchy;
+pub use hierarchy::*;
+
+mod child_builder;
+pub use child_builder::*;
+
+mod systems;
+pub use systems::*;
+
+#[doc(hidden)]
+pub mod prelude {
+    #[doc(hidden)]
+    pub use crate::{child_builder::*, components::*, hierarchy::*, HierarchyPlugin};
+}
+
+use bevy_app::prelude::*;
+use bevy_ecs::prelude::*;
+
+/// The base plugin for handling [`Parent`] and [`Children`] components
+#[derive(Default)]
+pub struct HierarchyPlugin;
+
+/// Label enum for the systems relating to hierarchy upkeep
+#[derive(Debug, Hash, PartialEq, Eq, Clone, SystemLabel)]
+pub enum HierarchySystem {
+    /// Updates [`Parent`] when changes in the hierarchy occur
+    ParentUpdate,
+}
+
+impl Plugin for HierarchyPlugin {
+    fn build(&self, app: &mut App) {
+        app.register_type::<Children>()
+            .register_type::<Parent>()
+            .register_type::<PreviousParent>()
+            .add_startup_system_to_stage(
+                StartupStage::PostStartup,
+                parent_update_system.label(HierarchySystem::ParentUpdate),
+            )
+            .add_system_to_stage(
+                CoreStage::PostUpdate,
+                parent_update_system.label(HierarchySystem::ParentUpdate),
+            );
+    }
+}

crates/bevy_hierarchy/src/systems.rs

@@ -70,101 +70,3 @@ pub fn parent_update_system(
         commands.entity(*e).insert(Children::with(v));
     });
 }
-#[cfg(test)]
-mod test {
-    use bevy_ecs::{
-        schedule::{Schedule, Stage, SystemStage},
-        system::CommandQueue,
-        world::World,
-    };
-
-    use super::*;
-    use crate::{hierarchy::BuildChildren, transform_propagate_system::transform_propagate_system};
-
-    #[test]
-    fn correct_children() {
-        let mut world = World::default();
-
-        let mut update_stage = SystemStage::parallel();
-        update_stage.add_system(parent_update_system);
-        update_stage.add_system(transform_propagate_system);
-
-        let mut schedule = Schedule::default();
-        schedule.add_stage("update", update_stage);
-
-        // Add parent entities
-        let mut command_queue = CommandQueue::default();
-        let mut commands = Commands::new(&mut command_queue, &world);
-        let mut children = Vec::new();
-        let parent = commands
-            .spawn()
-            .insert(Transform::from_xyz(1.0, 0.0, 0.0))
-            .id();
-        commands.entity(parent).with_children(|parent| {
-            children.push(
-                parent
-                    .spawn()
-                    .insert(Transform::from_xyz(0.0, 2.0, 0.0))
-                    .id(),
-            );
-            children.push(
-                parent
-                    .spawn()
-                    .insert(Transform::from_xyz(0.0, 3.0, 0.0))
-                    .id(),
-            );
-        });
-        command_queue.apply(&mut world);
-        schedule.run(&mut world);
-
-        assert_eq!(
-            world
-                .get::<Children>(parent)
-                .unwrap()
-                .0
-                .iter()
-                .cloned()
-                .collect::<Vec<_>>(),
-            children,
-        );
-
-        // Parent `e1` to `e2`.
-        (*world.get_mut::<Parent>(children[0]).unwrap()).0 = children[1];
-
-        schedule.run(&mut world);
-
-        assert_eq!(
-            world
-                .get::<Children>(parent)
-                .unwrap()
-                .iter()
-                .cloned()
-                .collect::<Vec<_>>(),
-            vec![children[1]]
-        );
-
-        assert_eq!(
-            world
-                .get::<Children>(children[1])
-                .unwrap()
-                .iter()
-                .cloned()
-                .collect::<Vec<_>>(),
-            vec![children[0]]
-        );
-
-        assert!(world.despawn(children[0]));
-
-        schedule.run(&mut world);
-
-        assert_eq!(
-            world
-                .get::<Children>(parent)
-                .unwrap()
-                .iter()
-                .cloned()
-                .collect::<Vec<_>>(),
-            vec![children[1]]
-        );
-    }
-}

crates/bevy_internal/Cargo.toml

@@ -56,6 +56,7 @@ bevy_core = { path = "../bevy_core", version = "0.6.0" }
 bevy_derive = { path = "../bevy_derive", version = "0.6.0" }
 bevy_diagnostic = { path = "../bevy_diagnostic", version = "0.6.0" }
 bevy_ecs = { path = "../bevy_ecs", version = "0.6.0" }
+bevy_hierarchy = { path = "../bevy_hierarchy", version = "0.6.0" }
 bevy_input = { path = "../bevy_input", version = "0.6.0" }
 bevy_log = { path = "../bevy_log", version = "0.6.0" }
 bevy_math = { path = "../bevy_math", version = "0.6.0" }

crates/bevy_internal/src/default_plugins.rs

@@ -4,6 +4,7 @@ use bevy_app::{PluginGroup, PluginGroupBuilder};
 /// * [`LogPlugin`](bevy_log::LogPlugin)
 /// * [`CorePlugin`](bevy_core::CorePlugin)
 /// * [`TransformPlugin`](bevy_transform::TransformPlugin)
+/// * [`HierarchyPlugin`](bevy_hierarchy::HierarchyPlugin)
 /// * [`DiagnosticsPlugin`](bevy_diagnostic::DiagnosticsPlugin)
 /// * [`InputPlugin`](bevy_input::InputPlugin)
 /// * [`WindowPlugin`](bevy_window::WindowPlugin)
@@ -27,6 +28,7 @@ impl PluginGroup for DefaultPlugins {
         group.add(bevy_log::LogPlugin::default());
         group.add(bevy_core::CorePlugin::default());
         group.add(bevy_transform::TransformPlugin::default());
+        group.add(bevy_hierarchy::HierarchyPlugin::default());
         group.add(bevy_diagnostic::DiagnosticsPlugin::default());
         group.add(bevy_input::InputPlugin::default());
         group.add(bevy_window::WindowPlugin::default());

crates/bevy_internal/src/lib.rs

@@ -65,6 +65,11 @@ pub mod tasks {
     pub use bevy_tasks::*;
 }
 
+pub mod hierarchy {
+    //! Entity hierarchies and property inheritance
+    pub use bevy_hierarchy::*;
+}
+
 pub mod transform {
     //! Local and global transforms (e.g. translation, scale, rotation).
     pub use bevy_transform::*;

crates/bevy_internal/src/prelude.rs

@@ -1,7 +1,7 @@
 #[doc(hidden)]
 pub use crate::{
-    app::prelude::*, asset::prelude::*, core::prelude::*, ecs::prelude::*, input::prelude::*,
+    app::prelude::*, asset::prelude::*, core::prelude::*, ecs::prelude::*, hierarchy::prelude::*,
-    log::prelude::*, math::prelude::*, reflect::prelude::*, scene::prelude::*,
+    input::prelude::*, log::prelude::*, math::prelude::*, reflect::prelude::*, scene::prelude::*,
     transform::prelude::*, utils::prelude::*, window::prelude::*, DefaultPlugins, MinimalPlugins,
 };
 

crates/bevy_render/src/view/visibility/mod.rs

@@ -6,7 +6,8 @@ use bevy_app::{CoreStage, Plugin};
 use bevy_asset::{Assets, Handle};
 use bevy_ecs::prelude::*;
 use bevy_reflect::Reflect;
-use bevy_transform::{components::GlobalTransform, TransformSystem};
+use bevy_transform::components::GlobalTransform;
+use bevy_transform::TransformSystem;
 
 use crate::{
     camera::{Camera, CameraProjection, OrthographicProjection, PerspectiveProjection},

crates/bevy_scene/Cargo.toml

@@ -14,7 +14,7 @@ bevy_app = { path = "../bevy_app", version = "0.6.0" }
 bevy_asset = { path = "../bevy_asset", version = "0.6.0" }
 bevy_ecs = { path = "../bevy_ecs", version = "0.6.0" }
 bevy_reflect = { path = "../bevy_reflect", version = "0.6.0", features = ["bevy"] }
-bevy_transform = { path = "../bevy_transform", version = "0.6.0" }
+bevy_hierarchy = { path = "../bevy_hierarchy", version = "0.6.0" }
 bevy_utils = { path = "../bevy_utils", version = "0.6.0" }
 
 # other

crates/bevy_scene/src/command.rs

@@ -4,7 +4,7 @@ use bevy_ecs::{
     system::{Command, Commands},
     world::World,
 };
-use bevy_transform::hierarchy::ChildBuilder;
+use bevy_hierarchy::ChildBuilder;
 
 use crate::{Scene, SceneSpawner};
 

crates/bevy_scene/src/scene_spawner.rs

@@ -7,8 +7,8 @@ use bevy_ecs::{
     system::Command,
     world::{Mut, World},
 };
+use bevy_hierarchy::{AddChild, Parent};
 use bevy_reflect::TypeRegistryArc;
-use bevy_transform::{hierarchy::AddChild, prelude::Parent};
 use bevy_utils::{tracing::error, HashMap};
 use thiserror::Error;
 use uuid::Uuid;

crates/bevy_transform/.gitignore

@@ -1,24 +0,0 @@
-book/book
-target
-Cargo.lock
-*.log
-
-# Backup files
-.DS_Store
-thumbs.db
-*~
-*.rs.bk
-*.swp
-
-# IDE / Editor files
-*.iml
-.idea
-.vscode
-
-
-#Added by cargo
-#
-#already existing elements are commented out
-
-/target
-**/*.rs.bk

crates/bevy_transform/Cargo.toml

@@ -2,7 +2,7 @@
 name = "bevy_transform"
 version = "0.6.0"
 edition = "2021"
-description = "Provides hierarchy and transform functionality for Bevy Engine"
+description = "Provides transform functionality for Bevy Engine"
 homepage = "https://bevyengine.org"
 repository = "https://github.com/bevyengine/bevy"
 license = "MIT OR Apache-2.0"
@@ -12,9 +12,6 @@ keywords = ["bevy"]
 # bevy
 bevy_app = { path = "../bevy_app", version = "0.6.0" }
 bevy_ecs = { path = "../bevy_ecs", version = "0.6.0", features = ["bevy_reflect"] }
+bevy_hierarchy = { path = "../bevy_hierarchy", version = "0.6.0"}
 bevy_math = { path = "../bevy_math", version = "0.6.0" }
 bevy_reflect = { path = "../bevy_reflect", version = "0.6.0", features = ["bevy"] }
-bevy_utils = { path = "../bevy_utils", version = "0.6.0" }
-
-# other
-smallvec = { version = "1.6", features = ["serde", "union", "const_generics"] }

crates/bevy_transform/src/components/global_transform.rs

@@ -14,12 +14,12 @@ use std::ops::Mul;
 /// ## [`Transform`] and [`GlobalTransform`]
 ///
 /// [`Transform`] is the position of an entity relative to its parent position, or the reference
-/// frame if it doesn't have a [`Parent`](super::Parent).
+/// frame if it doesn't have a [`Parent`](bevy_hierarchy::Parent).
 ///
 /// [`GlobalTransform`] is the position of an entity relative to the reference frame.
 ///
 /// [`GlobalTransform`] is updated from [`Transform`] in the system
-/// [`transform_propagate_system`](crate::transform_propagate_system::transform_propagate_system).
+/// [`transform_propagate_system`](crate::transform_propagate_system).
 ///
 /// This system runs in stage [`CoreStage::PostUpdate`](crate::CoreStage::PostUpdate). If you
 /// update the[`Transform`] of an entity in this stage or after, you will notice a 1 frame lag

crates/bevy_transform/src/components/mod.rs

@@ -1,9 +1,5 @@
-mod children;
 mod global_transform;
-mod parent;
 mod transform;
 
-pub use children::Children;
 pub use global_transform::*;
-pub use parent::{Parent, PreviousParent};
 pub use transform::*;

crates/bevy_transform/src/components/transform.rs

@@ -15,12 +15,12 @@ use std::ops::Mul;
 /// ## [`Transform`] and [`GlobalTransform`]
 ///
 /// [`Transform`] is the position of an entity relative to its parent position, or the reference
-/// frame if it doesn't have a [`Parent`](super::Parent).
+/// frame if it doesn't have a [`Parent`](bevy_hierarchy::Parent).
 ///
 /// [`GlobalTransform`] is the position of an entity relative to the reference frame.
 ///
 /// [`GlobalTransform`] is updated from [`Transform`] in the system
-/// [`transform_propagate_system`](crate::transform_propagate_system::transform_propagate_system).
+/// [`transform_propagate_system`](crate::transform_propagate_system).
 ///
 /// This system runs in stage [`CoreStage::PostUpdate`](crate::CoreStage::PostUpdate). If you
 /// update the[`Transform`] of an entity in this stage or after, you will notice a 1 frame lag

crates/bevy_transform/src/hierarchy/mod.rs

@@ -1,8 +0,0 @@
-mod child_builder;
-#[allow(clippy::module_inception)]
-mod hierarchy;
-mod hierarchy_maintenance_system;
-
-pub use child_builder::*;
-pub use hierarchy::*;
-pub use hierarchy_maintenance_system::*;

crates/bevy_transform/src/lib.rs

@@ -3,23 +3,19 @@
 
 /// The basic components of the transform crate
 pub mod components;
-/// Establishing and updating the transform hierarchy
+mod systems;
-pub mod hierarchy;
+pub use crate::systems::transform_propagate_system;
-/// Propagating transform changes down the transform hierarchy
-pub mod transform_propagate_system;
 
 #[doc(hidden)]
 pub mod prelude {
     #[doc(hidden)]
-    pub use crate::{components::*, hierarchy::*, TransformBundle, TransformPlugin};
+    pub use crate::{components::*, TransformBundle, TransformPlugin};
 }
 
 use bevy_app::prelude::*;
-use bevy_ecs::{
+use bevy_ecs::prelude::*;
-    bundle::Bundle,
+use bevy_hierarchy::HierarchySystem;
-    schedule::{ParallelSystemDescriptorCoercion, SystemLabel},
+use prelude::{GlobalTransform, Transform};
-};
-use prelude::{parent_update_system, Children, GlobalTransform, Parent, PreviousParent, Transform};
 
 /// A [`Bundle`] of the [`Transform`] and [`GlobalTransform`]
 /// [`Component`](bevy_ecs::component::Component)s, which describe the position of an entity.
@@ -32,12 +28,12 @@ use prelude::{parent_update_system, Children, GlobalTransform, Parent, PreviousP
 /// ## [`Transform`] and [`GlobalTransform`]
 ///
 /// [`Transform`] is the position of an entity relative to its parent position, or the reference
-/// frame if it doesn't have a [`Parent`](Parent).
+/// frame if it doesn't have a parent.
 ///
 /// [`GlobalTransform`] is the position of an entity relative to the reference frame.
 ///
 /// [`GlobalTransform`] is updated from [`Transform`] in the system
-/// [`transform_propagate_system`](crate::transform_propagate_system::transform_propagate_system).
+/// [`transform_propagate_system`].
 ///
 /// This system runs in stage [`CoreStage::PostUpdate`](crate::CoreStage::PostUpdate). If you
 /// update the[`Transform`] of an entity in this stage or after, you will notice a 1 frame lag
@@ -81,46 +77,33 @@ impl From<Transform> for TransformBundle {
         Self::from_transform(transform)
     }
 }
+/// Label enum for the systems relating to transform propagation
+#[derive(Debug, Hash, PartialEq, Eq, Clone, SystemLabel)]
+pub enum TransformSystem {
+    /// Propagates changes in transform to childrens' [`GlobalTransform`](crate::components::GlobalTransform)
+    TransformPropagate,
+}
+
 /// The base plugin for handling [`Transform`] components
 #[derive(Default)]
 pub struct TransformPlugin;
 
-/// Label enum for the types of systems relating to transform
-#[derive(Debug, Hash, PartialEq, Eq, Clone, SystemLabel)]
-pub enum TransformSystem {
-    /// Propagates changes in transform to childrens' [`GlobalTransform`]
-    TransformPropagate,
-    /// Updates [`Parent`] when changes in the hierarchy occur
-    ParentUpdate,
-}
-
 impl Plugin for TransformPlugin {
     fn build(&self, app: &mut App) {
-        app.register_type::<Children>()
+        app.register_type::<Transform>()
-            .register_type::<Parent>()
-            .register_type::<PreviousParent>()
-            .register_type::<Transform>()
             .register_type::<GlobalTransform>()
-            // add transform systems to startup so the first update is "correct"
+            // Adding these to startup ensures the first update is "correct"
             .add_startup_system_to_stage(
                 StartupStage::PostStartup,
-                parent_update_system.label(TransformSystem::ParentUpdate),
+                systems::transform_propagate_system
-            )
-            .add_startup_system_to_stage(
-                StartupStage::PostStartup,
-                transform_propagate_system::transform_propagate_system
                     .label(TransformSystem::TransformPropagate)
-                    .after(TransformSystem::ParentUpdate),
+                    .after(HierarchySystem::ParentUpdate),
             )
             .add_system_to_stage(
                 CoreStage::PostUpdate,
-                parent_update_system.label(TransformSystem::ParentUpdate),
+                systems::transform_propagate_system
-            )
-            .add_system_to_stage(
-                CoreStage::PostUpdate,
-                transform_propagate_system::transform_propagate_system
                     .label(TransformSystem::TransformPropagate)
-                    .after(TransformSystem::ParentUpdate),
+                    .after(HierarchySystem::ParentUpdate),
             );
     }
 }

crates/bevy_transform/src/systems.rs

@@ -1,9 +1,10 @@
-use crate::components::{Children, GlobalTransform, Parent, Transform};
+use crate::components::{GlobalTransform, Transform};
 use bevy_ecs::{
     entity::Entity,
     query::{Changed, With, Without},
     system::Query,
 };
+use bevy_hierarchy::{Children, Parent};
 
 /// Update [`GlobalTransform`] component of entities based on entity hierarchy and
 /// [`Transform`] component.
@@ -24,7 +25,7 @@ pub fn transform_propagate_system(
         }
 
         if let Some(children) = children {
-            for child in children.0.iter() {
+            for child in children.iter() {
                 propagate_recursive(
                     &global_transform,
                     &changed_transform_query,
@@ -60,7 +61,7 @@ fn propagate_recursive(
     };
 
     if let Ok(Some(children)) = children_query.get(entity) {
-        for child in children.0.iter() {
+        for child in children.iter() {
             propagate_recursive(
                 &global_matrix,
                 changed_transform_query,
@@ -81,10 +82,11 @@ mod test {
         world::World,
     };
 
-    use super::*;
+    use crate::components::{GlobalTransform, Transform};
-    use crate::{
+    use crate::systems::transform_propagate_system;
-        hierarchy::{parent_update_system, BuildChildren, BuildWorldChildren},
+    use crate::TransformBundle;
-        TransformBundle,
+    use bevy_hierarchy::{
+        parent_update_system, BuildChildren, BuildWorldChildren, Children, Parent,
     };
 
     #[test]
@@ -174,4 +176,90 @@ mod test {
             GlobalTransform::from_xyz(1.0, 0.0, 0.0) * Transform::from_xyz(0.0, 0.0, 3.0)
         );
     }
+
+    #[test]
+    fn correct_children() {
+        let mut world = World::default();
+
+        let mut update_stage = SystemStage::parallel();
+        update_stage.add_system(parent_update_system);
+        update_stage.add_system(transform_propagate_system);
+
+        let mut schedule = Schedule::default();
+        schedule.add_stage("update", update_stage);
+
+        // Add parent entities
+        let mut command_queue = CommandQueue::default();
+        let mut commands = Commands::new(&mut command_queue, &world);
+        let mut children = Vec::new();
+        let parent = commands
+            .spawn()
+            .insert(Transform::from_xyz(1.0, 0.0, 0.0))
+            .id();
+        commands.entity(parent).with_children(|parent| {
+            children.push(
+                parent
+                    .spawn()
+                    .insert(Transform::from_xyz(0.0, 2.0, 0.0))
+                    .id(),
+            );
+            children.push(
+                parent
+                    .spawn()
+                    .insert(Transform::from_xyz(0.0, 3.0, 0.0))
+                    .id(),
+            );
+        });
+        command_queue.apply(&mut world);
+        schedule.run(&mut world);
+
+        assert_eq!(
+            world
+                .get::<Children>(parent)
+                .unwrap()
+                .iter()
+                .cloned()
+                .collect::<Vec<_>>(),
+            children,
+        );
+
+        // Parent `e1` to `e2`.
+        (*world.get_mut::<Parent>(children[0]).unwrap()).0 = children[1];
+
+        schedule.run(&mut world);
+
+        assert_eq!(
+            world
+                .get::<Children>(parent)
+                .unwrap()
+                .iter()
+                .cloned()
+                .collect::<Vec<_>>(),
+            vec![children[1]]
+        );
+
+        assert_eq!(
+            world
+                .get::<Children>(children[1])
+                .unwrap()
+                .iter()
+                .cloned()
+                .collect::<Vec<_>>(),
+            vec![children[0]]
+        );
+
+        assert!(world.despawn(children[0]));
+
+        schedule.run(&mut world);
+
+        assert_eq!(
+            world
+                .get::<Children>(parent)
+                .unwrap()
+                .iter()
+                .cloned()
+                .collect::<Vec<_>>(),
+            vec![children[1]]
+        );
+    }
 }

crates/bevy_ui/Cargo.toml

@@ -16,6 +16,7 @@ bevy_core = { path = "../bevy_core", version = "0.6.0" }
 bevy_core_pipeline = { path = "../bevy_core_pipeline", version = "0.6.0" }
 bevy_derive = { path = "../bevy_derive", version = "0.6.0" }
 bevy_ecs = { path = "../bevy_ecs", version = "0.6.0" }
+bevy_hierarchy = { path = "../bevy_hierarchy", version = "0.6.0" }
 bevy_input = { path = "../bevy_input", version = "0.6.0" }
 bevy_log = { path = "../bevy_log", version = "0.6.0" }
 bevy_math = { path = "../bevy_math", version = "0.6.0" }

crates/bevy_ui/src/flex/mod.rs

@@ -7,9 +7,10 @@ use bevy_ecs::{
     query::{Changed, FilterFetch, With, Without, WorldQuery},
     system::{Query, Res, ResMut},
 };
+use bevy_hierarchy::{Children, Parent};
 use bevy_log::warn;
 use bevy_math::Vec2;
-use bevy_transform::prelude::{Children, Parent, Transform};
+use bevy_transform::components::Transform;
 use bevy_utils::HashMap;
 use bevy_window::{Window, WindowId, WindowScaleFactorChanged, Windows};
 use std::fmt;

crates/bevy_ui/src/update.rs

@@ -8,12 +8,10 @@ use bevy_ecs::{
     query::{With, Without},
     system::{Commands, Query},
 };
+use bevy_hierarchy::{Children, Parent};
 use bevy_math::Vec2;
 use bevy_sprite::Rect;
-use bevy_transform::{
+use bevy_transform::components::{GlobalTransform, Transform};
-    components::GlobalTransform,
-    prelude::{Children, Parent, Transform},
-};
 
 /// The resolution of Z values for UI
 pub const UI_Z_STEP: f32 = 0.001;
@@ -141,7 +139,8 @@ mod tests {
         system::{CommandQueue, Commands},
         world::World,
     };
-    use bevy_transform::{components::Transform, hierarchy::BuildChildren};
+    use bevy_hierarchy::BuildChildren;
+    use bevy_transform::components::Transform;
 
     use crate::Node;