Merge BuildWorldChildren and BuildChildren traits. (#14052)

# Objective

The `BuildChildren` and `BuildWorldChildren` traits are mostly
identical, so I decided to try and merge them. I'm not sure of the
history, maybe they were added before GATs existed.

## Solution

- Add an associated type to `BuildChildren` which reflects the prior
differences between the `BuildChildren` and `BuildWorldChildren` traits.
- Add `ChildBuild` trait that is the bounds for
`BuildChildren::Builder`, with impls for `ChildBuilder` and
`WorldChildBuilder`.
- Remove `BuildWorldChildren` trait and replace it with an impl of
`BuildChildren` for `EntityWorldMut`.

## Testing

I ran several of the examples that use entity hierarchies, mainly UI.

---

## Changelog

n/a

## Migration Guide

n/a

crates/bevy_dev_tools/src/fps_overlay.rs

@@ -10,7 +10,7 @@ use bevy_ecs::{
     schedule::{common_conditions::resource_changed, IntoSystemConfigs},
     system::{Commands, Query, Res, Resource},
 };
-use bevy_hierarchy::BuildChildren;
+use bevy_hierarchy::{BuildChildren, ChildBuild};
 use bevy_text::{Font, Text, TextSection, TextStyle};
 use bevy_ui::{
     node_bundles::{NodeBundle, TextBundle},

crates/bevy_gltf/src/loader.rs

@@ -13,7 +13,7 @@ use bevy_core::Name;
 use bevy_core_pipeline::prelude::Camera3dBundle;
 use bevy_ecs::entity::EntityHashMap;
 use bevy_ecs::{entity::Entity, world::World};
-use bevy_hierarchy::{BuildWorldChildren, WorldChildBuilder};
+use bevy_hierarchy::{BuildChildren, ChildBuild, WorldChildBuilder};
 use bevy_math::{Affine2, Mat4, Vec3};
 use bevy_pbr::{
     DirectionalLight, DirectionalLightBundle, PbrBundle, PointLight, PointLightBundle, SpotLight,

crates/bevy_hierarchy/src/child_builder.rs

@@ -261,7 +261,7 @@ impl Command for RemoveParent {
 /// ```
 /// # use bevy_ecs::bundle::Bundle;
 /// # use bevy_ecs::system::Commands;
-/// # use bevy_hierarchy::BuildChildren;
+/// # use bevy_hierarchy::{ChildBuild, BuildChildren};
 /// # #[derive(Bundle)]
 /// # struct MyBundle {}
 /// # #[derive(Bundle)]
@@ -279,30 +279,55 @@ pub struct ChildBuilder<'a> {
     push_children: PushChildren,
 }
 
-impl ChildBuilder<'_> {
+/// Trait for building children entities and adding them to a parent entity. This is used in
+/// implementations of [`BuildChildren`] as a bound on the [`Builder`](BuildChildren::Builder)
+/// associated type. The closure passed to [`BuildChildren::with_children`] accepts an
+/// implementation of `ChildBuild` so that children can be spawned via [`ChildBuild::spawn`].
+pub trait ChildBuild {
+    /// Spawn output type. Both [`spawn`](Self::spawn) and [`spawn_empty`](Self::spawn_empty) return
+    /// an implementation of this type so that children can be operated on via method-chaining.
+    /// Implementations of `ChildBuild` reborrow `self` when spawning entities (see
+    /// [`Commands::spawn_empty`] and [`World::get_entity_mut`]). Lifetime `'a` corresponds to this
+    /// reborrowed self, and `Self` outlives it.
+    type SpawnOutput<'a>: BuildChildren
+    where
+        Self: 'a;
+
     /// Spawns an entity with the given bundle and inserts it into the parent entity's [`Children`].
     /// Also adds [`Parent`] component to the created entity.
-    pub fn spawn(&mut self, bundle: impl Bundle) -> EntityCommands {
+    fn spawn(&mut self, bundle: impl Bundle) -> Self::SpawnOutput<'_>;
+
+    /// Spawns an [`Entity`] with no components and inserts it into the parent entity's [`Children`].
+    /// Also adds [`Parent`] component to the created entity.
+    fn spawn_empty(&mut self) -> Self::SpawnOutput<'_>;
+
+    /// Returns the parent entity.
+    fn parent_entity(&self) -> Entity;
+
+    /// Adds a command to be executed, like [`Commands::add`].
+    fn add_command<C: Command>(&mut self, command: C) -> &mut Self;
+}
+
+impl ChildBuild for ChildBuilder<'_> {
+    type SpawnOutput<'a> = EntityCommands<'a> where Self: 'a;
+
+    fn spawn(&mut self, bundle: impl Bundle) -> EntityCommands {
         let e = self.commands.spawn(bundle);
         self.push_children.children.push(e.id());
         e
     }
 
-    /// Spawns an [`Entity`] with no components and inserts it into the parent entity's [`Children`].
+    fn spawn_empty(&mut self) -> EntityCommands {
-    /// Also adds [`Parent`] component to the created entity.
-    pub fn spawn_empty(&mut self) -> EntityCommands {
         let e = self.commands.spawn_empty();
         self.push_children.children.push(e.id());
         e
     }
 
-    /// Returns the parent entity of this [`ChildBuilder`].
+    fn parent_entity(&self) -> Entity {
-    pub fn parent_entity(&self) -> Entity {
         self.push_children.parent
     }
 
-    /// Adds a command to be executed, like [`Commands::add`].
+    fn add_command<C: Command>(&mut self, command: C) -> &mut Self {
-    pub fn add_command<C: Command>(&mut self, command: C) -> &mut Self {
         self.commands.add(command);
         self
     }
@@ -310,8 +335,12 @@ impl ChildBuilder<'_> {
 
 /// Trait for removing, adding and replacing children and parents of an entity.
 pub trait BuildChildren {
-    /// Takes a closure which builds children for this entity using [`ChildBuilder`].
+    /// Child builder type.
-    fn with_children(&mut self, f: impl FnOnce(&mut ChildBuilder)) -> &mut Self;
+    type Builder<'a>: ChildBuild;
+
+    /// Takes a closure which builds children for this entity using [`ChildBuild`].
+    fn with_children(&mut self, f: impl FnOnce(&mut Self::Builder<'_>)) -> &mut Self;
+
     /// Pushes children to the back of the builder's children. For any entities that are
     /// already a child of this one, this method does nothing.
     ///
@@ -323,6 +352,7 @@ pub trait BuildChildren {
     ///
     /// Panics if any of the children are the same as the parent.
     fn push_children(&mut self, children: &[Entity]) -> &mut Self;
+
     /// Inserts children at the given index.
     ///
     /// If the children were previously children of another parent, that parent's [`Children`] component
@@ -333,10 +363,12 @@ pub trait BuildChildren {
     ///
     /// Panics if any of the children are the same as the parent.
     fn insert_children(&mut self, index: usize, children: &[Entity]) -> &mut Self;
+
     /// Removes the given children
     ///
     /// Removing all children from a parent causes its [`Children`] component to be removed from the entity.
     fn remove_children(&mut self, children: &[Entity]) -> &mut Self;
+
     /// Adds a single child.
     ///
     /// If the children were previously children of another parent, that parent's [`Children`] component
@@ -347,8 +379,10 @@ pub trait BuildChildren {
     ///
     /// Panics if the child is the same as the parent.
     fn add_child(&mut self, child: Entity) -> &mut Self;
+
     /// Removes all children from this entity. The [`Children`] component will be removed if it exists, otherwise this does nothing.
     fn clear_children(&mut self) -> &mut Self;
+
     /// Removes all current children from this entity, replacing them with the specified list of entities.
     ///
     /// The removed children will have their [`Parent`] component removed.
@@ -357,6 +391,7 @@ pub trait BuildChildren {
     ///
     /// Panics if any of the children are the same as the parent.
     fn replace_children(&mut self, children: &[Entity]) -> &mut Self;
+
     /// Sets the parent of this entity.
     ///
     /// If this entity already had a parent, the parent's [`Children`] component will have this
@@ -367,6 +402,7 @@ pub trait BuildChildren {
     ///
     /// Panics if the parent is the same as the child.
     fn set_parent(&mut self, parent: Entity) -> &mut Self;
+
     /// Removes the [`Parent`] of this entity.
     ///
     /// Also removes this entity from its parent's [`Children`] component. Removing all children from a parent causes
@@ -375,7 +411,9 @@ pub trait BuildChildren {
 }
 
 impl BuildChildren for EntityCommands<'_> {
-    fn with_children(&mut self, spawn_children: impl FnOnce(&mut ChildBuilder)) -> &mut Self {
+    type Builder<'a> = ChildBuilder<'a>;
+
+    fn with_children(&mut self, spawn_children: impl FnOnce(&mut Self::Builder<'_>)) -> &mut Self {
         let parent = self.id();
         let mut builder = ChildBuilder {
             commands: self.commands(),
@@ -478,10 +516,10 @@ pub struct WorldChildBuilder<'w> {
     parent: Entity,
 }
 
-impl<'w> WorldChildBuilder<'w> {
+impl ChildBuild for WorldChildBuilder<'_> {
-    /// Spawns an entity with the given bundle and inserts it into the parent entity's [`Children`].
+    type SpawnOutput<'a> = EntityWorldMut<'a> where Self: 'a;
-    /// Also adds [`Parent`] component to the created entity.
+
-    pub fn spawn(&mut self, bundle: impl Bundle) -> EntityWorldMut<'_> {
+    fn spawn(&mut self, bundle: impl Bundle) -> EntityWorldMut {
         let entity = self.world.spawn((bundle, Parent(self.parent))).id();
         push_child_unchecked(self.world, self.parent, entity);
         push_events(
@@ -494,9 +532,7 @@ impl<'w> WorldChildBuilder<'w> {
         self.world.entity_mut(entity)
     }
 
-    /// Spawns an [`Entity`] with no components and inserts it into the parent entity's [`Children`].
+    fn spawn_empty(&mut self) -> EntityWorldMut {
-    /// Also adds [`Parent`] component to the created entity.
-    pub fn spawn_empty(&mut self) -> EntityWorldMut<'_> {
         let entity = self.world.spawn(Parent(self.parent)).id();
         push_child_unchecked(self.world, self.parent, entity);
         push_events(
@@ -509,83 +545,19 @@ impl<'w> WorldChildBuilder<'w> {
         self.world.entity_mut(entity)
     }
 
-    /// Returns the parent entity of this [`WorldChildBuilder`].
+    fn parent_entity(&self) -> Entity {
-    pub fn parent_entity(&self) -> Entity {
         self.parent
     }
+
+    fn add_command<C: Command>(&mut self, command: C) -> &mut Self {
+        command.apply(self.world);
+        self
+    }
 }
 
-/// Trait that defines adding, changing and children and parents of an entity directly through the [`World`].
+impl BuildChildren for EntityWorldMut<'_> {
-pub trait BuildWorldChildren {
+    type Builder<'a> = WorldChildBuilder<'a>;
-    /// Takes a closure which builds children for this entity using [`WorldChildBuilder`].
-    fn with_children(&mut self, spawn_children: impl FnOnce(&mut WorldChildBuilder)) -> &mut Self;
 
-    /// Adds a single child.
-    ///
-    /// If the children were previously children of another parent, that parent's [`Children`] component
-    /// will have those children removed from its list. Removing all children from a parent causes its
-    /// [`Children`] component to be removed from the entity.
-    ///
-    /// # Panics
-    ///
-    /// Panics if the child is the same as the parent.
-    fn add_child(&mut self, child: Entity) -> &mut Self;
-
-    /// Pushes children to the back of the builder's children. For any entities that are
-    /// already a child of this one, this method does nothing.
-    ///
-    /// If the children were previously children of another parent, that parent's [`Children`] component
-    /// will have those children removed from its list. Removing all children from a parent causes its
-    /// [`Children`] component to be removed from the entity.
-    ///
-    /// # Panics
-    ///
-    /// Panics if any of the children are the same as the parent.
-    fn push_children(&mut self, children: &[Entity]) -> &mut Self;
-    /// Inserts children at the given index.
-    ///
-    /// If the children were previously children of another parent, that parent's [`Children`] component
-    /// will have those children removed from its list. Removing all children from a parent causes its
-    /// [`Children`] component to be removed from the entity.
-    ///
-    /// # Panics
-    ///
-    /// Panics if any of the children are the same as the parent.
-    fn insert_children(&mut self, index: usize, children: &[Entity]) -> &mut Self;
-    /// Removes the given children
-    ///
-    /// Removing all children from a parent causes its [`Children`] component to be removed from the entity.
-    fn remove_children(&mut self, children: &[Entity]) -> &mut Self;
-
-    /// Sets the parent of this entity.
-    ///
-    /// If this entity already had a parent, the parent's [`Children`] component will have this
-    /// child removed from its list. Removing all children from a parent causes its [`Children`]
-    /// component to be removed from the entity.
-    ///
-    /// # Panics
-    ///
-    /// Panics if the parent is the same as the child.
-    fn set_parent(&mut self, parent: Entity) -> &mut Self;
-
-    /// Removes the [`Parent`] of this entity.
-    ///
-    /// Also removes this entity from its parent's [`Children`] component. Removing all children from a parent causes
-    /// its [`Children`] component to be removed from the entity.
-    fn remove_parent(&mut self) -> &mut Self;
-    /// Removes all children from this entity. The [`Children`] component will be removed if it exists, otherwise this does nothing.
-    fn clear_children(&mut self) -> &mut Self;
-    /// Removes all current children from this entity, replacing them with the specified list of entities.
-    ///
-    /// The removed children will have their [`Parent`] component removed.
-    ///
-    /// # Panics
-    ///
-    /// Panics if any of the children are the same as the parent.
-    fn replace_children(&mut self, children: &[Entity]) -> &mut Self;
-}
-
-impl<'w> BuildWorldChildren for EntityWorldMut<'w> {
     fn with_children(&mut self, spawn_children: impl FnOnce(&mut WorldChildBuilder)) -> &mut Self {
         let parent = self.id();
         self.world_scope(|world| {
@@ -691,7 +663,7 @@ impl<'w> BuildWorldChildren for EntityWorldMut<'w> {
 
 #[cfg(test)]
 mod tests {
-    use super::{BuildChildren, BuildWorldChildren};
+    use super::{BuildChildren, ChildBuild};
     use crate::{
         components::{Children, Parent},
         HierarchyEvent::{self, ChildAdded, ChildMoved, ChildRemoved},

crates/bevy_hierarchy/src/hierarchy.rs

@@ -146,7 +146,10 @@ mod tests {
     };
 
     use super::DespawnRecursiveExt;
-    use crate::{child_builder::BuildChildren, components::Children};
+    use crate::{
+        child_builder::{BuildChildren, ChildBuild},
+        components::Children,
+    };
 
     #[derive(Component, Clone, Copy, PartialEq, Eq, Ord, PartialOrd, Debug)]
     struct Idx(u32);

crates/bevy_hierarchy/src/lib.rs

@@ -16,7 +16,7 @@
 //! for managing parent-child relationships between entities.
 //! It provides two components, [`Parent`] and [`Children`],
 //! to store references to related entities.
-//! It also provides [command] and [world] API extensions
+//! It also provides [command and world] API extensions
 //! to set and clear those relationships.
 //!
 //! More advanced users may also appreciate
@@ -44,13 +44,12 @@
 //! In most cases, these operations will invalidate the hierarchy.
 //! Instead, you should use the provided [hierarchical despawn extension methods].
 //!
-//! [command]: BuildChildren
+//! [command and world]: BuildChildren
 //! [diagnostic plugin]: ValidParentCheckPlugin
 //! [events]: HierarchyEvent
 //! [hierarchical despawn extension methods]: DespawnRecursiveExt
 //! [plugin]: HierarchyPlugin
 //! [query extension methods]: HierarchyQueryExt
-//! [world]: BuildWorldChildren
 
 mod components;
 pub use components::*;

crates/bevy_hierarchy/src/query_extension.rs

@@ -161,7 +161,7 @@ mod tests {
         world::World,
     };
 
-    use crate::{query_extension::HierarchyQueryExt, BuildWorldChildren, Children, Parent};
+    use crate::{query_extension::HierarchyQueryExt, BuildChildren, Children, Parent};
 
     #[derive(Component, PartialEq, Debug)]
     struct A(usize);

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

@@ -501,7 +501,7 @@ mod test {
 
     use super::*;
 
-    use bevy_hierarchy::BuildWorldChildren;
+    use bevy_hierarchy::BuildChildren;
 
     fn visibility_bundle(visibility: Visibility) -> VisibilityBundle {
         VisibilityBundle {

crates/bevy_scene/src/scene_spawner.rs

@@ -8,7 +8,7 @@ use bevy_ecs::{
     system::Resource,
     world::{Command, Mut, World},
 };
-use bevy_hierarchy::{BuildWorldChildren, DespawnRecursiveExt, Parent, PushChild};
+use bevy_hierarchy::{BuildChildren, DespawnRecursiveExt, Parent, PushChild};
 use bevy_utils::{tracing::error, HashMap, HashSet};
 use thiserror::Error;
 use uuid::Uuid;

crates/bevy_transform/src/helper.rs

@@ -84,7 +84,7 @@ mod tests {
 
     use bevy_app::App;
     use bevy_ecs::system::SystemState;
-    use bevy_hierarchy::BuildWorldChildren;
+    use bevy_hierarchy::BuildChildren;
     use bevy_math::{Quat, Vec3};
 
     use crate::{

crates/bevy_transform/src/systems.rs

@@ -190,7 +190,7 @@ mod test {
 
     use crate::bundles::TransformBundle;
     use crate::systems::*;
-    use bevy_hierarchy::{BuildChildren, BuildWorldChildren};
+    use bevy_hierarchy::{BuildChildren, ChildBuild};
 
     #[test]
     fn correct_parent_removed() {

crates/bevy_ui/src/layout/mod.rs

@@ -357,7 +357,9 @@ mod tests {
     use bevy_ecs::schedule::Schedule;
     use bevy_ecs::system::RunSystemOnce;
     use bevy_ecs::world::World;
-    use bevy_hierarchy::{despawn_with_children_recursive, BuildWorldChildren, Children, Parent};
+    use bevy_hierarchy::{
+        despawn_with_children_recursive, BuildChildren, ChildBuild, Children, Parent,
+    };
     use bevy_math::{vec2, Rect, UVec2, Vec2};
     use bevy_render::camera::ManualTextureViews;
     use bevy_render::camera::OrthographicProjection;

crates/bevy_ui/src/stack.rs

@@ -158,7 +158,7 @@ mod tests {
         system::Commands,
         world::{CommandQueue, World},
     };
-    use bevy_hierarchy::BuildChildren;
+    use bevy_hierarchy::{BuildChildren, ChildBuild};
 
     use crate::{Node, UiStack, ZIndex};