Switch ChildOf back to tuple struct (#18672)

# Objective

In #17905 we swapped to a named field on `ChildOf` to help resolve
variable naming ambiguity of child vs parent (ex: `child_of.parent`
clearly reads as "I am accessing the parent of the child_of
relationship", whereas `child_of.0` is less clear).

Unfortunately this has the side effect of making initialization less
ideal. `ChildOf { parent }` reads just as well as `ChildOf(parent)`, but
`ChildOf { parent: root }` doesn't read nearly as well as
`ChildOf(root)`.

## Solution

Move back to `ChildOf(pub Entity)` but add a `child_of.parent()`
function and use it for all accesses. The downside here is that users
are no longer "forced" to access the parent field with `parent`
nomenclature, but I think this strikes the right balance.

Take a look at the diff. I think the results provide strong evidence for
this change. Initialization has the benefit of reading much better _and_
of taking up significantly less space, as many lines go from 3 to 1, and
we're cutting out a bunch of syntax in some cases.

Sadly I do think this should land in 0.16 as the cost of doing this
_after_ the relationships migration is high.

Author: cart

benches/benches/bevy_ecs/entity_cloning.rs

@@ -155,7 +155,7 @@ fn bench_clone_hierarchy<B: Bundle + Default + GetTypeRegistration>(
 
         for parent in current_hierarchy_level {
             for _ in 0..children {
-                let child_id = world.spawn((B::default(), ChildOf { parent })).id();
+                let child_id = world.spawn((B::default(), ChildOf(parent))).id();
                 hierarchy_level.push(child_id);
             }
         }

crates/bevy_ecs/src/entity/clone_entities.rs

@@ -1337,9 +1337,9 @@ mod tests {
     fn recursive_clone() {
         let mut world = World::new();
         let root = world.spawn_empty().id();
-        let child1 = world.spawn(ChildOf { parent: root }).id();
-        let grandchild = world.spawn(ChildOf { parent: child1 }).id();
-        let child2 = world.spawn(ChildOf { parent: root }).id();
+        let child1 = world.spawn(ChildOf(root)).id();
+        let grandchild = world.spawn(ChildOf(child1)).id();
+        let child2 = world.spawn(ChildOf(root)).id();
 
         let clone_root = world.spawn_empty().id();
         EntityCloner::build(&mut world)

crates/bevy_ecs/src/hierarchy.rs

@@ -54,9 +54,9 @@ use log::warn;
 /// # use bevy_ecs::prelude::*;
 /// # let mut world = World::new();
 /// let root = world.spawn_empty().id();
-/// let child1 = world.spawn(ChildOf { parent: root }).id();
-/// let child2 = world.spawn(ChildOf { parent: root }).id();
-/// let grandchild = world.spawn(ChildOf { parent: child1 }).id();
+/// let child1 = world.spawn(ChildOf(root)).id();
+/// let child2 = world.spawn(ChildOf(root)).id();
+/// let grandchild = world.spawn(ChildOf(child1)).id();
 ///
 /// assert_eq!(&**world.entity(root).get::<Children>().unwrap(), &[child1, child2]);
 /// assert_eq!(&**world.entity(child1).get::<Children>().unwrap(), &[grandchild]);
@@ -96,9 +96,21 @@ use log::warn;
 )]
 #[relationship(relationship_target = Children)]
 #[doc(alias = "IsChild", alias = "Parent")]
-pub struct ChildOf {
+pub struct ChildOf(pub Entity);
+
+impl ChildOf {
+    /// The parent entity of this child entity.
+    #[inline]
+    pub fn parent(&self) -> Entity {
+        self.0
+    }
+
     /// The parent entity of this child entity.
-    pub parent: Entity,
+    #[deprecated(since = "0.16.0", note = "Use child_of.parent() instead")]
+    #[inline]
+    pub fn get(&self) -> Entity {
+        self.0
+    }
 }
 
 // TODO: We need to impl either FromWorld or Default so ChildOf can be registered as Reflect.
@@ -108,9 +120,7 @@ pub struct ChildOf {
 impl FromWorld for ChildOf {
     #[inline(always)]
     fn from_world(_world: &mut World) -> Self {
-        ChildOf {
-            parent: Entity::PLACEHOLDER,
-        }
+        ChildOf(Entity::PLACEHOLDER)
     }
 }
 
@@ -316,7 +326,7 @@ impl<'w> EntityWorldMut<'w> {
     pub fn with_child(&mut self, bundle: impl Bundle) -> &mut Self {
         let parent = self.id();
         self.world_scope(|world| {
-            world.spawn((bundle, ChildOf { parent }));
+            world.spawn((bundle, ChildOf(parent)));
         });
         self
     }
@@ -329,12 +339,9 @@ impl<'w> EntityWorldMut<'w> {
     }
 
     /// Inserts the [`ChildOf`] component with the given `parent` entity, if it exists.
-    #[deprecated(
-        since = "0.16.0",
-        note = "Use entity_mut.insert(ChildOf { parent: entity })"
-    )]
+    #[deprecated(since = "0.16.0", note = "Use entity_mut.insert(ChildOf(entity))")]
     pub fn set_parent(&mut self, parent: Entity) -> &mut Self {
-        self.insert(ChildOf { parent });
+        self.insert(ChildOf(parent));
         self
     }
 }
@@ -394,7 +401,7 @@ impl<'a> EntityCommands<'a> {
     /// [`with_children`]: EntityCommands::with_children
     pub fn with_child(&mut self, bundle: impl Bundle) -> &mut Self {
         let parent = self.id();
-        self.commands.spawn((bundle, ChildOf { parent }));
+        self.commands.spawn((bundle, ChildOf(parent)));
         self
     }
 
@@ -406,12 +413,9 @@ impl<'a> EntityCommands<'a> {
     }
 
     /// Inserts the [`ChildOf`] component with the given `parent` entity, if it exists.
-    #[deprecated(
-        since = "0.16.0",
-        note = "Use entity_commands.insert(ChildOf { parent: entity })"
-    )]
+    #[deprecated(since = "0.16.0", note = "Use entity_commands.insert(ChildOf(entity))")]
     pub fn set_parent(&mut self, parent: Entity) -> &mut Self {
-        self.insert(ChildOf { parent });
+        self.insert(ChildOf(parent));
         self
     }
 }
@@ -427,7 +431,7 @@ pub fn validate_parent_has_component<C: Component>(
         return;
     };
     if !world
-        .get_entity(child_of.parent)
+        .get_entity(child_of.parent())
         .is_ok_and(|e| e.contains::<C>())
     {
         // TODO: print name here once Name lives in bevy_ecs
@@ -527,9 +531,9 @@ mod tests {
     fn hierarchy() {
         let mut world = World::new();
         let root = world.spawn_empty().id();
-        let child1 = world.spawn(ChildOf { parent: root }).id();
-        let grandchild = world.spawn(ChildOf { parent: child1 }).id();
-        let child2 = world.spawn(ChildOf { parent: root }).id();
+        let child1 = world.spawn(ChildOf(root)).id();
+        let grandchild = world.spawn(ChildOf(child1)).id();
+        let child2 = world.spawn(ChildOf(root)).id();
 
         // Spawn
         let hierarchy = get_hierarchy(&world, root);
@@ -550,7 +554,7 @@ mod tests {
         assert_eq!(hierarchy, Node::new_with(root, vec![Node::new(child2)]));
 
         // Insert
-        world.entity_mut(child1).insert(ChildOf { parent: root });
+        world.entity_mut(child1).insert(ChildOf(root));
         let hierarchy = get_hierarchy(&world, root);
         assert_eq!(
             hierarchy,
@@ -638,7 +642,7 @@ mod tests {
     fn self_parenting_invalid() {
         let mut world = World::new();
         let id = world.spawn_empty().id();
-        world.entity_mut(id).insert(ChildOf { parent: id });
+        world.entity_mut(id).insert(ChildOf(id));
         assert!(
             world.entity(id).get::<ChildOf>().is_none(),
             "invalid ChildOf relationships should self-remove"
@@ -650,7 +654,7 @@ mod tests {
         let mut world = World::new();
         let parent = world.spawn_empty().id();
         world.entity_mut(parent).despawn();
-        let id = world.spawn(ChildOf { parent }).id();
+        let id = world.spawn(ChildOf(parent)).id();
         assert!(
             world.entity(id).get::<ChildOf>().is_none(),
             "invalid ChildOf relationships should self-remove"
@@ -661,10 +665,10 @@ mod tests {
     fn reinsert_same_parent() {
         let mut world = World::new();
         let parent = world.spawn_empty().id();
-        let id = world.spawn(ChildOf { parent }).id();
-        world.entity_mut(id).insert(ChildOf { parent });
+        let id = world.spawn(ChildOf(parent)).id();
+        world.entity_mut(id).insert(ChildOf(parent));
         assert_eq!(
-            Some(&ChildOf { parent }),
+            Some(&ChildOf(parent)),
             world.entity(id).get::<ChildOf>(),
             "ChildOf should still be there"
         );
@@ -699,11 +703,11 @@ mod tests {
 
         assert_eq!(
             world.entity(child_a).get::<ChildOf>().unwrap(),
-            &ChildOf { parent }
+            &ChildOf(parent)
         );
         assert_eq!(
             world.entity(child_c).get::<ChildOf>().unwrap(),
-            &ChildOf { parent }
+            &ChildOf(parent)
         );
         assert!(world.entity(child_b).get::<ChildOf>().is_none());
     }
@@ -739,7 +743,7 @@ mod tests {
         assert_eq!(children.0, [child]);
         assert_eq!(
             world.entity(child).get::<ChildOf>().unwrap(),
-            &ChildOf { parent }
+            &ChildOf(parent)
         );
     }
 
@@ -762,11 +766,11 @@ mod tests {
 
         assert_eq!(
             world.entity(child_a).get::<ChildOf>().unwrap(),
-            &ChildOf { parent }
+            &ChildOf(parent)
         );
         assert_eq!(
             world.entity(child_b).get::<ChildOf>().unwrap(),
-            &ChildOf { parent }
+            &ChildOf(parent)
         );
         assert_eq!(
             world.entity(parent).get::<Children>().unwrap().0,
@@ -781,15 +785,15 @@ mod tests {
         );
         assert_eq!(
             world.entity(child_a).get::<ChildOf>().unwrap(),
-            &ChildOf { parent }
+            &ChildOf(parent)
         );
         assert_eq!(
             world.entity(child_c).get::<ChildOf>().unwrap(),
-            &ChildOf { parent }
+            &ChildOf(parent)
         );
         assert_eq!(
             world.entity(child_d).get::<ChildOf>().unwrap(),
-            &ChildOf { parent }
+            &ChildOf(parent)
         );
         assert_eq!(
             world.entity(parent).get::<Children>().unwrap().0,
@@ -844,11 +848,11 @@ mod tests {
 
         assert_eq!(
             world.entity(child_a).get::<ChildOf>().unwrap(),
-            &ChildOf { parent }
+            &ChildOf(parent)
         );
         assert_eq!(
             world.entity(child_b).get::<ChildOf>().unwrap(),
-            &ChildOf { parent }
+            &ChildOf(parent)
         );
         assert_eq!(
             world.entity(parent).get::<Children>().unwrap().0,
@@ -863,11 +867,11 @@ mod tests {
         );
         assert_eq!(
             world.entity(child_c).get::<ChildOf>().unwrap(),
-            &ChildOf { parent }
+            &ChildOf(parent)
         );
         assert_eq!(
             world.entity(child_d).get::<ChildOf>().unwrap(),
-            &ChildOf { parent }
+            &ChildOf(parent)
         );
         assert_eq!(
             world.entity(parent).get::<Children>().unwrap().0,
@@ -974,11 +978,10 @@ mod tests {
         let mut world = World::new();
         let parent = world.spawn_empty().id();
         let other = world.spawn_empty().id();
-        let child = world.spawn(ChildOf { parent }).id();
-        world.entity_mut(child).insert_with_relationship_hook_mode(
-            ChildOf { parent: other },
-            RelationshipHookMode::Skip,
-        );
+        let child = world.spawn(ChildOf(parent)).id();
+        world
+            .entity_mut(child)
+            .insert_with_relationship_hook_mode(ChildOf(other), RelationshipHookMode::Skip);
         assert_eq!(
             &**world.entity(parent).get::<Children>().unwrap(),
             &[child],

crates/bevy_ecs/src/relationship/related_methods.rs

@@ -521,9 +521,9 @@ mod tests {
         let mut world = World::new();
 
         let a = world.spawn_empty().id();
-        let b = world.spawn(ChildOf { parent: a }).id();
-        let c = world.spawn(ChildOf { parent: a }).id();
-        let d = world.spawn(ChildOf { parent: b }).id();
+        let b = world.spawn(ChildOf(a)).id();
+        let c = world.spawn(ChildOf(a)).id();
+        let d = world.spawn(ChildOf(b)).id();
 
         world
             .entity_mut(a)

crates/bevy_input_focus/src/lib.rs

@@ -165,7 +165,7 @@ impl<E: Event + Clone> Traversal<FocusedInput<E>> for WindowTraversal {
 
         // Send event to parent, if it has one.
         if let Some(child_of) = child_of {
-            return Some(child_of.parent);
+            return Some(child_of.parent());
         };
 
         // Otherwise, send it to the window entity (unless this is a window entity).
@@ -338,7 +338,7 @@ impl IsFocused for World {
             if e == entity {
                 return true;
             }
-            if let Some(parent) = self.entity(e).get::<ChildOf>().map(|c| c.parent) {
+            if let Some(parent) = self.entity(e).get::<ChildOf>().map(ChildOf::parent) {
                 e = parent;
             } else {
                 return false;

crates/bevy_input_focus/src/tab_navigation.rs

@@ -375,22 +375,8 @@ mod tests {
         let world = app.world_mut();
 
         let tab_group_entity = world.spawn(TabGroup::new(0)).id();
-        let tab_entity_1 = world
-            .spawn((
-                TabIndex(0),
-                ChildOf {
-                    parent: tab_group_entity,
-                },
-            ))
-            .id();
-        let tab_entity_2 = world
-            .spawn((
-                TabIndex(1),
-                ChildOf {
-                    parent: tab_group_entity,
-                },
-            ))
-            .id();
+        let tab_entity_1 = world.spawn((TabIndex(0), ChildOf(tab_group_entity))).id();
+        let tab_entity_2 = world.spawn((TabIndex(1), ChildOf(tab_group_entity))).id();
 
         let mut system_state: SystemState<TabNavigation> = SystemState::new(world);
         let tab_navigation = system_state.get(world);

crates/bevy_picking/src/events.rs

@@ -92,7 +92,7 @@ where
 
         // Send event to parent, if it has one.
         if let Some(child_of) = child_of {
-            return Some(child_of.parent);
+            return Some(child_of.parent());
         };
 
         // Otherwise, send it to the window entity (unless this is a window entity).

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

@@ -411,7 +411,7 @@ fn visibility_propagate_system(
             Visibility::Hidden => false,
             // fall back to true if no parent is found or parent lacks components
             Visibility::Inherited => child_of
-                .and_then(|c| visibility_query.get(c.parent).ok())
+                .and_then(|c| visibility_query.get(c.parent()).ok())
                 .is_none_or(|(_, x)| x.get()),
         };
         let (_, mut inherited_visibility) = visibility_query
@@ -786,9 +786,7 @@ mod test {
             .entity_mut(parent2)
             .insert(Visibility::Visible);
         // Simulate a change in the parent component
-        app.world_mut()
-            .entity_mut(child2)
-            .insert(ChildOf { parent: parent2 }); // example of changing parent
+        app.world_mut().entity_mut(child2).insert(ChildOf(parent2)); // example of changing parent
 
         // Run the system again to propagate changes
         app.update();

crates/bevy_scene/src/dynamic_scene.rs

@@ -325,7 +325,7 @@ mod tests {
                 .unwrap()
                 .get::<ChildOf>()
                 .unwrap()
-                .parent,
+                .parent(),
             "something about reloading the scene is touching entities with the same scene Ids"
         );
         assert_eq!(
@@ -335,7 +335,7 @@ mod tests {
                 .unwrap()
                 .get::<ChildOf>()
                 .unwrap()
-                .parent,
+                .parent(),
             "something about reloading the scene is touching components not defined in the scene but on entities defined in the scene"
         );
         assert_eq!(
@@ -345,7 +345,7 @@ mod tests {
                 .unwrap()
                 .get::<ChildOf>()
                 .expect("something is wrong with this test, and the scene components don't have a parent/child relationship")
-                .parent,
+                .parent(),
             "something is wrong with this test or the code reloading scenes since the relationship between scene entities is broken"
         );
     }