Add with_child to simplify spawning when there will only be one child (#14594)

# Objective

This idea came up in the context of a hypothetical "text sections as
entities" where text sections are children of a text bundle.

```rust
commands
    .spawn(TextBundle::default())
    .with_children(|parent} {
        parent.spawn(TextSection::from("Hello"));
    });
```

This is a bit cumbersome (but powerful and probably the way things are
headed). [`bsn!`](https://github.com/bevyengine/bevy/discussions/14437)
will eventually make this nicer, but in the mean time, this might
improve ergonomics for the common case where there is only one
`TextSection`.

## Solution

Add a `with_child` method to the `BuildChildren` trait that spawns a
single bundle and adds it as a child to the entity.

```rust
commands
    .spawn(TextBundle::default())
    .with_child(TextSection::from("Hello"));
```

## Testing

I added some tests, and modified the `button` example to use the new
method.

If any potential co-authors want to improve the tests, that would be
great.

## Alternatives

- Some sort of macro. See
https://github.com/tigregalis/bevy_spans_ent/blob/main/examples/macro.rs#L20.
I don't love this, personally, and it would probably be obsoleted by
`bsn!`.
- Wait for `bsn!`
- Add `with_children_batch` that takes an `Into<Iterator>` of bundles.
  ```rust
  with_children_batch(vec![TextSection::from("Hello")])
  ```
This is maybe not as useful as it sounds -- it only works with
homogeneous bundles, so no marker components or styles.
- If this doesn't seem valuable, doing nothing is cool with me.

crates/bevy_hierarchy/src/child_builder.rs

@@ -341,6 +341,9 @@ pub trait BuildChildren {
     /// Takes a closure which builds children for this entity using [`ChildBuild`].
     fn with_children(&mut self, f: impl FnOnce(&mut Self::Builder<'_>)) -> &mut Self;
 
+    /// Spawns the passed bundle and adds it to this entity as a child.
+    fn with_child<B: Bundle>(&mut self, bundle: B) -> &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.
     ///
@@ -432,6 +435,13 @@ impl BuildChildren for EntityCommands<'_> {
         self
     }
 
+    fn with_child<B: Bundle>(&mut self, bundle: B) -> &mut Self {
+        let parent = self.id();
+        let child = self.commands().spawn(bundle).id();
+        self.commands().add(PushChild { parent, child });
+        self
+    }
+
     fn push_children(&mut self, children: &[Entity]) -> &mut Self {
         let parent = self.id();
         if children.contains(&parent) {
@@ -566,6 +576,17 @@ impl BuildChildren for EntityWorldMut<'_> {
         self
     }
 
+    fn with_child<B: Bundle>(&mut self, bundle: B) -> &mut Self {
+        let child = self.world_scope(|world| world.spawn(bundle).id());
+        if let Some(mut children_component) = self.get_mut::<Children>() {
+            children_component.0.retain(|value| child != *value);
+            children_component.0.push(child);
+        } else {
+            self.insert(Children::from_entities(&[child]));
+        }
+        self
+    }
+
     fn add_child(&mut self, child: Entity) -> &mut Self {
         let parent = self.id();
         if child == parent {
@@ -692,6 +713,14 @@ mod tests {
         assert_eq!(world.get::<Children>(parent).map(|c| &**c), children);
     }
 
+    /// Assert the number of children in the parent's [`Children`] component if it exists.
+    fn assert_num_children(world: &World, parent: Entity, num_children: usize) {
+        assert_eq!(
+            world.get::<Children>(parent).map(|c| c.len()).unwrap_or(0),
+            num_children
+        );
+    }
+
     /// Used to omit a number of events that are not relevant to a particular test.
     fn omit_events(world: &mut World, number: usize) {
         let mut events_resource = world.resource_mut::<Events<HierarchyEvent>>();
@@ -859,6 +888,19 @@ mod tests {
         assert_eq!(*world.get::<Parent>(children[1]).unwrap(), Parent(parent));
     }
 
+    #[test]
+    fn build_child() {
+        let mut world = World::default();
+        let mut queue = CommandQueue::default();
+        let mut commands = Commands::new(&mut queue, &world);
+
+        let parent = commands.spawn(C(1)).id();
+        commands.entity(parent).with_child(C(2));
+
+        queue.apply(&mut world);
+        assert_eq!(world.get::<Children>(parent).unwrap().0.len(), 1);
+    }
+
     #[test]
     fn push_and_insert_and_remove_children_commands() {
         let mut world = World::default();
@@ -1228,4 +1270,23 @@ mod tests {
         let children = query.get(&world, parent);
         assert!(children.is_err());
     }
+
+    #[test]
+    fn with_child() {
+        let world = &mut World::new();
+        world.insert_resource(Events::<HierarchyEvent>::default());
+
+        let a = world.spawn_empty().id();
+        let b = ();
+        let c = ();
+        let d = ();
+
+        world.entity_mut(a).with_child(b);
+
+        assert_num_children(world, a, 1);
+
+        world.entity_mut(a).with_child(c).with_child(d);
+
+        assert_num_children(world, a, 3);
+    }
 }

examples/ui/button.rs

@@ -83,15 +83,13 @@ fn setup(mut commands: Commands, asset_server: Res<AssetServer>) {
                     background_color: NORMAL_BUTTON.into(),
                     ..default()
                 })
-                .with_children(|parent| {
+                .with_child(TextBundle::from_section(
-                    parent.spawn(TextBundle::from_section(
+                    "Button",
-                        "Button",
+                    TextStyle {
-                        TextStyle {
+                        font: asset_server.load("fonts/FiraSans-Bold.ttf"),
-                            font: asset_server.load("fonts/FiraSans-Bold.ttf"),
+                        font_size: 40.0,
-                            font_size: 40.0,
+                        color: Color::srgb(0.9, 0.9, 0.9),
-                            color: Color::srgb(0.9, 0.9, 0.9),
+                    },
-                        },
+                ));
-                    ));
-                });
         });
 }