Insert children

crates/bevy_ecs/src/hierarchy.rs

@@ -185,6 +185,33 @@ impl<'w> EntityWorldMut<'w> {
         self.add_related::<ChildOf>(children)
     }
 
+    /// Insert children at specific index
+    pub fn insert_children(&mut self, index: usize, children: &[Entity]) -> &mut Self {
+        let parent = self.id();
+        if children.contains(&parent) {
+            panic!("Cannot insert entity as a child of itself.");
+        }
+        if let Some(mut children_component) = self.get_mut::<Children>() {
+            children_component
+                .0
+                .retain(|value| !children.contains(value));
+            if index >= children_component.len() {
+                panic!(
+                    "Index {} out of bounds! There are only {} children!",
+                    index,
+                    children.len()
+                );
+            }
+            children_component.0.reserve(children.len());
+            let mut v = children_component.0.split_off(index);
+            children_component.0.extend_from_slice(children);
+            children_component.0.append(&mut v);
+        } else {
+            self.insert(Children(children.to_vec()));
+        }
+        self
+    }
+
     /// Adds the given child to this entity
     pub fn add_child(&mut self, child: Entity) -> &mut Self {
         self.add_related::<ChildOf>(&[child])
@@ -416,6 +443,35 @@ mod tests {
         );
     }
 
+    #[test]
+    fn insert_children() {
+        let mut world = World::new();
+        let child1 = world.spawn_empty().id();
+        let child2 = world.spawn_empty().id();
+        let child3 = world.spawn_empty().id();
+        let child4 = world.spawn_empty().id();
+
+        let mut entity_world_mut = world.spawn_empty();
+
+        let first_children = entity_world_mut.add_children(&[child1, child2]);
+
+        let root = first_children.insert_children(1, &[child3, child4]).id();
+
+        let hierarchy = get_hierarchy(&world, root);
+        assert_eq!(
+            hierarchy,
+            Node::new_with(
+                root,
+                vec![
+                    Node::new(child1),
+                    Node::new(child3),
+                    Node::new(child4),
+                    Node::new(child2)
+                ]
+            )
+        );
+    }
+
     #[test]
     fn self_parenting_invalid() {
         let mut world = World::new();