Filter UI traversal to only Node and GhostNode (#15746)

# Objective

With the warning removed in
https://github.com/bevyengine/bevy/pull/15736, the rules for the UI tree
changes.
We no longer need to traverse non `Node`/`GhostNode` entities.

## Solution

- Added a filter `Or<(With<Node>, With<GhostNode>)>` to the child
traversal query so we don't unnecessarily traverse nodes that are not
part of the UI tree (like text nodes).
- Also moved the warning for NoUI->UI entities so it is actually
triggered (see comments)

## Testing

- Ran unit tests (still passing)
- Ran the ghost_nodes and ui examples, still works and looks fine 👍 
- Tested the warning by spawning a Node under an empty entity.

---

---------

Co-authored-by: UkoeHB <37489173+UkoeHB@users.noreply.github.com>

crates/bevy_ui/src/ghost_hierarchy.rs

@@ -44,7 +44,12 @@ impl<'w, 's> UiRootNodes<'w, 's> {
 /// System param that gives access to UI children utilities, skipping over [`GhostNode`].
 #[derive(SystemParam)]
 pub struct UiChildren<'w, 's> {
-    ui_children_query: Query<'w, 's, (Option<&'static Children>, Option<&'static GhostNode>)>,
+    ui_children_query: Query<
+        'w,
+        's,
+        (Option<&'static Children>, Has<GhostNode>),
+        Or<(With<Node>, With<GhostNode>)>,
+    >,
     changed_children_query: Query<'w, 's, Entity, Changed<Children>>,
     children_query: Query<'w, 's, &'static Children>,
     ghost_nodes_query: Query<'w, 's, Entity, With<GhostNode>>,
@@ -101,11 +106,21 @@ impl<'w, 's> UiChildren<'w, 's> {
                 .iter_ghost_nodes(entity)
                 .any(|entity| self.changed_children_query.contains(entity))
     }
+
+    /// Returns `true` if the given entity is either a [`Node`] or a [`GhostNode`].
+    pub fn is_ui_node(&'s self, entity: Entity) -> bool {
+        self.ui_children_query.contains(entity)
+    }
 }
 
 pub struct UiChildrenIter<'w, 's> {
     stack: SmallVec<[Entity; 8]>,
-    query: &'s Query<'w, 's, (Option<&'static Children>, Option<&'static GhostNode>)>,
+    query: &'s Query<
+        'w,
+        's,
+        (Option<&'static Children>, Has<GhostNode>),
+        Or<(With<Node>, With<GhostNode>)>,
+    >,
 }
 
 impl<'w, 's> Iterator for UiChildrenIter<'w, 's> {
@@ -113,12 +128,13 @@ impl<'w, 's> Iterator for UiChildrenIter<'w, 's> {
     fn next(&mut self) -> Option<Self::Item> {
         loop {
             let entity = self.stack.pop()?;
-            let (children, ghost_node) = self.query.get(entity).ok()?;
+            if let Ok((children, has_ghost_node)) = self.query.get(entity) {
-            if ghost_node.is_none() {
+                if !has_ghost_node {
-                return Some(entity);
+                    return Some(entity);
-            }
+                }
-            if let Some(children) = children {
+                if let Some(children) = children {
-                self.stack.extend(children.iter().rev().copied());
+                    self.stack.extend(children.iter().rev().copied());
+                }
             }
         }
     }
@@ -186,10 +202,12 @@ mod tests {
         let n9 = world.spawn((A(9), GhostNode)).id();
         let n10 = world.spawn((A(10), NodeBundle::default())).id();
 
+        let no_ui = world.spawn_empty().id();
+
         world.entity_mut(n1).add_children(&[n2, n3, n4, n6]);
         world.entity_mut(n2).add_children(&[n5]);
 
-        world.entity_mut(n6).add_children(&[n7, n9]);
+        world.entity_mut(n6).add_children(&[n7, no_ui, n9]);
         world.entity_mut(n7).add_children(&[n8]);
         world.entity_mut(n9).add_children(&[n10]);
 

crates/bevy_ui/src/layout/mod.rs

@@ -11,7 +11,7 @@ use bevy_ecs::{
     system::{Commands, Local, Query, Res, ResMut, SystemParam},
     world::Ref,
 };
-use bevy_hierarchy::Children;
+use bevy_hierarchy::{Children, Parent};
 use bevy_math::{UVec2, Vec2};
 use bevy_render::camera::{Camera, NormalizedRenderTarget};
 use bevy_sprite::BorderRect;
@@ -114,7 +114,7 @@ pub fn ui_layout_system(
         ),
         With<Node>,
     >,
-    node_query: Query<Entity, With<Node>>,
+    node_query: Query<(Entity, Option<Ref<Parent>>), With<Node>>,
     ui_children: UiChildren,
     mut removed_components: UiLayoutSystemRemovedComponentParam,
     mut node_transform_query: Query<(
@@ -249,7 +249,19 @@ pub fn ui_layout_system(
         ui_surface.try_remove_children(entity);
     }
 
-    node_query.iter().for_each(|entity| {
+    node_query.iter().for_each(|(entity, maybe_parent)| {
+        if let Some(parent) = maybe_parent {
+            // Note: This does not cover the case where a parent's Node component was removed.
+            // Users are responsible for fixing hierarchies if they do that (it is not recommended).
+            // Detecting it here would be a permanent perf burden on the hot path.
+            if parent.is_changed() && !ui_children.is_ui_node(parent.get()) {
+                warn!(
+                    "Styled child ({entity}) in a non-UI entity hierarchy. You are using an entity \
+with UI components as a child of an entity without UI components, your UI layout may be broken."
+                );
+            }
+        }
+
         if ui_children.is_changed(entity) {
             ui_surface.update_children(entity, ui_children.iter_ui_children(entity));
         }
@@ -261,7 +273,7 @@ pub fn ui_layout_system(
     ui_surface.remove_entities(removed_components.removed_nodes.read());
 
     // Re-sync changed children: avoid layout glitches caused by removed nodes that are still set as a child of another node
-    node_query.iter().for_each(|entity| {
+    node_query.iter().for_each(|(entity, _)| {
         if ui_children.is_changed(entity) {
             ui_surface.update_children(entity, ui_children.iter_ui_children(entity));
         }

crates/bevy_ui/src/layout/ui_surface.rs

@@ -7,7 +7,7 @@ use bevy_ecs::{
     prelude::Resource,
 };
 use bevy_math::UVec2;
-use bevy_utils::{default, tracing::warn};
+use bevy_utils::default;
 
 use crate::{
     layout::convert, LayoutContext, LayoutError, Measure, MeasureArgs, NodeMeasure, Style,
@@ -289,10 +289,6 @@ impl UiSurface {
                 .layout(*taffy_node)
                 .map_err(LayoutError::TaffyError)
         } else {
-            warn!(
-                "Styled child ({entity}) in a non-UI entity hierarchy. You are using an entity \
-with UI components as a child of an entity without UI components, your UI layout may be broken."
-            );
             Err(LayoutError::InvalidHierarchy)
         }
     }