Cleaning up NodeBundle, and some slight UI module re-organization (#6473)

# Objective

`NodeBundle` contains an `image` field, which can be misleading, because if you do supply an image there, nothing will be shown to screen. You need to use an `ImageBundle` instead.

## Solution

* `image` (`UiImage`) field is removed from `NodeBundle`, 
* extraction stage queries now make an optional query for `UiImage`, if one is not found, use the image handle that is used as a default by `UiImage`: c019a60b39/crates/bevy_ui/src/ui_node.rs (L464)
* touching up docs for `NodeBundle` to help guide what `NodeBundle` should be used for
* renamed `entity.rs` to `node_bundle.rs` as that gives more of a hint regarding the module's purpose
* separating `camera_config` stuff from the pre-made UI node bundles so that `node_bundle.rs` makes more sense as a module name.

crates/bevy_ui/src/camera_config.rs

@@ -0,0 +1,37 @@
+//! Configuration for cameras related to UI.
+
+use bevy_ecs::component::Component;
+use bevy_ecs::prelude::With;
+use bevy_ecs::query::QueryItem;
+use bevy_render::camera::Camera;
+use bevy_render::extract_component::ExtractComponent;
+
+/// Configuration for cameras related to UI.
+///
+/// When a [`Camera`] doesn't have the [`UiCameraConfig`] component,
+/// it will display the UI by default.
+///
+/// [`Camera`]: bevy_render::camera::Camera
+#[derive(Component, Clone)]
+pub struct UiCameraConfig {
+    /// Whether to output UI to this camera view.
+    ///
+    /// When a `Camera` doesn't have the [`UiCameraConfig`] component,
+    /// it will display the UI by default.
+    pub show_ui: bool,
+}
+
+impl Default for UiCameraConfig {
+    fn default() -> Self {
+        Self { show_ui: true }
+    }
+}
+
+impl ExtractComponent for UiCameraConfig {
+    type Query = &'static Self;
+    type Filter = With<Camera>;
+
+    fn extract_component(item: QueryItem<'_, Self::Query>) -> Self {
+        item.clone()
+    }
+}

crates/bevy_ui/src/focus.rs

@@ -1,4 +1,4 @@
-use crate::{entity::UiCameraConfig, CalculatedClip, Node, UiStack};
+use crate::{camera_config::UiCameraConfig, CalculatedClip, Node, UiStack};
 use bevy_ecs::{
     entity::Entity,
     prelude::Component,

crates/bevy_ui/src/lib.rs

@@ -1,6 +1,6 @@
 //! This crate contains Bevy's UI system, which can be used to create UI for both 2D and 3D games
 //! # Basic usage
-//! Spawn UI elements with [`entity::ButtonBundle`], [`entity::ImageBundle`], [`entity::TextBundle`] and [`entity::NodeBundle`]
+//! Spawn UI elements with [`node_bundles::ButtonBundle`], [`node_bundles::ImageBundle`], [`node_bundles::TextBundle`] and [`node_bundles::NodeBundle`]
 //! This UI is laid out with the Flexbox paradigm (see <https://cssreference.io/flexbox/>)
 mod flex;
 mod focus;
@@ -9,7 +9,8 @@ mod render;
 mod stack;
 mod ui_node;
 
-pub mod entity;
+pub mod camera_config;
+pub mod node_bundles;
 pub mod update;
 pub mod widget;
 
@@ -23,7 +24,10 @@ pub use ui_node::*;
 #[doc(hidden)]
 pub mod prelude {
     #[doc(hidden)]
-    pub use crate::{entity::*, geometry::*, ui_node::*, widget::Button, Interaction, UiScale};
+    pub use crate::{
+        camera_config::*, geometry::*, node_bundles::*, ui_node::*, widget::Button, Interaction,
+        UiScale,
+    };
 }
 
 use bevy_app::prelude::*;

crates/bevy_ui/src/node_bundles.rs

@@ -1,17 +1,11 @@
-//! This module contains the bundles used in Bevy's UI
+//! This module contains basic node bundles used to build UIs
 
 use crate::{
     widget::{Button, ImageMode},
     BackgroundColor, CalculatedSize, FocusPolicy, Interaction, Node, Style, UiImage, ZIndex,
 };
-use bevy_ecs::{
+use bevy_ecs::bundle::Bundle;
-    bundle::Bundle,
-    prelude::{Component, With},
-    query::QueryItem,
-};
 use bevy_render::{
-    camera::Camera,
-    extract_component::ExtractComponent,
     prelude::{Color, ComputedVisibility},
     view::Visibility,
 };
@@ -19,6 +13,8 @@ use bevy_text::{Text, TextAlignment, TextSection, TextStyle};
 use bevy_transform::prelude::{GlobalTransform, Transform};
 
 /// The basic UI node
+///
+/// Useful as a container for a variety of child nodes.
 #[derive(Bundle, Clone, Debug)]
 pub struct NodeBundle {
     /// Describes the size of the node
@@ -27,8 +23,6 @@ pub struct NodeBundle {
     pub style: Style,
     /// The background color, which serves as a "fill" for this node
     pub background_color: BackgroundColor,
-    /// Describes the image of the node
-    pub image: UiImage,
     /// Whether this node should block interaction with lower nodes
     pub focus_policy: FocusPolicy,
     /// The transform of the node
@@ -56,7 +50,6 @@ impl Default for NodeBundle {
             background_color: Color::NONE.into(),
             node: Default::default(),
             style: Default::default(),
-            image: Default::default(),
             focus_policy: Default::default(),
             transform: Default::default(),
             global_transform: Default::default(),
@@ -241,32 +234,3 @@ impl Default for ButtonBundle {
         }
     }
 }
-/// Configuration for cameras related to UI.
-///
-/// When a [`Camera`] doesn't have the [`UiCameraConfig`] component,
-/// it will display the UI by default.
-///
-/// [`Camera`]: bevy_render::camera::Camera
-#[derive(Component, Clone)]
-pub struct UiCameraConfig {
-    /// Whether to output UI to this camera view.
-    ///
-    /// When a `Camera` doesn't have the [`UiCameraConfig`] component,
-    /// it will display the UI by default.
-    pub show_ui: bool,
-}
-
-impl Default for UiCameraConfig {
-    fn default() -> Self {
-        Self { show_ui: true }
-    }
-}
-
-impl ExtractComponent for UiCameraConfig {
-    type Query = &'static Self;
-    type Filter = With<Camera>;
-
-    fn extract_component(item: QueryItem<'_, Self::Query>) -> Self {
-        item.clone()
-    }
-}

crates/bevy_ui/src/render/mod.rs

@@ -11,6 +11,7 @@ use bevy_asset::{load_internal_asset, AssetEvent, Assets, Handle, HandleUntyped}
 use bevy_ecs::prelude::*;
 use bevy_math::{Mat4, Rect, UVec4, Vec2, Vec3, Vec4Swizzles};
 use bevy_reflect::TypeUuid;
+use bevy_render::texture::DEFAULT_IMAGE_HANDLE;
 use bevy_render::{
     camera::Camera,
     color::Color,
@@ -209,7 +210,7 @@ pub fn extract_uinodes(
             &Node,
             &GlobalTransform,
             &BackgroundColor,
-            &UiImage,
+            Option<&UiImage>,
             &ComputedVisibility,
             Option<&CalculatedClip>,
         )>,
@@ -218,11 +219,19 @@ pub fn extract_uinodes(
     let scale_factor = windows.scale_factor(WindowId::primary()) as f32;
     extracted_uinodes.uinodes.clear();
     for (stack_index, entity) in ui_stack.uinodes.iter().enumerate() {
-        if let Ok((uinode, transform, color, image, visibility, clip)) = uinode_query.get(*entity) {
+        if let Ok((uinode, transform, color, maybe_image, visibility, clip)) =
+            uinode_query.get(*entity)
+        {
             if !visibility.is_visible() {
                 continue;
             }
-            let image = image.0.clone_weak();
+
+            let image = if let Some(image) = maybe_image {
+                image.0.clone_weak()
+            } else {
+                DEFAULT_IMAGE_HANDLE.typed().clone_weak()
+            };
+
             // Skip loading images
             if !images.contains(&image) {
                 continue;
@@ -231,6 +240,7 @@ pub fn extract_uinodes(
             if color.0.a() == 0.0 {
                 continue;
             }
+
             extracted_uinodes.uinodes.push(ExtractedUiNode {
                 stack_index,
                 transform: transform.compute_matrix(),