Auto add default UI camera

crates/bevy_ui/src/lib.rs

@@ -1,11 +1,13 @@
 //! This crate contains Bevy's UI system, which can be used to create UI for both 2D and 3D games
 //! # Basic usage
 //! Spawn [`entity::UiCameraBundle`] and spawn UI elements with [`entity::ButtonBundle`], [`entity::ImageBundle`], [`entity::TextBundle`] and [`entity::NodeBundle`]
+//! When UI elements are present during startup, the [`entity::UiCameraBundle`] will be added automatically.
 //! This UI is laid out with the Flexbox paradigm (see <https://cssreference.io/flexbox/> ) except the vertical axis is inverted
 mod flex;
 mod focus;
 mod margins;
 mod render;
+mod startup;
 mod ui_node;
 
 pub mod entity;
@@ -72,6 +74,10 @@ impl Plugin for UiPlugin {
             .register_type::<Val>()
             .register_type::<widget::Button>()
             .register_type::<widget::ImageMode>()
+            .add_startup_system_to_stage(
+                StartupStage::PostStartup,
+                startup::add_default_ui_cam_if_needed,
+            )
             .add_system_to_stage(
                 CoreStage::PreUpdate,
                 ui_focus_system.label(UiSystem::Focus).after(InputSystem),

crates/bevy_ui/src/startup.rs

@@ -0,0 +1,96 @@
+use crate::prelude::UiCameraBundle;
+use crate::ui_node::Node;
+use crate::CAMERA_UI;
+use bevy_ecs::prelude::{Commands, Entity, Query, With};
+use bevy_render::prelude::{Camera, OrthographicProjection};
+
+pub fn add_default_ui_cam_if_needed(
+    mut commands: Commands,
+    node_query: Query<Entity, With<Node>>,
+    ui_cam_query: Query<&Camera, With<OrthographicProjection>>,
+) {
+    let world_contains_nodes = node_query.iter().next().is_some();
+    let world_contains_ui_cam = ui_cam_query
+        .iter()
+        .any(|cam| cam.name.as_deref() == Some(CAMERA_UI));
+
+    if world_contains_nodes && !world_contains_ui_cam {
+        commands.spawn_bundle(UiCameraBundle::default());
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use crate::entity::{NodeBundle, UiCameraBundle};
+    use crate::startup::add_default_ui_cam_if_needed;
+    use bevy_ecs::prelude::{Stage, SystemStage, World};
+    use bevy_render::prelude::{Camera, OrthographicProjection};
+
+    #[test]
+    fn no_ui_camera_added_when_no_ui_nodes_exist() {
+        let mut world = World::default();
+
+        let mut startup_stage = SystemStage::parallel();
+        startup_stage.add_system(add_default_ui_cam_if_needed);
+
+        startup_stage.run(&mut world);
+
+        assert_eq!(
+            world
+                .query::<(&Camera, &OrthographicProjection)>()
+                .iter(&world)
+                .len(),
+            0
+        );
+    }
+
+    #[test]
+    fn ui_camera_added_when_ui_nodes_exist() {
+        let mut world = World::default();
+
+        world.spawn().insert_bundle(NodeBundle::default());
+
+        let mut startup_stage = SystemStage::parallel();
+        startup_stage.add_system(add_default_ui_cam_if_needed);
+
+        startup_stage.run(&mut world);
+
+        assert_eq!(
+            world
+                .query::<(&Camera, &OrthographicProjection)>()
+                .iter(&world)
+                .len(),
+            1
+        );
+    }
+
+    #[test]
+    fn no_duplicate_ui_camera_added_when_one_is_already_present() {
+        let mut world = World::default();
+
+        let cam_id = world.spawn().insert_bundle(UiCameraBundle::default()).id();
+
+        assert_eq!(
+            world
+                .query::<(&Camera, &OrthographicProjection)>()
+                .iter(&world)
+                .len(),
+            1
+        );
+
+        let mut startup_stage = SystemStage::parallel();
+        startup_stage.add_system(add_default_ui_cam_if_needed);
+
+        startup_stage.run(&mut world);
+
+        assert_eq!(
+            world
+                .query::<(&Camera, &OrthographicProjection)>()
+                .iter(&world)
+                .len(),
+            1
+        );
+
+        assert!(world.get::<Camera>(cam_id).is_some());
+    }
+}

examples/2d/contributors.rs

@@ -118,7 +118,6 @@ fn setup_contributor_selection(mut commands: Commands, asset_server: Res<AssetSe
 
 fn setup(mut commands: Commands, asset_server: Res<AssetServer>) {
     commands.spawn_bundle(OrthographicCameraBundle::new_2d());
-    commands.spawn_bundle(UiCameraBundle::default());
 
     commands.spawn_bundle((SelectTimer, Timer::from_seconds(SHOWCASE_TIMER_SECS, true)));
 

examples/game/breakout.rs

@@ -50,7 +50,6 @@ fn setup(mut commands: Commands, asset_server: Res<AssetServer>) {
 
     // cameras
     commands.spawn_bundle(OrthographicCameraBundle::new_2d());
-    commands.spawn_bundle(UiCameraBundle::default());
     // paddle
     commands
         .spawn_bundle(SpriteBundle {

examples/scene/scene.rs

@@ -103,7 +103,6 @@ fn save_scene_system(world: &mut World) {
 // This is only necessary for the info message in the UI. See examples/ui/text.rs for a standalone
 // text example.
 fn infotext_system(mut commands: Commands, asset_server: Res<AssetServer>) {
-    commands.spawn_bundle(UiCameraBundle::default());
     commands.spawn_bundle(TextBundle {
         style: Style {
             align_self: AlignSelf::FlexEnd,

examples/tools/bevymark.rs

@@ -83,7 +83,6 @@ fn setup(mut commands: Commands, asset_server: Res<AssetServer>) {
     let texture = asset_server.load("branding/icon.png");
 
     commands.spawn_bundle(OrthographicCameraBundle::new_2d());
-    commands.spawn_bundle(UiCameraBundle::default());
     commands.spawn_bundle(TextBundle {
         text: Text {
             sections: vec![

examples/ui/button.rs

@@ -41,8 +41,6 @@ fn button_system(
 }
 
 fn setup(mut commands: Commands, asset_server: Res<AssetServer>) {
-    // ui camera
-    commands.spawn_bundle(UiCameraBundle::default());
     commands
         .spawn_bundle(ButtonBundle {
             style: Style {

examples/ui/font_atlas_debug.rs

@@ -79,7 +79,6 @@ fn text_update_system(mut state: ResMut<State>, time: Res<Time>, mut query: Quer
 fn setup(mut commands: Commands, asset_server: Res<AssetServer>, mut state: ResMut<State>) {
     let font_handle = asset_server.load("fonts/FiraSans-Bold.ttf");
     state.handle = font_handle.clone();
-    commands.spawn_bundle(UiCameraBundle::default());
     commands.spawn_bundle(TextBundle {
         text: Text::with_section(
             "a",

examples/ui/text.rs

@@ -25,8 +25,6 @@ struct FpsText;
 struct ColorText;
 
 fn setup(mut commands: Commands, asset_server: Res<AssetServer>) {
-    // UI camera
-    commands.spawn_bundle(UiCameraBundle::default());
     // Text with one section
     commands
         .spawn_bundle(TextBundle {

examples/ui/text_debug.rs

@@ -22,7 +22,6 @@ struct TextChanges;
 
 fn infotext_system(mut commands: Commands, asset_server: Res<AssetServer>) {
     let font = asset_server.load("fonts/FiraSans-Bold.ttf");
-    commands.spawn_bundle(UiCameraBundle::default());
     commands.spawn_bundle(TextBundle {
         style: Style {
             align_self: AlignSelf::FlexEnd,

examples/ui/ui.rs

@@ -13,9 +13,6 @@ fn main() {
 }
 
 fn setup(mut commands: Commands, asset_server: Res<AssetServer>) {
-    // ui camera
-    commands.spawn_bundle(UiCameraBundle::default());
-
     // root node
     commands
         .spawn_bundle(NodeBundle {

examples/window/scale_factor_override.rs

@@ -16,8 +16,6 @@ fn main() {
 }
 
 fn setup(mut commands: Commands, asset_server: Res<AssetServer>) {
-    // ui camera
-    commands.spawn_bundle(UiCameraBundle::default());
     // root node
     commands
         .spawn_bundle(NodeBundle {