Warn if there is no UI camera

crates/bevy_core/src/lib.rs

@@ -37,6 +37,7 @@ impl Plugin for CorePlugin {
         app.init_resource::<Time>()
             .init_resource::<EntityLabels>()
             .init_resource::<FixedTimesteps>()
+            .init_resource::<ValidationConfig>()
             .register_type::<Option<String>>()
             .register_type::<Name>()
             .register_type::<Labels>()
@@ -47,3 +48,29 @@ impl Plugin for CorePlugin {
             .add_system_to_stage(stage::POST_UPDATE, entity_labels_system.system());
     }
 }
+
+/// Configures which validation checks are run.
+///
+/// By default, they are all enabled in debug mode but
+/// disabled in release mode.
+pub struct ValidationConfig {
+    /// Checks whether the world contains no ui camera but ui nodes.
+    pub missing_ui_cam: bool,
+    #[doc(hidden)]
+    #[allow(unused)]
+    // we cannot use `#[non_exhaustive]`, since that wouldn't allow `ValidationConfig { ..Default::default() }`
+    pub non_exhaustive: (),
+}
+
+impl Default for ValidationConfig {
+    fn default() -> Self {
+        #[cfg(debug_assertions)]
+        let default_value = true;
+        #[cfg(not(debug_assertions))]
+        let default_value = false;
+        ValidationConfig {
+            missing_ui_cam: default_value,
+            non_exhaustive: (),
+        }
+    }
+}

crates/bevy_ui/src/lib.rs

@@ -20,8 +20,11 @@ pub mod prelude {
 }
 
 use bevy_app::prelude::*;
-use bevy_ecs::{IntoSystem, ParallelSystemDescriptorCoercion, SystemStage};
-use bevy_render::render_graph::RenderGraph;
+use bevy_ecs::{prelude::*, IntoSystem, ParallelSystemDescriptorCoercion, SystemStage};
+use bevy_render::{
+    camera::{Camera, OrthographicProjection},
+    render_graph::RenderGraph,
+};
 use update::ui_z_system;
 
 #[derive(Default)]
@@ -54,8 +57,36 @@ impl Plugin for UiPlugin {
             .add_system_to_stage(stage::UI, ui_z_system.system())
             .add_system_to_stage(bevy_render::stage::DRAW, widget::draw_text_system.system());
 
+        let add_ui_cam_validation_system = app
+            .resources()
+            .get::<bevy_core::ValidationConfig>()
+            .map_or(false, |config| config.missing_ui_cam);
+
+        if add_ui_cam_validation_system {
+            app.add_startup_system_to_stage(
+                bevy_app::startup_stage::POST_STARTUP,
+                warn_no_ui_cam.system(),
+            );
+        }
+
         let resources = app.resources();
         let mut render_graph = resources.get_mut::<RenderGraph>().unwrap();
         render_graph.add_ui_graph(resources);
     }
 }
+
+fn warn_no_ui_cam(
+    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(crate::camera::CAMERA_UI));
+
+    if world_contains_nodes && !world_contains_ui_cam {
+        bevy_log::warn!(
+            "The world contains ui nodes but no ui camera. Consider spawning a UiCameraBundle."
+        );
+    }
+}