make sub_app return an &App and add sub_app_mut() -> &mut App (#3309)

It's sometimes useful to have a reference to an app a sub app at the same time, which is only possible with an immutable reference.

crates/bevy_app/src/app.rs

@@ -854,7 +854,24 @@ impl App {
     }
 
     /// Retrieves a "sub app" stored inside this [App]. This will panic if the sub app does not exist.
-    pub fn sub_app(&mut self, label: impl AppLabel) -> &mut App {
+    pub fn sub_app_mut(&mut self, label: impl AppLabel) -> &mut App {
+        match self.get_sub_app_mut(label) {
+            Ok(app) => app,
+            Err(label) => panic!("Sub-App with label '{:?}' does not exist", label),
+        }
+    }
+
+    /// Retrieves a "sub app" inside this [App] with the given label, if it exists. Otherwise returns
+    /// an [Err] containing the given label.
+    pub fn get_sub_app_mut(&mut self, label: impl AppLabel) -> Result<&mut App, impl AppLabel> {
+        self.sub_apps
+            .get_mut((&label) as &dyn AppLabel)
+            .map(|sub_app| &mut sub_app.app)
+            .ok_or(label)
+    }
+
+    /// Retrieves a "sub app" stored inside this [App]. This will panic if the sub app does not exist.
+    pub fn sub_app(&self, label: impl AppLabel) -> &App {
         match self.get_sub_app(label) {
             Ok(app) => app,
             Err(label) => panic!("Sub-App with label '{:?}' does not exist", label),
@@ -863,10 +880,10 @@ impl App {
 
     /// Retrieves a "sub app" inside this [App] with the given label, if it exists. Otherwise returns
     /// an [Err] containing the given label.
-    pub fn get_sub_app(&mut self, label: impl AppLabel) -> Result<&mut App, impl AppLabel> {
+    pub fn get_sub_app(&self, label: impl AppLabel) -> Result<&App, impl AppLabel> {
         self.sub_apps
-            .get_mut((&label) as &dyn AppLabel)
+            .get((&label) as &dyn AppLabel)
-            .map(|sub_app| &mut sub_app.app)
+            .map(|sub_app| &sub_app.app)
             .ok_or(label)
     }
 }

crates/bevy_core_pipeline/src/lib.rs

@@ -88,7 +88,7 @@ impl Plugin for CorePipelinePlugin {
     fn build(&self, app: &mut App) {
         app.init_resource::<ClearColor>();
 
-        let render_app = app.sub_app(RenderApp);
+        let render_app = app.sub_app_mut(RenderApp);
         render_app
             .init_resource::<DrawFunctions<Transparent2d>>()
             .init_resource::<DrawFunctions<Opaque3d>>()

crates/bevy_pbr/src/lib.rs

@@ -135,7 +135,7 @@ impl Plugin for PbrPlugin {
                 },
             );
 
-        let render_app = app.sub_app(RenderApp);
+        let render_app = app.sub_app_mut(RenderApp);
         render_app
             .add_system_to_stage(
                 RenderStage::Extract,

crates/bevy_pbr/src/render/mesh.rs

@@ -57,7 +57,7 @@ impl Plugin for MeshRenderPlugin {
 
         app.add_plugin(UniformComponentPlugin::<MeshUniform>::default());
 
-        app.sub_app(RenderApp)
+        app.sub_app_mut(RenderApp)
             .init_resource::<MeshPipeline>()
             .add_system_to_stage(RenderStage::Extract, extract_meshes)
             .add_system_to_stage(RenderStage::Queue, queue_mesh_bind_group)

crates/bevy_pbr/src/wireframe.rs

@@ -31,7 +31,7 @@ impl Plugin for WireframePlugin {
 
         app.init_resource::<WireframeConfig>();
 
-        app.sub_app(RenderApp)
+        app.sub_app_mut(RenderApp)
             .add_render_command::<Opaque3d, DrawWireframes>()
             .init_resource::<WireframePipeline>()
             .init_resource::<SpecializedPipelines<WireframePipeline>>()

crates/bevy_render/src/camera/mod.rs

@@ -53,7 +53,7 @@ impl Plugin for CameraPlugin {
                 CoreStage::PostUpdate,
                 crate::camera::camera_system::<PerspectiveProjection>,
             );
-        app.sub_app(RenderApp)
+        app.sub_app_mut(RenderApp)
             .init_resource::<ExtractedCameraNames>()
             .add_system_to_stage(RenderStage::Extract, extract_cameras);
     }

crates/bevy_render/src/render_asset.rs

@@ -54,7 +54,7 @@ impl<A: RenderAsset> Default for RenderAssetPlugin<A> {
 
 impl<A: RenderAsset> Plugin for RenderAssetPlugin<A> {
     fn build(&self, app: &mut App) {
-        let render_app = app.sub_app(RenderApp);
+        let render_app = app.sub_app_mut(RenderApp);
         let prepare_asset_system = PrepareAssetSystem::<A>::system(&mut render_app.world);
         render_app
             .init_resource::<ExtractedAssets<A>>()

crates/bevy_render/src/render_component.rs

@@ -62,7 +62,7 @@ impl<C> Default for UniformComponentPlugin<C> {
 
 impl<C: Component + AsStd140 + Clone> Plugin for UniformComponentPlugin<C> {
     fn build(&self, app: &mut App) {
-        app.sub_app(RenderApp)
+        app.sub_app_mut(RenderApp)
             .insert_resource(ComponentUniforms::<C>::default())
             .add_system_to_stage(
                 RenderStage::Prepare,
@@ -144,7 +144,7 @@ where
 {
     fn build(&self, app: &mut App) {
         let system = ExtractComponentSystem::<C>::system(&mut app.world);
-        let render_app = app.sub_app(RenderApp);
+        let render_app = app.sub_app_mut(RenderApp);
         render_app.add_system_to_stage(RenderStage::Extract, system);
     }
 }

crates/bevy_render/src/texture/mod.rs

@@ -35,7 +35,7 @@ impl Plugin for ImagePlugin {
             .unwrap()
             .set_untracked(DEFAULT_IMAGE_HANDLE, Image::default());
 
-        app.sub_app(RenderApp)
+        app.sub_app_mut(RenderApp)
             .init_resource::<TextureCache>()
             .add_system_to_stage(RenderStage::Cleanup, update_texture_cache_system);
     }

crates/bevy_render/src/view/mod.rs

@@ -26,7 +26,7 @@ impl Plugin for ViewPlugin {
     fn build(&self, app: &mut App) {
         app.init_resource::<Msaa>().add_plugin(VisibilityPlugin);
 
-        app.sub_app(RenderApp)
+        app.sub_app_mut(RenderApp)
             .init_resource::<ViewUniforms>()
             .add_system_to_stage(RenderStage::Extract, extract_msaa)
             .add_system_to_stage(RenderStage::Prepare, prepare_view_uniforms)

crates/bevy_render/src/view/window.rs

@@ -24,7 +24,7 @@ pub enum WindowSystem {
 
 impl Plugin for WindowRenderPlugin {
     fn build(&self, app: &mut App) {
-        app.sub_app(RenderApp)
+        app.sub_app_mut(RenderApp)
             .init_resource::<ExtractedWindows>()
             .init_resource::<WindowSurfaces>()
             .init_resource::<NonSendMarker>()

crates/bevy_sprite/src/lib.rs

@@ -54,7 +54,7 @@ impl Plugin for SpritePlugin {
         let sprite_shader = Shader::from_wgsl(include_str!("render/sprite.wgsl"));
         shaders.set_untracked(SPRITE_SHADER_HANDLE, sprite_shader);
         app.add_asset::<TextureAtlas>().register_type::<Sprite>();
-        let render_app = app.sub_app(RenderApp);
+        let render_app = app.sub_app_mut(RenderApp);
         render_app
             .init_resource::<ImageBindGroups>()
             .init_resource::<SpritePipeline>()

crates/bevy_text/src/lib.rs

@@ -49,7 +49,7 @@ impl Plugin for TextPlugin {
             .insert_resource(DefaultTextPipeline::default())
             .add_system_to_stage(CoreStage::PostUpdate, text2d_system);
 
-        let render_app = app.sub_app(RenderApp);
+        let render_app = app.sub_app_mut(RenderApp);
         render_app.add_system_to_stage(
             RenderStage::Extract,
             extract_text2d_sprite.after(SpriteSystem::ExtractSprite),

crates/bevy_ui/src/render/mod.rs

@@ -66,7 +66,7 @@ pub fn build_ui_render(app: &mut App) {
     let mut active_cameras = app.world.get_resource_mut::<ActiveCameras>().unwrap();
     active_cameras.add(CAMERA_UI);
 
-    let render_app = app.sub_app(RenderApp);
+    let render_app = app.sub_app_mut(RenderApp);
     render_app
         .init_resource::<UiPipeline>()
         .init_resource::<SpecializedPipelines<UiPipeline>>()

examples/shader/shader_defs.rs

@@ -23,7 +23,7 @@ pub struct IsRedPlugin;
 impl Plugin for IsRedPlugin {
     fn build(&self, app: &mut App) {
         app.add_plugin(ExtractComponentPlugin::<IsRed>::default());
-        app.sub_app(RenderApp)
+        app.sub_app_mut(RenderApp)
             .add_render_command::<Transparent3d, DrawIsRed>()
             .init_resource::<IsRedPipeline>()
             .init_resource::<SpecializedPipelines<IsRedPipeline>>()

examples/shader/shader_material.rs

@@ -110,7 +110,7 @@ impl Plugin for CustomMaterialPlugin {
         app.add_asset::<CustomMaterial>()
             .add_plugin(ExtractComponentPlugin::<Handle<CustomMaterial>>::default())
             .add_plugin(RenderAssetPlugin::<CustomMaterial>::default());
-        app.sub_app(RenderApp)
+        app.sub_app_mut(RenderApp)
             .add_render_command::<Transparent3d, DrawCustom>()
             .init_resource::<CustomPipeline>()
             .init_resource::<SpecializedPipelines<CustomPipeline>>()

examples/window/multiple_windows.rs

@@ -18,7 +18,7 @@ fn main() {
         .add_startup_system(setup)
         .add_startup_system(create_new_window);
 
-    let render_app = app.sub_app(RenderApp);
+    let render_app = app.sub_app_mut(RenderApp);
     render_app.add_system_to_stage(RenderStage::Extract, extract_secondary_camera_phases);
     let mut graph = render_app.world.get_resource_mut::<RenderGraph>().unwrap();
     graph.add_node(SECONDARY_PASS_DRIVER, SecondaryCameraDriver);