Add an example 'showcasing' using multiple windows (bevyengine#3367)

# Objective

- The multiple windows example which was viciously murdered in bevyengine#3175.
- cart asked me to

## Solution

- Rework the example to work on pipelined-rendering, based on the work from bevyengine#2898

Cargo.toml

@@ -25,7 +25,7 @@ default = [
   "hdr",
   "mp3",
   "x11",
-  "filesystem_watcher"
+  "filesystem_watcher",
 ]
 
 # Force dynamic linking, which improves iterative compile times
@@ -441,6 +441,10 @@ path = "examples/ui/ui.rs"
 name = "clear_color"
 path = "examples/window/clear_color.rs"
 
+[[example]]
+name = "multiple_windows"
+path = "examples/window/multiple_windows.rs"
+
 [[example]]
 name = "scale_factor_override"
 path = "examples/window/scale_factor_override.rs"

crates/bevy_render/src/camera/mod.rs

@@ -82,8 +82,8 @@ fn extract_cameras(
         if let Some((entity, camera, transform, visible_entities)) =
             camera.entity.and_then(|e| query.get(e).ok())
         {
-            entities.insert(name.clone(), entity);
             if let Some(window) = windows.get(camera.window) {
+                entities.insert(name.clone(), entity);
                 commands.get_or_spawn(entity).insert_bundle((
                     ExtractedCamera {
                         window_id: camera.window,

examples/README.md

@@ -243,6 +243,7 @@ Example | File | Description
 Example | File | Description
 --- | --- | ---
 `clear_color` | [`window/clear_color.rs`](./window/clear_color.rs) | Creates a solid color window
+`multiple_windows` | [`window/multiple_windows.rs`](./window/multiple_windows.rs) | Demonstrates creating multiple windows, and rendering to them
 `scale_factor_override` | [`window/scale_factor_override.rs`](./window/scale_factor_override.rs) | Illustrates how to customize the default window settings
 `transparent_window` | [`window/transparent_window.rs`](./window/transparent_window.rs) | Illustrates making the window transparent and hiding the window decoration
 `window_settings` | [`window/window_settings.rs`](./window/window_settings.rs) | Demonstrates customizing default window settings

examples/window/multiple_windows.rs

@@ -0,0 +1,111 @@
+use bevy::{
+    core_pipeline::{draw_3d_graph, node, AlphaMask3d, Opaque3d, Transparent3d},
+    prelude::*,
+    render::{
+        camera::{ActiveCameras, ExtractedCameraNames},
+        render_graph::{Node, NodeRunError, RenderGraph, RenderGraphContext, SlotValue},
+        render_phase::RenderPhase,
+        renderer::RenderContext,
+        RenderApp, RenderStage,
+    },
+    window::{CreateWindow, WindowId},
+};
+
+/// This example creates a second window and draws a mesh from two different cameras, one in each window
+fn main() {
+    let mut app = App::new();
+    app.add_plugins(DefaultPlugins)
+        .add_startup_system(setup)
+        .add_startup_system(create_new_window);
+
+    let render_app = app.sub_app(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);
+    graph
+        .add_node_edge(node::MAIN_PASS_DEPENDENCIES, SECONDARY_PASS_DRIVER)
+        .unwrap();
+    app.run();
+}
+
+fn extract_secondary_camera_phases(mut commands: Commands, active_cameras: Res<ActiveCameras>) {
+    if let Some(secondary) = active_cameras.get(SECONDARY_CAMERA_NAME) {
+        if let Some(entity) = secondary.entity {
+            commands.get_or_spawn(entity).insert_bundle((
+                RenderPhase::<Opaque3d>::default(),
+                RenderPhase::<AlphaMask3d>::default(),
+                RenderPhase::<Transparent3d>::default(),
+            ));
+        }
+    }
+}
+
+const SECONDARY_CAMERA_NAME: &str = "Secondary";
+const SECONDARY_PASS_DRIVER: &str = "secondary_pass_driver";
+
+fn create_new_window(
+    mut create_window_events: EventWriter<CreateWindow>,
+
+    mut commands: Commands,
+    mut active_cameras: ResMut<ActiveCameras>,
+) {
+    let window_id = WindowId::new();
+
+    // sends out a "CreateWindow" event, which will be received by the windowing backend
+    create_window_events.send(CreateWindow {
+        id: window_id,
+        descriptor: WindowDescriptor {
+            width: 800.,
+            height: 600.,
+            vsync: false,
+            title: "Second window".to_string(),
+            ..Default::default()
+        },
+    });
+    // second window camera
+    commands.spawn_bundle(PerspectiveCameraBundle {
+        camera: Camera {
+            window: window_id,
+            name: Some(SECONDARY_CAMERA_NAME.into()),
+            ..Default::default()
+        },
+        transform: Transform::from_xyz(6.0, 0.0, 0.0).looking_at(Vec3::ZERO, Vec3::Y),
+        ..Default::default()
+    });
+
+    active_cameras.add(SECONDARY_CAMERA_NAME);
+}
+
+struct SecondaryCameraDriver;
+impl Node for SecondaryCameraDriver {
+    fn run(
+        &self,
+        graph: &mut RenderGraphContext,
+        _render_context: &mut RenderContext,
+        world: &World,
+    ) -> Result<(), NodeRunError> {
+        let extracted_cameras = world.get_resource::<ExtractedCameraNames>().unwrap();
+        if let Some(camera_3d) = extracted_cameras.entities.get(SECONDARY_CAMERA_NAME) {
+            graph.run_sub_graph(
+                crate::draw_3d_graph::NAME,
+                vec![SlotValue::Entity(*camera_3d)],
+            )?;
+        }
+        Ok(())
+    }
+}
+
+fn setup(mut commands: Commands, asset_server: Res<AssetServer>) {
+    // add entities to the world
+    commands.spawn_scene(asset_server.load("models/monkey/Monkey.gltf#Scene0"));
+    // light
+    commands.spawn_bundle(PointLightBundle {
+        transform: Transform::from_xyz(4.0, 5.0, 4.0),
+        ..Default::default()
+    });
+    // main camera
+    commands.spawn_bundle(PerspectiveCameraBundle {
+        transform: Transform::from_xyz(0.0, 0.0, 6.0).looking_at(Vec3::ZERO, Vec3::Y),
+        ..Default::default()
+    });
+}