bevy 0.9.0

A refreshingly simple data-driven game engine and app framework
Documentation 
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//! Renders two cameras to the same window to accomplish "split screen".

use std::f32::consts::PI;

use bevy::{
    core_pipeline::clear_color::ClearColorConfig,
    prelude::*,
    render::camera::Viewport,
    window::{WindowId, WindowResized},
};

fn main() {
    App::new()
        .add_plugins(DefaultPlugins)
        .add_startup_system(setup)
        .add_system(set_camera_viewports)
        .run();
}

/// set up a simple 3D scene
fn setup(
    mut commands: Commands,
    asset_server: Res<AssetServer>,
    mut meshes: ResMut<Assets<Mesh>>,
    mut materials: ResMut<Assets<StandardMaterial>>,
) {
    // plane
    commands.spawn(PbrBundle {
        mesh: meshes.add(Mesh::from(shape::Plane { size: 100.0 })),
        material: materials.add(Color::rgb(0.3, 0.5, 0.3).into()),
        ..default()
    });

    commands.spawn(SceneBundle {
        scene: asset_server.load("models/animated/Fox.glb#Scene0"),
        ..default()
    });

    // Light
    commands.spawn(DirectionalLightBundle {
        transform: Transform::from_rotation(Quat::from_euler(EulerRot::ZYX, 0.0, 1.0, -PI / 4.)),
        directional_light: DirectionalLight {
            shadows_enabled: true,
            ..default()
        },
        ..default()
    });

    // Left Camera
    commands.spawn((
        Camera3dBundle {
            transform: Transform::from_xyz(0.0, 200.0, -100.0).looking_at(Vec3::ZERO, Vec3::Y),
            ..default()
        },
        LeftCamera,
    ));

    // Right Camera
    commands.spawn((
        Camera3dBundle {
            transform: Transform::from_xyz(100.0, 100., 150.0).looking_at(Vec3::ZERO, Vec3::Y),
            camera: Camera {
                // Renders the right camera after the left camera, which has a default priority of 0
                priority: 1,
                ..default()
            },
            camera_3d: Camera3d {
                // don't clear on the second camera because the first camera already cleared the window
                clear_color: ClearColorConfig::None,
                ..default()
            },
            ..default()
        },
        RightCamera,
    ));
}

#[derive(Component)]
struct LeftCamera;

#[derive(Component)]
struct RightCamera;

fn set_camera_viewports(
    windows: Res<Windows>,
    mut resize_events: EventReader<WindowResized>,
    mut left_camera: Query<&mut Camera, (With<LeftCamera>, Without<RightCamera>)>,
    mut right_camera: Query<&mut Camera, With<RightCamera>>,
) {
    // We need to dynamically resize the camera's viewports whenever the window size changes
    // so then each camera always takes up half the screen.
    // A resize_event is sent when the window is first created, allowing us to reuse this system for initial setup.
    for resize_event in resize_events.iter() {
        if resize_event.id == WindowId::primary() {
            let window = windows.primary();
            let mut left_camera = left_camera.single_mut();
            left_camera.viewport = Some(Viewport {
                physical_position: UVec2::new(0, 0),
                physical_size: UVec2::new(window.physical_width() / 2, window.physical_height()),
                ..default()
            });

            let mut right_camera = right_camera.single_mut();
            right_camera.viewport = Some(Viewport {
                physical_position: UVec2::new(window.physical_width() / 2, 0),
                physical_size: UVec2::new(window.physical_width() / 2, window.physical_height()),
                ..default()
            });
        }
    }
}