Bevy 0.9 + bevy_egui: changing a cube's color with sliders

main.rs

// Mashup of
// https://github.com/mvlabat/bevy_egui/blob/v0.19.0/examples/simple.rs
// https://github.com/bevyengine/bevy/blob/v0.9.1/examples/3d/3d_scene.rs

// In cargo.toml:
// [dependencies]
// bevy = "0.9"
// bevy_egui = "0.19"

use bevy::prelude::*;
use bevy_egui::{egui, EguiContext, EguiPlugin};

#[derive(Resource, Default)]
struct CubeSettings {
    r: f32,
    g: f32,
    b: f32,
}

fn main() {
    App::new()
        .add_plugins(DefaultPlugins)
        .add_plugin(EguiPlugin)
        .add_system(ui)
        .add_system(update_cube)
        .add_startup_system(setup)
        .init_resource::<CubeSettings>()
        .run();
}

fn setup(
    mut commands: Commands,
    mut meshes: ResMut<Assets<Mesh>>,
    mut materials: ResMut<Assets<StandardMaterial>>,
    settings: Res<CubeSettings>,
) {
    // cube
    commands.spawn(PbrBundle {
        mesh: meshes.add(Mesh::from(shape::Cube { size: 1.0 })),
        material: materials.add(Color::rgb(settings.r, settings.g, settings.b).into()),
        transform: Transform::from_xyz(0.0, 0.5, 0.0),
        ..default()
    });
    // light
    commands.spawn(PointLightBundle {
        point_light: PointLight {
            intensity: 1500.0,
            shadows_enabled: true,
            ..default()
        },
        transform: Transform::from_xyz(4.0, 8.0, 4.0),
        ..default()
    });
    // camera
    commands.spawn(Camera3dBundle {
        transform: Transform::from_xyz(-2.0, 2.5, 5.0).looking_at(Vec3::ZERO, Vec3::Y),
        ..default()
    });
}

fn ui(mut egui_context: ResMut<EguiContext>, mut settings: ResMut<CubeSettings>) {
    egui::Window::new("Color").show(egui_context.ctx_mut(), |ui| {
        ui.add(egui::Slider::new(&mut settings.r, 0.0..=1.0).text("red"));
        ui.add(egui::Slider::new(&mut settings.g, 0.0..=1.0).text("green"));
        ui.add(egui::Slider::new(&mut settings.b, 0.0..=1.0).text("blue"));
    });
}

fn update_cube(
    settings: Res<CubeSettings>,
    query: Query<&Handle<StandardMaterial>>,
    mut materials: ResMut<Assets<StandardMaterial>>,
) {
    if !settings.is_changed() {
        return;
    }

    let handle = query.single();

    if let Some(mut material) = materials.get_mut(handle) {
        material.base_color = Color::rgb(settings.r, settings.g, settings.b);
    }
}