Bevy 0.14: Modify a material in GLTF file after spawning it without changing other GLTFs

modify_gltf_material.rs

use bevy::{
    color::palettes::basic,
    prelude::*,
    scene::{SceneInstance, SceneInstanceReady},
};

/// A marker component that we will add to the particular fox that we want to be red.
#[derive(Component)]
struct RedFox;

fn setup(
    mut commands: Commands,
    asset_server: Res<AssetServer>,
    mut meshes: ResMut<Assets<Mesh>>,
    mut materials: ResMut<Assets<StandardMaterial>>,
) {
    commands.spawn(PbrBundle {
        mesh: meshes.add(Circle::new(4.0)),
        material: materials.add(Color::WHITE),
        transform: Transform::from_rotation(Quat::from_rotation_x(-std::f32::consts::FRAC_PI_2)),
        ..default()
    });

    commands.spawn((
        SceneBundle {
            scene: asset_server.load("models/animated/Fox.glb#Scene0"),
            transform: Transform::from_scale(Vec3::splat(0.02)).with_translation(Vec3::NEG_X),
            ..default()
        },
        RedFox,
    ));

    commands.spawn(SceneBundle {
        scene: asset_server.load("models/animated/Fox.glb#Scene0"),
        transform: Transform::from_scale(Vec3::splat(0.02)).with_translation(Vec3::X),
        ..default()
    });

    commands.spawn(PointLightBundle {
        point_light: PointLight {
            shadows_enabled: true,
            ..default()
        },
        transform: Transform::from_xyz(4.0, 8.0, 4.0),
        ..default()
    });

    commands.spawn(Camera3dBundle {
        transform: Transform::from_xyz(-2.0, 2.5, 5.0).looking_at(Vec3::ZERO, Vec3::Y),
        ..default()
    });
}

fn main() {
    App::new()
        .add_plugins(DefaultPlugins)
        .add_systems(Startup, setup)
        .add_systems(PostUpdate, decorate_red_fox)
        .run();
}

fn decorate_red_fox(
    mut events: EventReader<SceneInstanceReady>,
    instances: Query<&SceneInstance, With<RedFox>>,
    spawner: Res<SceneSpawner>,
    mut material_query: Query<&mut Handle<StandardMaterial>>,
    mut materials: ResMut<Assets<StandardMaterial>>,
    mut red_material: Local<Option<Handle<StandardMaterial>>>,
) {
    for event in events.read() {
        let Ok(instance) = instances.get(event.parent) else {
            continue;
        };

        for instance_entity in spawner.iter_instance_entities(**instance) {
            let Ok(mut original_material) = material_query.get_mut(instance_entity) else {
                continue;
            };

            let red_material = red_material.get_or_insert_with(|| {
                let mut red = materials.get(&*original_material).unwrap().clone();
                red.base_color = basic::RED.into();
                materials.add(red)
            });

            *original_material = red_material.clone();
        }
    }
}