How to swap color materials in an update call?

Question

During bevy setup, I've spawned multiple rectangles with two different colors. How do I update these rectangles with a new color material?

// in setup
commands.spawn(
    (MaterialMesh2dBundle {
        mesh: Mesh2dHandle(meshes.add(Rectangle::new(10., 10.))),
        material: if dull_world.is_live(row_index, col_index) {
            materials.add(Color::PURPLE)
        } else {
            materials.add(Color::GREEN)
        },
        transform: Transform::from_xyz(
            (row_index as f32 - row_len as f32 / 2.) * 10.,
            (col_index as f32 - col_len as f32 / 2.) * 10.,
            0.0,
        ),
        ..default()
    }),
);

full code

Answer 1

One way to accomplish this is to reuse the material handles you're creating by inserting them as Resources.

Then during your update call, query the entities that match Query<&mut Handle<ColorMaterial>> with the resource you want Res<CellDeadColor>. Since the handles exist already, you simply need to clone them and apply them to the color material handler.

// global
#[derive(Resource)]
struct CellDeadColor(Handle<ColorMaterial>);

#[derive(Resource)]
struct CellLiveColor(Handle<ColorMaterial>);

// setup
let live_color_handle = materials.add(Color::GREEN); // Handle<ColorMaterial>
let dead_color_handle = materials.add(Color::PURPLE); // Handle<ColorMaterial>

let cell_live_color = CellLiveColor(live_color_handle.clone());
let cell_dead_color = CellDeadColor(dead_color_handle.clone());

commands.insert_resource(cell_live_color);
commands.insert_resource(cell_dead_color);

commands.spawn(
    (MaterialMesh2dBundle {
        mesh: Mesh2dHandle(meshes.add(Rectangle::new(10., 10.))),
        material: if dull_world.is_live(row_index, col_index) {
            live_color_handle.clone()
        } else {
            dead_color_handle.clone()
        },
        transform: Transform::from_xyz(
            (row_index as f32 - row_len as f32 / 2.) * 10.,
            (col_index as f32 - col_len as f32 / 2.) * 10.,
            0.0,
        ),
        ..default()
    }),
);

// update
fn render_world(
    mut q_entities: Query<(&mut Handle<ColorMaterial>, &Cell)>,
    q_dull_world: Query<&DullWorld>,
    q_dead_cell_color: Res<CellDeadColor>,
    q_live_cell_color: Res<CellLiveColor>,
) {
    let dull_world = q_dull_world.iter().next().unwrap();
    for (mut entity, cell) in q_entities.iter_mut() {
        if dull_world.is_live(cell.row_index, cell.col_index) {
            *entity = q_dead_cell_color.0.clone();
        } else {
            *entity = q_live_cell_color.0.clone();
        };
    }
}