An engine written as documents.

use press_start_test_app::{advance, fixed_step_app};

#[derive(Resource, Default)]
struct TickCount(u64);

fn count_ticks(mut count: ResMut<TickCount>) {
    count.0 += 1;
}

#[test]
fn three_fixed_steps_a_frame_are_thirty_after_ten_frames() {
    let mut app = fixed_step_app(3);
    app.init_resource::<TickCount>();
    app.add_systems(FixedUpdate, count_ticks);

    app.update(); // priming: Startup runs, zero fixed steps
    assert_eq!(app.world().resource::<TickCount>().0, 0);

    advance(&mut app, 10);
    assert_eq!(app.world().resource::<TickCount>().0, 30);
}