Name the profiles to not suggest they're WASM-only

This way I can `cargo run --target ...` and Cargo will spawn a dummy
webserver instead of attempting to execute the WASM file on my x86/arm
CPU.

.cargo/config.toml

@@ -0,0 +1,4 @@
+
+[target.wasm32-unknown-unknown]
+runner = "wasm-server-runner"
+rustflags = ["--cfg", "getrandom_backend=\"wasm_js\""]

Cargo.toml

@@ -1,7 +1,33 @@
 [package]
 name = "asteroids"
-version = "0.1.0"
+version = "0.3.0"
-edition = "2021"
+edition = "2024"
 
 [dependencies]
-bevy = "0.14.2"
+bevy = "0.16"
+bevy-inspector-egui = "0.32.0"
+bevy_rapier2d = "0.31.0"
+rand = "0.9.2"
+
+[features]
+dynamic_linking = ["bevy/dynamic_linking"]
+debug_ui = ["bevy_rapier2d/debug-render-2d"]
+
+[target.'cfg(target_arch = "wasm32")'.dependencies]
+getrandom = { version = "0.3.3", features = ["wasm_js"] }
+
+[profile.speedy]
+inherits = "release"
+codegen-units = 1
+lto = "fat"
+opt-level = 3
+strip = "symbols"
+panic = "abort"
+
+[profile.tiny]
+inherits = "release"
+codegen-units = 1
+lto = "fat"
+opt-level = "z"
+strip = "symbols"
+panic = "abort"

src/config.rs

@@ -1,16 +1,28 @@
-/*
+//! Global constants used all over the program. Rather than leaving them scattered
- Global constants used all over the program. Rather than leaving them scattered
+//! where ever they happen to be needed, I'm concentrating them here.
-where ever they happen to be needed, I'm concentrating them here.
-*/
 
 use bevy::color::Color;
 
 pub const WINDOW_SIZE: bevy::prelude::Vec2 = bevy::prelude::Vec2::new(800.0, 600.0);
 
+pub const UI_BUTTON_NORMAL: Color = Color::srgb(0.15, 0.15, 0.15); // Button color when it's just hanging out
+pub const UI_BUTTON_HOVERED: Color = Color::srgb(0.25, 0.25, 0.25); // ... when it's hovered
+pub const UI_BUTTON_PRESSED: Color = Color::srgb(0.55, 0.55, 0.55); // ... when it's pressed
+
 pub(crate) const BACKGROUND_COLOR: Color = Color::srgb(0.3, 0.3, 0.3);
 pub(crate) const PLAYER_SHIP_COLOR: Color = Color::srgb(1.0, 1.0, 1.0);
 pub(crate) const SHIP_THRUSTER_COLOR_ACTIVE: Color = Color::srgb(1.0, 0.2, 0.2);
 pub(crate) const SHIP_THRUSTER_COLOR_INACTIVE: Color = Color::srgb(0.5, 0.5, 0.5);
+pub(crate) const ASTEROID_SMALL_COLOR: Color = Color::srgb(1.0, 0., 0.);
+pub(crate) const BULLET_COLOR: Color = Color::srgb(0.0, 0.1, 0.9);
+// TODO: asteroid medium & large
 
 pub(crate) const SHIP_THRUST: f32 = 1.0;
-pub(crate) const SHIP_ROTATION: f32 = 0.1; // +/- rotation speed in... radians per frame
+pub(crate) const SHIP_ROTATION: f32 = 4.0; // +/- rotation speed in... radians per frame
+
+pub(crate) const BULLET_SPEED: f32 = 150.0;
+pub(crate) const BULLET_LIFETIME: f32 = 2.0;
+
+pub(crate) const ASTEROID_LIFETIME: f32 = 40.0;
+
+pub const RNG_SEED: [u8; 32] = *b"12345678909876543210123456789098";

src/events.rs

@@ -0,0 +1,36 @@
+use bevy::prelude::*;
+
+use crate::objects::AsteroidSize;
+
+/// Signals that the player's ship has been destroyed.
+///
+/// Produced by the [`fn collision_listener(...)`](`crate::physics::collision_listener`)
+/// system when the player collides with an asteroid. They are consumed by [`fn ship_impact_listener(...)`](`crate::objects::ship_impact_listener`).
+#[derive(Event)]
+pub(crate) struct ShipDestroy;
+
+/// Signals that a particular asteroid has been destroyed.
+///
+/// Produced by the [`fn collision_listener(...)`](`crate::physics::collision_listener`)
+/// system when bullets contact asteroids. It is consumed by [`fn split_asteroid(...)`](`crate::objects::spawn_asteroid`)
+/// system, which re-emits [spawn events](`SpawnAsteroid`).
+#[derive(Event)]
+pub(crate) struct AsteroidDestroy(pub Entity);
+
+/// Signals that an asteroid needs to be spawned and provides the parameters
+/// for that action.
+///
+/// Produced by the [`tick_asteroid_manager(...)`](`crate::machinery::tick_asteroid_manager`)
+/// system and consumed by the [`spawn_asteroid(...)`](`crate::objects::spawn_asteroid`) system.
+#[derive(Event)]
+pub struct SpawnAsteroid {
+    pub pos: Vec2,
+    pub vel: Vec2,
+    pub size: AsteroidSize,
+}
+
+/// Signals that a particular bullet has been destroyed (after it strikes an Asteroid).
+///
+/// TODO: Maybe use it for lifetime expiration (which is also a TODO item).
+#[derive(Event)]
+pub(crate) struct BulletDestroy(pub Entity);

src/lib.rs

@@ -1,111 +1,116 @@
+//! Asteroids, implemented as a Bevy plugin.
+//!
+//! Compile-time configurables can be found in the [`config`] module.
+
 pub mod config;
+mod events;
+mod machinery;
+mod objects;
+mod physics;
+mod resources;
+mod widgets;
 
-use crate::config::{BACKGROUND_COLOR, PLAYER_SHIP_COLOR, SHIP_ROTATION, SHIP_THRUST, WINDOW_SIZE};
+use crate::config::{
+    ASTEROID_SMALL_COLOR, BACKGROUND_COLOR, BULLET_COLOR, BULLET_LIFETIME, BULLET_SPEED,
+    PLAYER_SHIP_COLOR, SHIP_ROTATION, SHIP_THRUST, SHIP_THRUSTER_COLOR_ACTIVE,
+    SHIP_THRUSTER_COLOR_INACTIVE,
+};
+use crate::machinery::AsteroidSpawner;
+use crate::objects::{Bullet, Ship};
+use crate::physics::AngularVelocity;
 
-use bevy::{prelude::*, sprite::MaterialMesh2dBundle};
+use bevy::prelude::*;
-use config::{SHIP_THRUSTER_COLOR_ACTIVE, SHIP_THRUSTER_COLOR_INACTIVE};
+use bevy_rapier2d::{
+    plugin::{NoUserData, RapierPhysicsPlugin},
+    prelude::*,
+    render::RapierDebugRenderPlugin,
+};
+use machinery::Lifetime;
+use resources::{GameAssets, Lives, Score, WorldSize};
 
+/// The main game plugin.
 pub struct AsteroidPlugin;
 
 impl Plugin for AsteroidPlugin {
     fn build(&self, app: &mut App) {
-        app.add_systems(Startup, (spawn_camera, spawn_player))
+        app.add_plugins((
-            .insert_resource(ClearColor(BACKGROUND_COLOR))
+            widgets::PluginGameMenu,
-            .insert_resource(WorldSize {
+            widgets::PluginGameOver,
-                width: WINDOW_SIZE.x,
+            widgets::PluginGetReady,
-                height: WINDOW_SIZE.y,
+            RapierPhysicsPlugin::<NoUserData>::pixels_per_meter(10.0),
-            })
+            RapierDebugRenderPlugin::default(),
-            .add_systems(
+        ))
-                FixedUpdate,
+        .insert_resource(ClearColor(BACKGROUND_COLOR))
-                (input_ship_thruster, input_ship_rotation, wrap_entities),
+        .insert_resource(WorldSize::default())
+        .insert_resource(Lives(3))
+        .register_type::<Lives>()
+        .insert_resource(Score(0))
+        .insert_resource(AsteroidSpawner::new())
+        .init_resource::<GameAssets>()
+        .add_systems(Startup, spawn_camera)
+        .add_systems(
+            OnEnter(GameState::Playing),
+            (objects::spawn_player, widgets::spawn_ui),
+        )
+        .add_systems(
+            FixedUpdate,
+            (
+                input_ship_thruster,
+                input_ship_rotation,
+                input_ship_shoot,
+                physics::wrap_entities,
+                machinery::tick_asteroid_manager,
+                objects::spawn_asteroid.after(machinery::tick_asteroid_manager),
+                objects::split_asteroids,
+                objects::bullet_impact_listener,
+                objects::ship_impact_listener,
+                physics::collision_listener,
+                machinery::tick_lifetimes,
             )
-            .add_systems(
+                .run_if(in_state(GameState::Playing)),
-                FixedPostUpdate,
+        )
-                (integrate_velocity, update_positions, apply_rotation_to_mesh),
+        .add_systems(
-            );
+            FixedPostUpdate,
+            (
+                physics::integrate_velocity,
+                physics::integrate_angular_velocity,
+            )
+                .run_if(in_state(GameState::Playing)),
+        )
+        .add_event::<events::SpawnAsteroid>()
+        .add_event::<events::AsteroidDestroy>()
+        .add_event::<events::ShipDestroy>()
+        .add_event::<events::BulletDestroy>();
+        app.insert_state(GameState::TitleScreen);
     }
 }
 
-#[derive(Component)]
+/// The game's main state tracking mechanism, registered with Bevy as a [`State`](`bevy::prelude::State`).
-struct Position(bevy::math::Vec2);
+#[derive(Clone, Debug, Eq, Hash, PartialEq, States)]
-
+pub enum GameState {
-#[derive(Component)]
+    TitleScreen, // Program is started. Present title screen and await user start
-struct Velocity(bevy::math::Vec2);
+    GetReady,    // Short timer to let the player get ready after pressing start
-
+    Playing,     // Player has started the game. Run the main loop
-#[derive(Component)]
+    GameOver,    // Game has ended. Present game over dialogue and await user restart
-struct Rotation(f32);
-
-#[derive(Component)]
-struct Ship;
-
-// Data component to store color properties attached to an entity
-// This was easier (and imo better) than holding global consts with
-// UUID assets.
-#[derive(Component)]
-struct ThrusterColors(Handle<ColorMaterial>, Handle<ColorMaterial>);
-
-#[derive(Resource)]
-struct WorldSize {
-    width: f32,
-    height: f32,
 }
 
 fn spawn_camera(mut commands: Commands) {
-    commands.spawn(Camera2dBundle::default());
+    commands.spawn(Camera2d);
 }
 
-fn spawn_player(
+/// Player's thruster control system.
-    mut commands: Commands,
+///
-    mut meshes: ResMut<Assets<Mesh>>,
+/// Checks if "W" is pressed and increases velocity accordingly.
-    mut materials: ResMut<Assets<ColorMaterial>>,
-) {
-    let triangle = Triangle2d::new(
-        Vec2::new(0.5, 0.0),
-        Vec2::new(-0.5, 0.45),
-        Vec2::new(-0.5, -0.45),
-    );
-    let thruster_firing_id = materials.add(SHIP_THRUSTER_COLOR_ACTIVE);
-    let thruster_stopped_id = materials.add(SHIP_THRUSTER_COLOR_INACTIVE);
-
-    let ship_mesh = MaterialMesh2dBundle {
-        mesh: meshes.add(triangle).into(),
-        material: materials.add(PLAYER_SHIP_COLOR),
-        transform: Transform::default().with_scale(Vec3::new(20.0, 20.0, 20.0)),
-        ..default()
-    };
-
-    let thruster_mesh = MaterialMesh2dBundle {
-        mesh: meshes.add(triangle).into(),
-        material: materials.add(PLAYER_SHIP_COLOR),
-        transform: Transform::default()
-            .with_scale(Vec3::splat(0.5))
-            .with_translation(Vec3::new(-0.5, 0.0, -0.1)),
-        ..default()
-    };
-
-    let thruster = commands.spawn(thruster_mesh).id();
-
-    let mut ship_id = commands.spawn((
-        Ship,
-        Position(Vec2::default()),
-        Velocity(Vec2::ZERO),
-        Rotation(0.0),
-        ship_mesh,
-        ThrusterColors(thruster_firing_id, thruster_stopped_id),
-    ));
-
-    ship_id.add_child(thruster);
-}
-
-/*
- Checks if "W" is pressed and increases velocity accordingly.
-*/
 fn input_ship_thruster(
     keyboard_input: Res<ButtonInput<KeyCode>>,
-    mut query: Query<(&mut Velocity, &Rotation, &mut Children, &ThrusterColors), With<Ship>>,
+    mut query: Query<(&mut physics::Velocity, &Transform, &mut Children), With<Ship>>,
     mut commands: Commands,
+    game_assets: Res<GameAssets>,
 ) {
-    let Ok((mut velocity, rotation, children, colors)) = query.get_single_mut() else {
+    // TODO: Maybe change for a Single<Ship>> so this only runs for the one ship
+    // buuut... that would silently do nothing if there are 0 or >1 ships, and
+    // I might want to crash on purpose in that case.
+    let Ok((mut velocity, transform, children)) = query.single_mut() else {
         let count = query.iter().count();
         panic!("There should be exactly one player ship! Instead, there seems to be {count}.");
     };
@@ -115,75 +120,71 @@ fn input_ship_thruster(
         .expect("Couldn't find first child, which should be the thruster");
 
     if keyboard_input.pressed(KeyCode::KeyW) {
-        velocity.0 += Vec2::from_angle(rotation.0) * SHIP_THRUST;
+        velocity.0 += (transform.rotation * Vec3::X).xy() * SHIP_THRUST;
-        commands.entity(*thrusters).insert(colors.0.clone());
+        commands
+            .entity(*thrusters)
+            .insert(MeshMaterial2d(game_assets.thruster_mat_active()));
     } else {
-        commands.entity(*thrusters).insert(colors.1.clone());
+        commands
+            .entity(*thrusters)
+            .insert(MeshMaterial2d(game_assets.thruster_mat_inactive()));
     }
 }
 
-/*
+/// Player's rotation control system.
- Checks if "A" or "D" is pressed and updates the player's Rotation component accordingly
+///
- Does *not* rotate the graphical widget! (that's done by the `apply_rotation_to_mesh` system)
+/// Checks if "A" or "D" is pressed and updates the player's [`AngularVelocity`]
-*/
+/// component accordingly.
 fn input_ship_rotation(
     keyboard_input: Res<ButtonInput<KeyCode>>,
-    mut query: Query<&mut Rotation, With<Ship>>,
+    mut query: Query<&mut AngularVelocity, With<Ship>>,
 ) {
-    let Ok(mut rotation) = query.get_single_mut() else {
+    let Ok(mut angular_vel) = query.single_mut() else {
         let count = query.iter().count();
         panic!("There should be exactly one player ship! Instead, there seems to be {count}.");
     };
 
     if keyboard_input.pressed(KeyCode::KeyA) {
-        rotation.0 += SHIP_ROTATION;
+        angular_vel.0 = SHIP_ROTATION;
     } else if keyboard_input.pressed(KeyCode::KeyD) {
-        rotation.0 -= SHIP_ROTATION;
+        angular_vel.0 = -SHIP_ROTATION;
+    } else {
+        angular_vel.0 = 0.0;
     }
 }
 
-/*
+/// Player's gun trigger.
- Add velocity to position
+///
-*/
+/// Checks if the spacebar has just been pressed, spawning a bullet if so.
-fn integrate_velocity(mut query: Query<(&mut Position, &Velocity)>, time: Res<Time>) {
+///
-    for (mut position, velocity) in &mut query {
+/// TODO: Hook up a timer to control weapon fire-rate. Something will have to
-        position.0 += velocity.0 * time.delta_seconds();
+/// tick those timers. Maybe this system?
-    }
+fn input_ship_shoot(
-}
+    keyboard_input: Res<ButtonInput<KeyCode>>,
-
+    ship: Single<(&Transform, &physics::Velocity), With<Ship>>,
-fn update_positions(mut query: Query<(&mut Transform, &Position)>) {
+    game_assets: Res<GameAssets>,
-    for (mut transform, position) in &mut query {
+    mut commands: Commands,
-        transform.translation.x = position.0.x;
+) {
-        transform.translation.y = position.0.y;
+    let (ship_pos, ship_vel) = *ship;
-    }
+
-}
+    // Derive bullet velocity, add to the ship's velocity
-
+    let bullet_vel = (ship_pos.rotation * Vec3::X).xy() * BULLET_SPEED;
-/*
+    let bullet_vel = bullet_vel + ship_vel.0;
- Assigns the rotation to the transform by copying it from the Rotation component.
+
-*/
+    // TODO: create a timer for the gun fire rate.
-fn apply_rotation_to_mesh(mut query: Query<(&mut Transform, &Rotation)>) {
+    // For now, spawn one for each press of the spacebar.
-    for (mut transform, rotation) in &mut query {
+    if keyboard_input.just_pressed(KeyCode::Space) {
-        transform.rotation = Quat::from_rotation_z(rotation.0);
+        commands.spawn((
-    }
+            Bullet,
-}
+            Collider::ball(0.2),
-
+            Sensor,
-fn wrap_entities(mut query: Query<&mut Position>, world_size: Res<WorldSize>) {
+            ActiveEvents::COLLISION_EVENTS,
-    let right = world_size.width / 2.0;
+            ActiveCollisionTypes::STATIC_STATIC,
-    let left = -right;
+            physics::Velocity(bullet_vel),
-    let top = world_size.height / 2.0;
+            Mesh2d(game_assets.bullet().0),
-    let bottom = -top;
+            MeshMaterial2d(game_assets.bullet().1),
-
+            ship_pos.clone(), // clone ship transform
-    for mut pos in query.iter_mut() {
+            Lifetime(Timer::from_seconds(BULLET_LIFETIME, TimerMode::Once)),
-        if pos.0.x > right {
+        ));
-            pos.0.x = left;
-        } else if pos.0.x < left {
-            pos.0.x = right;
-        }
-
-        if pos.0.y > top {
-            pos.0.y = bottom;
-        } else if pos.0.y < bottom {
-            pos.0.y = top;
-        }
     }
 }

src/machinery.rs

@@ -0,0 +1,105 @@
+//! Systems, Components, and any other items for powering the game logic.
+//!
+//! Where the objects (ship, asteroid, etc) carry their own behavioral systems,
+//! the *game* keeps its main logic here. Its for ambient behaviors, like
+//! asteroid spawning, or eventually the flying saucer spawns.
+use rand::{Rng, SeedableRng};
+use std::time::Duration;
+
+use bevy::prelude::*;
+
+use crate::{WorldSize, events::SpawnAsteroid, objects::AsteroidSize};
+
+/// Asteroid spawning parameters and state.
+///
+/// This struct keeps track of the rng and timer for spawning asteroids. In the
+/// future it may contain additional fields to allow for more control.
+///
+/// It's values are operated by the [`tick_asteroid_manager`] system.
+#[derive(Resource)]
+pub struct AsteroidSpawner {
+    rng: std::sync::Mutex<rand::rngs::StdRng>,
+    timer: Timer,
+    // TODO: Configurables?
+    // - interval
+    // - density
+    // - size distribution
+}
+
+impl AsteroidSpawner {
+    pub fn new() -> Self {
+        Self {
+            rng: std::sync::Mutex::new(rand::rngs::StdRng::from_seed(crate::config::RNG_SEED)),
+            timer: Timer::new(Duration::from_secs(3), TimerMode::Repeating),
+        }
+    }
+}
+
+/// Update the asteroid spawn timer in the [`AsteroidSpawner`] resource, and
+/// spawns any asteroids that are due this frame.
+pub fn tick_asteroid_manager(
+    mut events: EventWriter<SpawnAsteroid>,
+    mut spawner: ResMut<AsteroidSpawner>,
+    time: Res<Time>,
+    play_area: Res<WorldSize>,
+) {
+    spawner.timer.tick(time.delta());
+    if spawner.timer.just_finished() {
+        let mut rng = spawner
+            .rng
+            .lock()
+            .expect("Expected to acquire lock on the AsteroidSpawner's RNG field.");
+
+        // Use polar coordinate to decide where the asteroid will spawn
+        // Theta will be random between 0 to 2pi
+        let spawn_angle = rng.random_range(0.0..(std::f32::consts::PI * 2.0));
+        // Rho will be the radius of a circle bordering the viewport, multiplied by 1.2
+        // TODO: Use view diagonal to get a minimally sized circle around the play area
+        let spawn_distance = play_area.x.max(play_area.y) / 2.0;
+
+        // Convert polar to Cartesian, use as position
+        let pos = Vec2::new(
+            spawn_distance * spawn_angle.cos(),
+            spawn_distance * spawn_angle.sin(),
+        );
+
+        // Right now, I'm thinking I can use the opposite signs attached to the position Vec components.
+        // pos.x == -100, then vel.x = + <random>
+        // pos.x == 100, then vel.x = - <random>
+        // etc,
+        let mut vel = Vec2::new(rng.random_range(0.0..100.0), rng.random_range(0.0..100.0));
+        if pos.x > 0.0 {
+            vel.x *= -1.0;
+        }
+        if pos.y > 0.0 {
+            vel.y *= -1.0;
+        }
+
+        let size = match rng.random_range(0..=2) {
+            0 => AsteroidSize::Small,
+            1 => AsteroidSize::Medium,
+            2 => AsteroidSize::Large,
+            _ => unreachable!(),
+        };
+
+        events.write(SpawnAsteroid { pos, vel, size });
+    }
+}
+
+/// Sets a lifetime on an entity to automatically delete it after expiration.
+#[derive(Component)]
+pub struct Lifetime(pub Timer);
+
+/// System to tick the [`Lifetime`] timers and despawn expired entities.
+pub fn tick_lifetimes(
+    mut commands: Commands,
+    time: Res<Time>,
+    query: Query<(Entity, &mut Lifetime)>,
+) {
+    for (e, mut life) in query {
+        life.0.tick(time.delta());
+        if life.0.just_finished() {
+            commands.entity(e).despawn();
+        }
+    }
+}

src/main.rs

@@ -1,6 +1,7 @@
 use bevy::{prelude::*, window::WindowResolution};
 
-use asteroids::{config::WINDOW_SIZE, AsteroidPlugin};
+use asteroids::{AsteroidPlugin, config::WINDOW_SIZE};
+use bevy_inspector_egui::{bevy_egui::EguiPlugin, quick::WorldInspectorPlugin};
 
 fn main() {
     App::new()
@@ -12,5 +13,7 @@ fn main() {
             ..default()
         }))
         .add_plugins(AsteroidPlugin)
+        .add_plugins(EguiPlugin::default())
+        .add_plugins(WorldInspectorPlugin::new())
         .run();
 }

src/objects.rs

@@ -0,0 +1,207 @@
+//! This module contains all the "things" in the game.
+//!
+//! Asteroids, the player's ship, and such.
+
+use bevy::{
+    ecs::{
+        component::Component,
+        entity::Entity,
+        event::{EventReader, EventWriter},
+        query::With,
+        system::{Commands, Query, Res, ResMut, Single},
+    },
+    math::{Vec2, Vec3, Vec3Swizzles},
+    prelude::{Deref, DerefMut},
+    render::mesh::Mesh2d,
+    sprite::MeshMaterial2d,
+    state::state::NextState,
+    time::{Timer, TimerMode},
+    transform::components::Transform,
+};
+use bevy_rapier2d::prelude::{ActiveCollisionTypes, ActiveEvents, Collider, Sensor};
+
+use crate::{
+    AngularVelocity, GameAssets, GameState, Lives,
+    config::ASTEROID_LIFETIME,
+    events::{AsteroidDestroy, BulletDestroy, ShipDestroy, SpawnAsteroid},
+    machinery::Lifetime,
+    physics::{Velocity, Wrapping},
+};
+
+/// The asteroid, defined entirely by [it's size](`AsteroidSize`).
+#[derive(Component, Deref, DerefMut)]
+pub struct Asteroid(pub AsteroidSize);
+
+#[derive(Clone, Copy, Debug)]
+pub enum AsteroidSize {
+    Small,
+    Medium,
+    Large,
+}
+
+impl AsteroidSize {
+    /// Convenience util to get the "next smallest" size. Useful for splitting
+    /// after collision.
+    pub fn next(&self) -> Option<Self> {
+        match self {
+            AsteroidSize::Small => None,
+            AsteroidSize::Medium => Some(AsteroidSize::Small),
+            AsteroidSize::Large => Some(AsteroidSize::Medium),
+        }
+    }
+}
+
+/// Marker component for the player's ship.
+#[derive(Component)]
+pub struct Ship;
+
+/// Marker component for bullets.
+#[derive(Component)]
+pub struct Bullet;
+
+/// Responds to [`SpawnAsteroid`] events, spawning as specified
+pub fn spawn_asteroid(
+    mut events: EventReader<SpawnAsteroid>,
+    mut commands: Commands,
+    game_assets: Res<GameAssets>,
+) {
+    for spawn in events.read() {
+        let (mesh, material) = match spawn.size {
+            AsteroidSize::Small => game_assets.asteroid_small(),
+            AsteroidSize::Medium => game_assets.asteroid_medium(),
+            AsteroidSize::Large => game_assets.asteroid_large(),
+        };
+
+        let collider_radius = match spawn.size {
+            AsteroidSize::Small => 10.0,
+            AsteroidSize::Medium => 20.0,
+            AsteroidSize::Large => 40.0,
+        };
+
+        commands.spawn((
+            Asteroid(spawn.size),
+            Collider::ball(collider_radius),
+            Sensor,
+            Transform::from_translation(spawn.pos.extend(0.0)),
+            Velocity(spawn.vel),
+            Mesh2d(mesh),
+            MeshMaterial2d(material),
+            Lifetime(Timer::from_seconds(ASTEROID_LIFETIME, TimerMode::Once)),
+        ));
+    }
+}
+
+/// Event listener for asteroid destruction events. Shrinks and multiplies
+/// asteroids until they vanish.
+///
+/// - Large -> 2x Medium
+/// - Medium -> 2x Small
+/// - Small -> (despawned)
+///
+/// The velocity of the child asteroids is scattered somewhat, as if they were
+/// explosively pushed apart.
+pub fn split_asteroids(
+    mut destroy_events: EventReader<AsteroidDestroy>,
+    mut respawn_events: EventWriter<SpawnAsteroid>,
+    mut commands: Commands,
+    query: Query<(&Transform, &Asteroid, &Velocity)>,
+) {
+    for event in destroy_events.read() {
+        if let Ok((transform, rock, velocity)) = query.get(event.0) {
+            let next_size = rock.0.next();
+            if let Some(size) = next_size {
+                let pos = transform.translation.xy();
+                let left_offset = Vec2::from_angle(0.4);
+                let right_offset = Vec2::from_angle(-0.4);
+                respawn_events.write(SpawnAsteroid {
+                    pos,
+                    vel: left_offset.rotate(velocity.0),
+                    size,
+                });
+                respawn_events.write(SpawnAsteroid {
+                    pos,
+                    vel: right_offset.rotate(velocity.0),
+                    size,
+                });
+            }
+            // Always despawn the asteroid. New ones (may) be spawned in it's
+            // place, but this one is gone.
+            commands.entity(event.0).despawn();
+        }
+    }
+}
+
+/// Spawns the player at the world origin. Used during the state change to
+/// [`GameState::Playing`] to spawn the player.
+///
+/// This only spawns the player. For player **re**-spawn activity, see the
+/// [`ship_impact_listener()`] system.
+pub fn spawn_player(mut commands: Commands, game_assets: Res<GameAssets>) {
+    commands
+        .spawn((
+            Collider::ball(0.7),
+            Sensor,
+            ActiveEvents::COLLISION_EVENTS,
+            ActiveCollisionTypes::STATIC_STATIC,
+            Ship,
+            Wrapping,
+            Velocity(Vec2::ZERO),
+            AngularVelocity(0.0),
+            Mesh2d(game_assets.ship().0),
+            MeshMaterial2d(game_assets.ship().1),
+            Transform::default().with_scale(Vec3::new(20.0, 20.0, 20.0)),
+        ))
+        .with_child((
+            Mesh2d(game_assets.thruster_mesh()),
+            MeshMaterial2d(game_assets.thruster_mat_inactive()),
+            Transform::default()
+                .with_scale(Vec3::splat(0.5))
+                .with_translation(Vec3::new(-0.5, 0.0, -0.1)),
+        ));
+}
+
+/// Watch for [`BulletDestroy`] events and despawn
+/// the associated bullet.
+pub fn bullet_impact_listener(mut commands: Commands, mut events: EventReader<BulletDestroy>) {
+    for event in events.read() {
+        commands.entity(event.0).despawn();
+    }
+}
+
+/// Watch for [`ShipDestroy`] events and update game state accordingly.
+///
+/// One life is taken from the counter, asteroids are cleared, and the player
+/// is placed back at the origin. If lives reach 0, this system will change
+/// states to [`GameState::GameOver`].
+/// - Subtract a life
+/// - Check life count. If 0, go to game-over state
+/// - Clear all asteroids
+/// - Respawn player
+pub fn ship_impact_listener(
+    mut events: EventReader<ShipDestroy>,
+    mut commands: Commands,
+    mut lives: ResMut<Lives>,
+    rocks: Query<Entity, With<Asteroid>>,
+    mut player: Single<(&mut Transform, &mut Velocity), With<Ship>>,
+    mut next_state: ResMut<NextState<GameState>>,
+) {
+    for _ in events.read() {
+        // STEP 1: Decrement lives (and maybe go to game over)
+        if lives.0 == 0 {
+            // If already at 0, game is over.
+            next_state.set(GameState::GameOver);
+        } else {
+            // Decrease life count.
+            lives.0 -= 1;
+        }
+
+        // STEP 2: Clear asteroids
+        for rock in rocks {
+            commands.entity(rock).despawn();
+        }
+
+        // STEP 3: Respawn player (teleport them to the origin)
+        player.0.translation = Vec3::ZERO;
+        player.1.0 = Vec2::ZERO;
+    }
+}

src/physics.rs

@@ -0,0 +1,140 @@
+//! Custom physics items
+//!
+//! TODO: Refactor in terms of Rapier2D, *or* implement colliders and remove it.
+//!
+//! Position, both translation and rotation, are tracked by the built-in
+//! Bevy [`Transform`] component. This module adds a (Linear) [`Velocity`] and
+//! [`AngularVelocity`] component to the mix.
+//! These two components are operated by simple integrator systems to
+//! accumulate translation and rotation from the velocities.
+//!
+//! The [`Wrapping`] component marks things that should wrap around the world
+//! boundaries. It's a kind of physical property of this world, so I'm putting
+//! it here.
+//!
+//! Collisions are also considered physics. After all, I got *a physics engine*
+//! to detect them for me (so that I don't have to write clipping code).
+
+use crate::{
+    WorldSize, events,
+    objects::{Asteroid, Bullet, Ship},
+};
+
+use bevy::prelude::*;
+use bevy_rapier2d::pipeline::CollisionEvent;
+
+#[derive(Clone, Component)]
+pub(crate) struct Velocity(pub(crate) bevy::math::Vec2);
+
+#[derive(Component)]
+pub(crate) struct AngularVelocity(pub(crate) f32);
+
+/// Marker for any entity that should wrap on screen edges
+#[derive(Component)]
+pub(crate) struct Wrapping;
+
+/// Integrate linear velocity and update the entity's transform.
+pub(crate) fn integrate_velocity(mut query: Query<(&mut Transform, &Velocity)>, time: Res<Time>) {
+    for (mut transform, velocity) in &mut query {
+        let delta = velocity.0 * time.delta_secs();
+        transform.translation += delta.extend(0.0);
+    }
+}
+
+/// Integrate angular velocity and update the entity's transform.
+pub(crate) fn integrate_angular_velocity(
+    mut objects: Query<(&mut Transform, &AngularVelocity)>,
+    time: Res<Time>,
+) {
+    for (mut transform, ang_vel) in &mut objects {
+        let delta = ang_vel.0 * time.delta_secs();
+        transform.rotate_z(delta);
+    }
+}
+
+pub(crate) fn wrap_entities(
+    mut query: Query<&mut Transform, With<Wrapping>>,
+    world_size: Res<WorldSize>,
+) {
+    let right = world_size.x / 2.0;
+    let left = -right;
+    let top = world_size.y / 2.0;
+    let bottom = -top;
+
+    for mut pos in query.iter_mut() {
+        if pos.translation.x > right {
+            pos.translation.x = left;
+        } else if pos.translation.x < left {
+            pos.translation.x = right;
+        }
+
+        if pos.translation.y > top {
+            pos.translation.y = bottom;
+        } else if pos.translation.y < bottom {
+            pos.translation.y = top;
+        }
+    }
+}
+
+/// The collision event routing system.
+///
+/// When a `CollisionEvent` occurrs, this system checks which things collided
+/// and emits secondary events accordignly.
+///
+/// | Objects | Response |
+/// |-|-|
+/// | Ship & Asteroid | emits event [`ShipDestroy`](`crate::events::ShipDestroy`) |
+/// | Asteroid & Bullet | emits event [`AsteroidDestroy`](`crate::events::AsteroidDestroy`) |
+/// | Asteroid & Asteroid | Nothing. Asteroids won't collide with each other |
+/// | Bullet & Bullet | Nothing. Bullets won't collide with each other (and probably can't under normal gameplay conditions) |
+/// | Bullet & Ship | Nothing. The player shouldn't be able to shoot themselves (and the Flying Saucer hasn't been impl.'d, so it's bullets don't count) |
+pub fn collision_listener(
+    mut collisions: EventReader<CollisionEvent>,
+    mut ship_writer: EventWriter<events::ShipDestroy>,
+    mut asteroid_writer: EventWriter<events::AsteroidDestroy>,
+    mut bullet_writer: EventWriter<events::BulletDestroy>,
+    player: Single<Entity, With<Ship>>,
+    bullets: Query<&Bullet>,
+    rocks: Query<&Asteroid>,
+) {
+    for event in collisions.read() {
+        if let CollisionEvent::Started(one, two, _flags) = event {
+            // Valid collisions are:
+            //
+            // - Ship & Asteroid
+            // - Bullet & Asteroid
+            //
+            // Asteroids don't collide with each other, bullets don't collide
+            // with each other, and bullets don't collide with the player ship.
+
+            // Option 1: Ship & Asteroid
+            if *one == *player {
+                if rocks.contains(*two) {
+                    // player-asteroid collision
+                    dbg!("Writing ShipDestroy event");
+                    ship_writer.write(events::ShipDestroy);
+                } // else, we don't care
+            } else if *two == *player {
+                if rocks.contains(*one) {
+                    dbg!("Writing ShipDestroy event");
+                    ship_writer.write(events::ShipDestroy);
+                }
+            }
+
+            // Option 2: Bullet & Asteroid
+            if bullets.contains(*one) {
+                if rocks.contains(*two) {
+                    dbg!("Writing AsteroidDestroy & BulletDestroy events");
+                    asteroid_writer.write(events::AsteroidDestroy(*two));
+                    bullet_writer.write(events::BulletDestroy(*one));
+                }
+            } else if rocks.contains(*one) {
+                if bullets.contains(*two) {
+                    dbg!("Writing AsteroidDestroy & BulletDestroy events");
+                    asteroid_writer.write(events::AsteroidDestroy(*one));
+                    bullet_writer.write(events::BulletDestroy(*two));
+                }
+            }
+        }
+    }
+}

src/resources.rs

@@ -0,0 +1,124 @@
+//! All the resources for the game
+
+use bevy::{
+    asset::{Assets, Handle},
+    ecs::{
+        resource::Resource,
+        world::{FromWorld, World},
+    },
+    math::{
+        Vec2,
+        primitives::{Circle, Triangle2d},
+    },
+    prelude::{Deref, DerefMut, Reflect, ReflectResource},
+    render::mesh::Mesh,
+    sprite::ColorMaterial,
+};
+use bevy_inspector_egui::InspectorOptions;
+use bevy_inspector_egui::inspector_options::ReflectInspectorOptions;
+
+use crate::{
+    ASTEROID_SMALL_COLOR, BULLET_COLOR, PLAYER_SHIP_COLOR, SHIP_THRUSTER_COLOR_ACTIVE,
+    SHIP_THRUSTER_COLOR_INACTIVE, config::WINDOW_SIZE,
+};
+
+#[derive(Resource, Debug, Deref, Clone, Copy)]
+pub struct Score(pub i32);
+
+impl From<Score> for String {
+    fn from(value: Score) -> Self {
+        value.to_string()
+    }
+}
+
+#[derive(InspectorOptions, Reflect, Resource, Debug, Deref, Clone, Copy)]
+#[reflect(Resource, InspectorOptions)]
+pub struct Lives(pub i32);
+
+impl From<Lives> for String {
+    fn from(value: Lives) -> Self {
+        value.to_string()
+    }
+}
+
+#[derive(Deref, DerefMut, Resource)]
+pub struct WorldSize(Vec2);
+
+impl Default for WorldSize {
+    fn default() -> Self {
+        WorldSize(Vec2::new(WINDOW_SIZE.x, WINDOW_SIZE.y))
+    }
+}
+
+#[derive(Resource)]
+pub struct GameAssets {
+    meshes: [Handle<Mesh>; 5],
+    materials: [Handle<ColorMaterial>; 7],
+}
+
+impl GameAssets {
+    pub fn ship(&self) -> (Handle<Mesh>, Handle<ColorMaterial>) {
+        (self.meshes[0].clone(), self.materials[0].clone())
+    }
+
+    // The thruster mesh is actually just the ship mesh
+    pub fn thruster_mesh(&self) -> Handle<Mesh> {
+        self.meshes[0].clone()
+    }
+
+    // TODO: Look into parameterizing the material
+    // A shader uniform should be able to do this, but I don't know how to
+    // load those in Bevy.
+    pub fn thruster_mat_inactive(&self) -> Handle<ColorMaterial> {
+        self.materials[1].clone()
+    }
+
+    pub fn thruster_mat_active(&self) -> Handle<ColorMaterial> {
+        self.materials[2].clone()
+    }
+
+    pub fn asteroid_small(&self) -> (Handle<Mesh>, Handle<ColorMaterial>) {
+        (self.meshes[1].clone(), self.materials[1].clone())
+    }
+
+    pub fn asteroid_medium(&self) -> (Handle<Mesh>, Handle<ColorMaterial>) {
+        (self.meshes[2].clone(), self.materials[2].clone())
+    }
+
+    pub fn asteroid_large(&self) -> (Handle<Mesh>, Handle<ColorMaterial>) {
+        (self.meshes[3].clone(), self.materials[3].clone())
+    }
+
+    pub fn bullet(&self) -> (Handle<Mesh>, Handle<ColorMaterial>) {
+        (self.meshes[4].clone(), self.materials[6].clone())
+    }
+}
+
+impl FromWorld for GameAssets {
+    fn from_world(world: &mut World) -> Self {
+        let mut world_meshes = world.resource_mut::<Assets<Mesh>>();
+        let meshes = [
+            world_meshes.add(Triangle2d::new(
+                Vec2::new(0.5, 0.0),
+                Vec2::new(-0.5, 0.45),
+                Vec2::new(-0.5, -0.45),
+            )),
+            world_meshes.add(Circle::new(10.0)),
+            world_meshes.add(Circle::new(20.0)),
+            world_meshes.add(Circle::new(40.0)),
+            world_meshes.add(Circle::new(0.2)),
+        ];
+        let mut world_materials = world.resource_mut::<Assets<ColorMaterial>>();
+        let materials = [
+            world_materials.add(PLAYER_SHIP_COLOR),
+            world_materials.add(SHIP_THRUSTER_COLOR_INACTIVE),
+            world_materials.add(SHIP_THRUSTER_COLOR_ACTIVE),
+            world_materials.add(ASTEROID_SMALL_COLOR),
+            // TODO: asteroid medium and large colors
+            world_materials.add(ASTEROID_SMALL_COLOR),
+            world_materials.add(ASTEROID_SMALL_COLOR),
+            world_materials.add(BULLET_COLOR),
+        ];
+        GameAssets { meshes, materials }
+    }
+}

src/widgets.rs

@@ -0,0 +1,301 @@
+use crate::{
+    GameState,
+    config::{UI_BUTTON_HOVERED, UI_BUTTON_NORMAL, UI_BUTTON_PRESSED},
+    resources::{Lives, Score},
+};
+
+use bevy::{
+    color::palettes::css::{BLACK, DARK_GRAY, GREEN, LIGHT_BLUE, RED, WHITE},
+    prelude::*,
+};
+
+/// Plugin for the main menu
+pub struct PluginGameMenu;
+
+impl Plugin for PluginGameMenu {
+    fn build(&self, app: &mut App) {
+        app.add_systems(OnEnter(GameState::TitleScreen), spawn_menu)
+            .add_systems(OnExit(GameState::TitleScreen), despawn::<MarkerMainMenu>)
+            .add_systems(
+                Update,
+                (handle_spacebar, operate_buttons).run_if(in_state(GameState::TitleScreen)),
+            );
+    }
+}
+
+/// Plugin for the "get ready" period as gameplay starts.
+pub struct PluginGetReady;
+
+impl Plugin for PluginGetReady {
+    fn build(&self, app: &mut App) {
+        app.add_systems(OnEnter(GameState::GetReady), spawn_get_ready)
+            .add_systems(OnExit(GameState::GetReady), despawn::<OnReadySetGo>)
+            .add_systems(
+                Update,
+                (animate_get_ready_widget).run_if(in_state(GameState::GetReady)),
+            )
+            .insert_resource(ReadySetGoTimer(Timer::from_seconds(3.0, TimerMode::Once)));
+    }
+}
+
+/// Plugin for the game-over screen
+pub struct PluginGameOver;
+
+impl Plugin for PluginGameOver {
+    fn build(&self, app: &mut App) {
+        app.add_systems(OnEnter(GameState::GameOver), spawn_gameover_ui)
+            .add_systems(OnExit(GameState::GameOver), despawn::<MarkerGameOver>)
+            .add_systems(
+                Update,
+                operate_buttons.run_if(in_state(GameState::GameOver)),
+            );
+    }
+}
+
+// Marker component for the title screen UI entity.
+// This way, a query for the TitleUI can be used to despawn the title screen
+#[derive(Component)]
+struct MarkerMainMenu;
+
+/// Marker component for things on the get-ready indicator
+#[derive(Component)]
+struct OnReadySetGo;
+
+/// Marker for things on the game-over screen
+#[derive(Component)]
+struct MarkerGameOver;
+
+/// Action specifier for the game-over menu's buttons.
+///
+/// Attach this component to a button and [`PluginGameOver`] will use it to
+/// decide what to do when that button is pressed.
+#[derive(Component)]
+enum ButtonMenuAction {
+    ToMainMenu,
+    StartGame,
+    Quit,
+}
+
+/// Newtype wrapper for `Timer`. Used to count down during the "get ready" phase.
+#[derive(Deref, DerefMut, Resource)]
+struct ReadySetGoTimer(Timer);
+
+/// Marker for the counter text segment
+#[derive(Component)]
+struct CountdownText;
+
+/// Marker for the counter bar segment
+#[derive(Component)]
+struct CountdownBar;
+
+/// Despawns entities matching the generic argument. Intended to remove UI
+/// elements.
+fn despawn<T: Component>(mut commands: Commands, to_despawn: Query<Entity, With<T>>) {
+    for entity in to_despawn {
+        commands.entity(entity).despawn();
+    }
+}
+
+/// Utility function for creating a standard button.
+fn button_bundle(text: &str) -> impl Bundle {
+    (
+        Button,
+        // TODO: Generic action
+        Node {
+            width: Val::Px(150.0),
+            height: Val::Px(65.0),
+            border: UiRect::all(Val::Px(2.0)),
+            justify_content: JustifyContent::Center,
+            align_items: AlignItems::Center,
+            margin: UiRect::all(Val::Px(5.0)),
+            ..default()
+        },
+        BorderColor(Color::BLACK),
+        BorderRadius::MAX,
+        BackgroundColor(UI_BUTTON_NORMAL),
+        children![(
+            Text::new(text),
+            TextColor(Color::srgb(0.9, 0.9, 0.9)),
+            TextShadow::default(),
+        )],
+    )
+}
+
+fn spawn_menu(mut commands: Commands) {
+    commands
+        .spawn((
+            MarkerMainMenu,
+            Node {
+                width: Val::Percent(100.0),
+                height: Val::Percent(100.0),
+                align_items: AlignItems::Center,
+                justify_content: JustifyContent::Center,
+                flex_direction: FlexDirection::Column,
+                ..Default::default()
+            },
+        ))
+        .with_children(|cmds| {
+            cmds.spawn((
+                Text::new("Robert's Bad Asteroids Game"),
+                TextFont::from_font_size(50.0),
+                TextLayout::new_with_justify(JustifyText::Center),
+                TextShadow::default(),
+            ));
+            cmds.spawn((
+                Text::new("Press space to begin"),
+                TextFont::from_font_size(20.0),
+                TextColor(Color::srgb(0.7, 0.7, 0.7)),
+                TextShadow::default(),
+            ));
+            cmds.spawn((button_bundle("Start Game"), ButtonMenuAction::StartGame));
+            cmds.spawn((button_bundle("Quit"), ButtonMenuAction::Quit));
+        });
+}
+
+fn spawn_get_ready(mut commands: Commands) {
+    commands.spawn((
+        OnReadySetGo, // marker, so this can be de-spawned properly
+        Node {
+            align_self: AlignSelf::Center,
+            justify_self: JustifySelf::Center,
+            align_items: AlignItems::Center,
+            justify_content: JustifyContent::Center,
+            flex_direction: FlexDirection::Column,
+            width: Val::Percent(30.),
+            height: Val::Percent(30.),
+            ..default()
+        },
+        BackgroundColor(LIGHT_BLUE.into()),
+        children![
+            (Text::new("Get Ready!"), TextColor(BLACK.into())),
+            (
+                CountdownBar,
+                Node {
+                    width: Val::Percent(90.0),
+                    height: Val::Percent(10.),
+                    ..default()
+                },
+                BackgroundColor(GREEN.into()),
+            ),
+            (
+                CountdownText,
+                Text::new("<uninit timer>"),
+                TextColor(RED.into()),
+            )
+        ],
+    ));
+}
+
+/// Spawns the game over screen.
+///
+/// Used by [`PluginGameOver`] when entering the [`GameState::GameOver`] state.
+fn spawn_gameover_ui(mut commands: Commands) {
+    commands.spawn((
+        MarkerGameOver, // Marker, so `despawn<T>` can remove this on state exit.
+        Node {
+            width: Val::Percent(100.0),
+            height: Val::Percent(100.0),
+            align_items: AlignItems::Center,
+            justify_content: JustifyContent::Center,
+            flex_direction: FlexDirection::Column,
+            ..default()
+        },
+        children![
+            (button_bundle("Main Menu"), ButtonMenuAction::ToMainMenu,),
+            (button_bundle("Quit"), ButtonMenuAction::Quit),
+        ],
+    ));
+}
+
+fn animate_get_ready_widget(
+    mut text_segment: Single<&mut Text, With<CountdownText>>,
+    mut bar_segment: Single<&mut Node, With<CountdownBar>>,
+    time: Res<Time>,
+    mut timer: ResMut<ReadySetGoTimer>,
+    mut game_state: ResMut<NextState<GameState>>,
+) {
+    // Advance the timer, read the remaining time and write it onto the label.
+    timer.tick(time.delta());
+
+    // Add one to the visual value so the countdown starts at 3 and stops at 1.
+    // Otherwise it starts at 2 and disappears after showing 0.
+    // That feels wrong even though it's functionally identical.
+    let tval = timer.0.remaining().as_secs() + 1;
+    **text_segment = format!("{tval}").into();
+
+    // Shrink the progress bar Node
+    bar_segment.width = Val::Percent(100.0 * (1.0 - timer.fraction()));
+
+    // If the timer has expired, change state to playing.
+    if timer.finished() {
+        game_state.set(GameState::Playing);
+    }
+}
+
+/// Handles interactions with the menu buttons.
+///
+/// The buttons are used by the main menu and the game-over menu to change
+/// between game states.
+///
+/// Button animation and action handling is done entirely within this system.
+///
+/// There are no checks for current state. If a "quit" button was put somewhere
+/// on the HUD, this system would quit the game. The same will happen for
+/// returning to the title screen. This should be useful for making a pause
+/// menu, too.
+fn operate_buttons(
+    mut interactions: Query<
+        (
+            &Interaction,
+            &mut BackgroundColor,
+            &mut BorderColor,
+            &ButtonMenuAction,
+        ),
+        (Changed<Interaction>, With<Button>),
+    >,
+    mut game_state: ResMut<NextState<GameState>>,
+    mut app_exit_events: EventWriter<AppExit>,
+) {
+    // TODO: Better colors. These are taken from the example and they're ugly.
+    for (interaction, mut color, mut border_color, menu_action) in &mut interactions {
+        match *interaction {
+            Interaction::Pressed => {
+                *color = UI_BUTTON_PRESSED.into();
+                border_color.0 = DARK_GRAY.into();
+                match menu_action {
+                    ButtonMenuAction::ToMainMenu => {
+                        game_state.set(GameState::TitleScreen);
+                    }
+                    ButtonMenuAction::StartGame => {
+                        game_state.set(GameState::Playing);
+                    }
+                    ButtonMenuAction::Quit => {
+                        app_exit_events.write(AppExit::Success);
+                    }
+                }
+            }
+            Interaction::Hovered => {
+                *color = UI_BUTTON_HOVERED.into();
+                border_color.0 = WHITE.into();
+            }
+            Interaction::None => {
+                *color = UI_BUTTON_NORMAL.into();
+                border_color.0 = BLACK.into();
+            }
+        }
+    }
+}
+
+/// Main menu input listener. Starts game when the spacebar is pressed.
+fn handle_spacebar(input: Res<ButtonInput<KeyCode>>, mut game_state: ResMut<NextState<GameState>>) {
+    if input.just_pressed(KeyCode::Space) {
+        game_state.set(GameState::GetReady);
+    }
+}
+
+pub fn spawn_ui(mut commands: Commands, score: Res<Score>, lives: Res<Lives>) {
+    commands.spawn((
+        Text::new(format!("Score: {score:?} | Lives: {lives:?}")),
+        TextFont::from_font_size(25.0),
+    ));
+}