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

@@ -1,10 +1,33 @@
 [package]
 name = "asteroids"
-version = "0.2.0"
+version = "0.3.0"
 edition = "2024"
 
 [dependencies]
-bevy = { version = "0.16", features = ["dynamic_linking"] }
+bevy = "0.16"
 bevy-inspector-egui = "0.32.0"
-bevy_rapier2d = { version = "0.31.0", features = ["debug-render-2d"] }
+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

@@ -5,6 +5,10 @@ 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);
@@ -17,5 +21,8 @@ pub(crate) const SHIP_THRUST: f32 = 1.0;
 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/event.rs

@@ -1,21 +0,0 @@
-use bevy::prelude::*;
-
-/// Signals that the player's ship has been destroyed.
-/// Used when the player collides with an asteroid.
-#[derive(Event)]
-pub(crate) struct ShipDestroy;
-
-/// Signals that a particular asteroid has been destroyed.
-/// Used to split (or vanish) an asteroid when a bullet strikes it.
-#[derive(Event)]
-pub(crate) struct AsteroidDestroy(pub Entity);
-
-// TODO: BulletDestroy
-// Which depends on the still-pending Bullet component creation.
-
-/// Signals that a particular bullet has been destroyed.
-/// Used to despawn the bullet 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/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

@@ -1,51 +1,57 @@
-mod asteroids;
+//! Asteroids, implemented as a Bevy plugin.
-pub mod config;
+//!
-mod event;
+//! Compile-time configurables can be found in the [`config`] module.
-mod physics;
+
-mod preparation_widget;
+pub mod config;
-mod ship;
+mod events;
-mod title_screen;
+mod machinery;
+mod objects;
+mod physics;
+mod resources;
+mod widgets;
 
-use crate::asteroids::{Asteroid, AsteroidSpawner};
 use crate::config::{
-    ASTEROID_SMALL_COLOR, BACKGROUND_COLOR, BULLET_COLOR, BULLET_SPEED, PLAYER_SHIP_COLOR,
+    ASTEROID_SMALL_COLOR, BACKGROUND_COLOR, BULLET_COLOR, BULLET_LIFETIME, BULLET_SPEED,
-    SHIP_ROTATION, SHIP_THRUST, SHIP_THRUSTER_COLOR_ACTIVE, SHIP_THRUSTER_COLOR_INACTIVE,
+    PLAYER_SHIP_COLOR, SHIP_ROTATION, SHIP_THRUST, SHIP_THRUSTER_COLOR_ACTIVE,
-    WINDOW_SIZE,
+    SHIP_THRUSTER_COLOR_INACTIVE,
 };
+use crate::machinery::AsteroidSpawner;
+use crate::objects::{Bullet, Ship};
 use crate::physics::AngularVelocity;
-use crate::ship::Ship;
 
 use bevy::prelude::*;
-use bevy_inspector_egui::InspectorOptions;
-use bevy_inspector_egui::prelude::ReflectInspectorOptions;
 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_plugins((
-            title_screen::GameMenuPlugin,
+            widgets::PluginGameMenu,
-            preparation_widget::preparation_widget_plugin,
+            widgets::PluginGameOver,
+            widgets::PluginGetReady,
             RapierPhysicsPlugin::<NoUserData>::pixels_per_meter(10.0),
             RapierDebugRenderPlugin::default(),
         ))
         .insert_resource(ClearColor(BACKGROUND_COLOR))
-        .insert_resource(WorldSize {
+        .insert_resource(WorldSize::default())
-            width: WINDOW_SIZE.x,
-            height: WINDOW_SIZE.y,
-        })
         .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), (ship::spawn_player, spawn_ui))
+        .add_systems(
+            OnEnter(GameState::Playing),
+            (objects::spawn_player, widgets::spawn_ui),
+        )
         .add_systems(
             FixedUpdate,
             (
@@ -53,11 +59,13 @@ impl Plugin for AsteroidPlugin {
                 input_ship_rotation,
                 input_ship_shoot,
                 physics::wrap_entities,
-                asteroids::tick_asteroid_manager,
+                machinery::tick_asteroid_manager,
-                asteroids::spawn_asteroid.after(asteroids::tick_asteroid_manager),
+                objects::spawn_asteroid.after(machinery::tick_asteroid_manager),
-                collision_listener,
+                objects::split_asteroids,
-                // TODO: Remove debug printing
+                objects::bullet_impact_listener,
-                debug_collision_event_printer,
+                objects::ship_impact_listener,
+                physics::collision_listener,
+                machinery::tick_lifetimes,
             )
                 .run_if(in_state(GameState::Playing)),
         )
@@ -69,67 +77,15 @@ impl Plugin for AsteroidPlugin {
             )
                 .run_if(in_state(GameState::Playing)),
         )
-        .add_event::<asteroids::SpawnAsteroid>()
+        .add_event::<events::SpawnAsteroid>()
-        .add_event::<event::AsteroidDestroy>()
+        .add_event::<events::AsteroidDestroy>()
-        .add_event::<event::ShipDestroy>();
+        .add_event::<events::ShipDestroy>()
-        app.insert_state(GameState::Playing);
+        .add_event::<events::BulletDestroy>();
-    }
+        app.insert_state(GameState::TitleScreen);
-}
-
-fn debug_collision_event_printer(mut collision_events: EventReader<CollisionEvent>) {
-    for event in collision_events.read() {
-        dbg!(event);
-    }
-}
-
-/// 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::event::ShipDestroy`) |
-/// | Asteroid & Bullet | emits event [`AsteroidDestroy`](`crate::event::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) |
-fn collision_listener(
-    mut collisions: EventReader<CollisionEvent>,
-    mut ship_writer: EventWriter<event::ShipDestroy>,
-    mut asteroid_writer: EventWriter<event::AsteroidDestroy>,
-    player: Single<Entity, With<Ship>>,
-    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(event::ShipDestroy);
-                } // else, we don't care
-            } else if *two == *player {
-                if rocks.contains(*one) {
-                    dbg!("Writing ShipDestroy event");
-                    ship_writer.write(event::ShipDestroy);
-                }
-            }
-
-            // TODO: Bullet-asteroid collisions
-        }
     }
 }
 
+/// The game's main state tracking mechanism, registered with Bevy as a [`State`](`bevy::prelude::State`).
 #[derive(Clone, Debug, Eq, Hash, PartialEq, States)]
 pub enum GameState {
     TitleScreen, // Program is started. Present title screen and await user start
@@ -138,111 +94,13 @@ pub enum GameState {
     GameOver,    // Game has ended. Present game over dialogue and await user restart
 }
 
-#[derive(Resource, Debug, Deref, Clone, Copy)]
-struct Score(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)]
-struct Lives(i32);
-
-impl From<Lives> for String {
-    fn from(value: Lives) -> Self {
-        value.to_string()
-    }
-}
-
-#[derive(Resource)]
-struct WorldSize {
-    width: f32,
-    height: f32,
-}
-
-#[derive(Resource)]
-struct GameAssets {
-    meshes: [Handle<Mesh>; 5],
-    materials: [Handle<ColorMaterial>; 7],
-}
-
-impl GameAssets {
-    fn ship(&self) -> (Handle<Mesh>, Handle<ColorMaterial>) {
-        (self.meshes[0].clone(), self.materials[0].clone())
-    }
-
-    // The thruster mesh is actually just the ship mesh
-    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.
-    fn thruster_mat_inactive(&self) -> Handle<ColorMaterial> {
-        self.materials[1].clone()
-    }
-
-    fn thruster_mat_active(&self) -> Handle<ColorMaterial> {
-        self.materials[2].clone()
-    }
-
-    fn asteroid_small(&self) -> (Handle<Mesh>, Handle<ColorMaterial>) {
-        (self.meshes[1].clone(), self.materials[1].clone())
-    }
-
-    fn asteroid_medium(&self) -> (Handle<Mesh>, Handle<ColorMaterial>) {
-        (self.meshes[2].clone(), self.materials[2].clone())
-    }
-
-    fn asteroid_large(&self) -> (Handle<Mesh>, Handle<ColorMaterial>) {
-        (self.meshes[3].clone(), self.materials[3].clone())
-    }
-
-    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 }
-    }
-}
-
 fn spawn_camera(mut commands: Commands) {
     commands.spawn(Camera2d);
 }
 
-/*
+/// Player's thruster control system.
- Checks if "W" is pressed and increases velocity accordingly.
+///
-*/
+/// Checks if "W" is pressed and increases velocity accordingly.
 fn input_ship_thruster(
     keyboard_input: Res<ButtonInput<KeyCode>>,
     mut query: Query<(&mut physics::Velocity, &Transform, &mut Children), With<Ship>>,
@@ -273,10 +131,10 @@ fn input_ship_thruster(
     }
 }
 
-/*
+/// 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 AngularVelocity, With<Ship>>,
@@ -295,6 +153,12 @@ fn input_ship_rotation(
     }
 }
 
+/// Player's gun trigger.
+///
+/// Checks if the spacebar has just been pressed, spawning a bullet if so.
+///
+/// TODO: Hook up a timer to control weapon fire-rate. Something will have to
+/// tick those timers. Maybe this system?
 fn input_ship_shoot(
     keyboard_input: Res<ButtonInput<KeyCode>>,
     ship: Single<(&Transform, &physics::Velocity), With<Ship>>,
@@ -311,7 +175,7 @@ fn input_ship_shoot(
     // For now, spawn one for each press of the spacebar.
     if keyboard_input.just_pressed(KeyCode::Space) {
         commands.spawn((
-            ship::Bullet,
+            Bullet,
             Collider::ball(0.2),
             Sensor,
             ActiveEvents::COLLISION_EVENTS,
@@ -320,13 +184,7 @@ fn input_ship_shoot(
             Mesh2d(game_assets.bullet().0),
             MeshMaterial2d(game_assets.bullet().1),
             ship_pos.clone(), // clone ship transform
+            Lifetime(Timer::from_seconds(BULLET_LIFETIME, TimerMode::Once)),
         ));
     }
 }
-
-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),
-    ));
-}

src/machinery.rs

@@ -1,23 +1,21 @@
-//! This is the module containing all the rock-related things.
+//! Systems, Components, and any other items for powering the game logic.
-//! Not... not the whole game.
+//!
-
+//! Where the objects (ship, asteroid, etc) carry their own behavioral systems,
-use bevy_rapier2d::prelude::*;
+//! 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::{GameAssets, WorldSize, physics::Velocity};
+use crate::{WorldSize, events::SpawnAsteroid, objects::AsteroidSize};
-
-#[derive(Component, Deref, DerefMut)]
-pub struct Asteroid(AsteroidSize);
-
-pub enum AsteroidSize {
-    Small,
-    Medium,
-    Large,
-}
 
+/// 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>,
@@ -37,15 +35,8 @@ impl AsteroidSpawner {
     }
 }
 
-#[derive(Event)]
+/// Update the asteroid spawn timer in the [`AsteroidSpawner`] resource, and
-pub struct SpawnAsteroid {
+/// spawns any asteroids that are due this frame.
-    pos: Vec2,
-    vel: Vec2,
-    size: AsteroidSize,
-}
-
-/// Update the asteroid spawn timer and spawn any asteroids
-/// that are due this frame.
 pub fn tick_asteroid_manager(
     mut events: EventWriter<SpawnAsteroid>,
     mut spawner: ResMut<AsteroidSpawner>,
@@ -64,7 +55,7 @@ pub fn tick_asteroid_manager(
         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.width.max(play_area.height) / 2.0;
+        let spawn_distance = play_area.x.max(play_area.y) / 2.0;
 
         // Convert polar to Cartesian, use as position
         let pos = Vec2::new(
@@ -95,35 +86,20 @@ pub fn tick_asteroid_manager(
     }
 }
 
-/// Utility function to spawn a single asteroid of a given type
+/// Sets a lifetime on an entity to automatically delete it after expiration.
-/// TODO: convert to an event listener monitoring for "spawn asteroid" events
+#[derive(Component)]
-/// from the `fn tick_asteroid_manager(...)` system.
+pub struct Lifetime(pub Timer);
-pub fn spawn_asteroid(
+
-    mut events: EventReader<SpawnAsteroid>,
+/// System to tick the [`Lifetime`] timers and despawn expired entities.
+pub fn tick_lifetimes(
     mut commands: Commands,
-    game_assets: Res<GameAssets>,
+    time: Res<Time>,
+    query: Query<(Entity, &mut Lifetime)>,
 ) {
-    for spawn in events.read() {
+    for (e, mut life) in query {
-        let (mesh, material) = match spawn.size {
+        life.0.tick(time.delta());
-            AsteroidSize::Small => game_assets.asteroid_small(),
+        if life.0.just_finished() {
-            AsteroidSize::Medium => game_assets.asteroid_medium(),
+            commands.entity(e).despawn();
-            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(AsteroidSize::Small),
-            Collider::ball(collider_radius),
-            Sensor,
-            Transform::from_translation(spawn.pos.extend(0.0)),
-            Velocity(spawn.vel),
-            Mesh2d(mesh),
-            MeshMaterial2d(material),
-        ));
     }
 }

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

@@ -1,9 +1,27 @@
 //! 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;
+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);
@@ -38,9 +56,9 @@ pub(crate) fn wrap_entities(
     mut query: Query<&mut Transform, With<Wrapping>>,
     world_size: Res<WorldSize>,
 ) {
-    let right = world_size.width / 2.0;
+    let right = world_size.x / 2.0;
     let left = -right;
-    let top = world_size.height / 2.0;
+    let top = world_size.y / 2.0;
     let bottom = -top;
 
     for mut pos in query.iter_mut() {
@@ -57,3 +75,66 @@ pub(crate) fn wrap_entities(
         }
     }
 }
+
+/// 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/preparation_widget.rs

@@ -1,99 +0,0 @@
-use crate::GameState;
-
-use bevy::{
-    color::palettes::css::{BLACK, GREEN, LIGHT_BLUE, RED},
-    prelude::*,
-};
-
-pub fn preparation_widget_plugin(app: &mut App) {
-    app.add_systems(OnEnter(GameState::GetReady), spawn_get_ready)
-        .add_systems(OnExit(GameState::GetReady), despawn_get_ready)
-        .add_systems(
-            Update,
-            (animate_get_ready_widget).run_if(in_state(GameState::GetReady)),
-        )
-        .insert_resource(ReadySetGoTimer(Timer::from_seconds(3.0, TimerMode::Once)));
-}
-
-/// Marker component for things on the get-ready indicator
-#[derive(Component)]
-struct OnReadySetGo;
-
-/// 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;
-
-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()),
-            )
-        ],
-    ));
-}
-
-// TODO: Replace this with a generic somewhere else in the crate
-// want: `despawn_screen::<OnReadySetGo>>()`
-fn despawn_get_ready(mut commands: Commands, to_despawn: Query<Entity, With<OnReadySetGo>>) {
-    for entity in to_despawn {
-        commands.entity(entity).despawn();
-    }
-}
-
-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);
-    }
-}

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/ship.rs

@@ -1,37 +0,0 @@
-use crate::{
-    AngularVelocity, GameAssets,
-    physics::{Velocity, Wrapping},
-};
-
-use bevy::prelude::*;
-use bevy_rapier2d::prelude::*;
-
-#[derive(Component)]
-pub struct Ship;
-
-#[derive(Component)]
-pub struct Bullet;
-
-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)),
-        ));
-}

src/title_screen.rs

@@ -1,54 +0,0 @@
-use crate::GameState;
-
-use bevy::prelude::*;
-
-pub struct GameMenuPlugin;
-
-impl Plugin for GameMenuPlugin {
-    fn build(&self, app: &mut App) {
-        app.add_systems(OnEnter(GameState::TitleScreen), spawn_menu)
-            .add_systems(OnExit(GameState::TitleScreen), despawn_menu)
-            .add_systems(
-                Update,
-                handle_spacebar.run_if(in_state(GameState::TitleScreen)),
-            );
-    }
-}
-
-// 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 TitleUI;
-
-fn spawn_menu(mut commands: Commands) {
-    commands
-        .spawn((
-            TitleUI,
-            Node {
-                flex_direction: FlexDirection::Column,
-                ..Default::default()
-            },
-        ))
-        .with_children(|cmds| {
-            cmds.spawn((
-                Text::new("Robert's Bad Asteroids Game"),
-                TextFont::from_font_size(50.0),
-            ));
-            cmds.spawn((
-                Text::new("Press space to begin"),
-                TextFont::from_font_size(40.0),
-            ));
-        });
-}
-
-fn despawn_menu(mut commands: Commands, to_despawn: Query<Entity, With<TitleUI>>) {
-    for entity in &to_despawn {
-        commands.entity(entity).despawn();
-    }
-}
-
-fn handle_spacebar(input: Res<ButtonInput<KeyCode>>, mut game_state: ResMut<NextState<GameState>>) {
-    if input.just_pressed(KeyCode::Space) {
-        game_state.set(GameState::GetReady);
-    }
-}

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),
+    ));
+}