Move asteroid spawning & splitting systems

They deal with events, but are directly related to how the Asteroid
object interacts with the events, so I'm putting them here.

src/objects.rs

@@ -3,8 +3,25 @@
 //! Asteroids, the player's ship, and such.
 
 use bevy::{
-    ecs::component::Component,
+    ecs::{
+        component::Component,
+        event::{EventReader, EventWriter},
+        system::{Commands, Query, Res},
+    },
+    math::{Vec2, Vec3Swizzles},
     prelude::{Deref, DerefMut},
+    render::mesh::Mesh2d,
+    sprite::MeshMaterial2d,
+    time::{Timer, TimerMode},
+    transform::components::Transform,
+};
+use bevy_rapier2d::prelude::{Collider, Sensor};
+
+use crate::{
+    GameAssets, Lifetime,
+    config::ASTEROID_LIFETIME,
+    events::{AsteroidDestroy, SpawnAsteroid},
+    physics::Velocity,
 };
 
 #[derive(Component, Deref, DerefMut)]
@@ -32,3 +49,75 @@ pub struct Ship;
 
 #[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();
+        }
+    }
+}