Begin work on event-based asteroid spawning

The events are being emitted by the spawn manager, and consumed by the
spawn_asteroid system. Now to wire in the spawn properties, and then
make a good spawning manager.

src/asteroids.rs

@@ -2,7 +2,7 @@ use std::time::Duration;
 
 /// This is the module containing all the rock-related things
 /// not... not the whole game.
-use bevy::prelude::*;
+use bevy::{math::VectorSpace, prelude::*};
 
 use crate::{GameAssets, Position, Rotation, Velocity};
 
@@ -32,16 +32,40 @@ impl AsteroidSpawner {
     }
 }
 
+#[derive(Event)]
+pub struct SpawnAsteroid {
+    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 commands: Commands,
+    mut events: EventWriter<SpawnAsteroid>,
     mut spawner: ResMut<AsteroidSpawner>,
-    game_assets: Res<GameAssets>,
     time: Res<Time>,
 ) {
     spawner.timer.tick(time.delta());
     if spawner.timer.just_finished() {
+        events.write(SpawnAsteroid {
+            pos: Vec2::ZERO,
+            vel: Vec2::ZERO,
+            size: AsteroidSize::SMALL,
+        });
+    }
+}
+
+/// Utility function to spawn a single asteroid of a given type
+/// TODO: convert to an event listener monitoring for "spawn asteroid" events
+/// from the `fn tick_asteroid_manager(...)` system.
+pub fn spawn_asteroid(
+    mut events: EventReader<SpawnAsteroid>,
+    mut commands: Commands,
+    game_assets: Res<GameAssets>,
+) {
+    for spawn in events.read() {
+        // TODO: Use the asteroid properties defined in the event to spawn the entity
         commands.spawn((
             Asteroid(AsteroidSize::SMALL),
             Position(Vec2::new(40.0, 40.0)),
@@ -52,10 +76,3 @@ pub fn tick_asteroid_manager(
         ));
     }
 }
-
-/// Utility function to spawn a single asteroid of a given type
-/// TODO: convert to an event listener monitoring for "spawn asteroid" events
-/// from the `fn tick_asteroid_manager(...)` system.
-fn spawn_asteroid(mut commands: Commands) {
-    todo!();
-}