First parts of a collision event handler

I've added a new `event` module to contain all the game's events (which
it does not. I'll collect them all later).

Using these events, the CollisionEvent listening system can watch for
any two colliders intersecting, check on what role they play (asteroid,
ship, bullet, etc), and re-emit an event for that specific thing. Each
component can then handle it's own destruction process.

src/event.rs

@@ -0,0 +1,14 @@
+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.

src/lib.rs

@@ -1,12 +1,13 @@
 mod asteroids;
 pub mod config;
+mod event;
 mod preparation_widget;
 mod ship;
 mod title_screen;
 
 use crate::config::{BACKGROUND_COLOR, PLAYER_SHIP_COLOR, SHIP_ROTATION, SHIP_THRUST, WINDOW_SIZE};
 
-use asteroids::AsteroidSpawner;
+use asteroids::{Asteroid, AsteroidSpawner};
 use bevy::prelude::*;
 use bevy_inspector_egui::InspectorOptions;
 use bevy_inspector_egui::prelude::ReflectInspectorOptions;
@@ -49,6 +50,7 @@ impl Plugin for AsteroidPlugin {
                 wrap_entities,
                 asteroids::tick_asteroid_manager,
                 asteroids::spawn_asteroid.after(asteroids::tick_asteroid_manager),
+                collision_listener,
                 // TODO: Remove debug printing
                 debug_collision_event_printer,
             )
@@ -59,7 +61,9 @@ impl Plugin for AsteroidPlugin {
             (integrate_velocity, update_positions, apply_rotation_to_mesh)
                 .run_if(in_state(GameState::Playing)),
         )
-        .add_event::<asteroids::SpawnAsteroid>();
+        .add_event::<asteroids::SpawnAsteroid>()
+        .add_event::<event::AsteroidDestroy>()
+        .add_event::<event::ShipDestroy>();
         app.insert_state(GameState::TitleScreen);
     }
 }
@@ -70,6 +74,54 @@ fn debug_collision_event_printer(mut collision_events: EventReader<CollisionEven
     }
 }
 
+/// 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
+        }
+    }
+}
+
 #[derive(Clone, Debug, Eq, Hash, PartialEq, States)]
 pub enum GameState {
     TitleScreen, // Program is started. Present title screen and await user start