simple hitpoint mechanic

src/npc.rs

@@ -0,0 +1,41 @@
+use crate::tb_entities::EnemySpawn;
+use bevy::prelude::*;
+
+#[derive(Event, Reflect)]
+pub struct Hit {
+    pub damage: u32,
+}
+
+// TODO: add actual keys
+#[derive(Event, Reflect)]
+pub struct KeyCollected;
+
+#[derive(Component, Reflect)]
+struct Hp(i32);
+
+pub fn plugin(app: &mut App) {
+    app.add_systems(Update, init);
+}
+
+fn init(mut commands: Commands, query: Query<Entity, (With<EnemySpawn>, Without<Hp>)>) {
+    for e in query.iter() {
+        commands.entity(e).insert(Hp(100)).observe(on_hit);
+    }
+}
+
+fn on_hit(trigger: Trigger<Hit>, mut commands: Commands, mut query: Query<&mut Hp>) {
+    let Hit { damage } = trigger.event();
+
+    let Ok(mut hp) = query.get_mut(trigger.entity()) else {
+        return;
+    };
+
+    hp.0 = hp.0.saturating_sub(*damage as i32);
+
+    info!("npc hp changed: {} [{}]", hp.0, trigger.entity());
+
+    if hp.0 <= 0 {
+        commands.entity(trigger.entity()).despawn_recursive();
+        commands.trigger(KeyCollected);
+    }
+}