Snake growth

Author: marcusbuffett

src/main.rs

@@ -34,6 +34,11 @@ struct SnakeHead {
     direction: Direction,
 }
 
+struct GrowthEvent;
+
+#[derive(Default)]
+struct LastTailPosition(Option<Position>);
+
 #[derive(Component)]
 struct SnakeSegment;
 
@@ -103,6 +108,7 @@ fn spawn_segment(mut commands: Commands, position: Position) -> Entity {
 }
 
 fn snake_movement(
+    mut last_tail_position: ResMut<LastTailPosition>,
     segments: ResMut<SnakeSegments>,
     mut heads: Query<(Entity, &SnakeHead)>,
     mut positions: Query<&mut Position>,
@@ -133,6 +139,7 @@ fn snake_movement(
             .for_each(|(pos, segment)| {
                 *positions.get_mut(*segment).unwrap() = *pos;
             });
+        *last_tail_position = LastTailPosition(Some(*segment_positions.last().unwrap()));
     }
 }
 
@@ -154,6 +161,32 @@ fn snake_movement_input(keyboard_input: Res<Input<KeyCode>>, mut heads: Query<&m
         }
     }
 }
+fn snake_eating(
+    mut commands: Commands,
+    mut growth_writer: EventWriter<GrowthEvent>,
+    food_positions: Query<(Entity, &Position), With<Food>>,
+    head_positions: Query<&Position, With<SnakeHead>>,
+) {
+    for head_pos in head_positions.iter() {
+        for (ent, food_pos) in food_positions.iter() {
+            if food_pos == head_pos {
+                commands.entity(ent).despawn();
+                growth_writer.send(GrowthEvent);
+            }
+        }
+    }
+}
+
+fn snake_growth(
+    commands: Commands,
+    last_tail_position: Res<LastTailPosition>,
+    mut segments: ResMut<SnakeSegments>,
+    mut growth_reader: EventReader<GrowthEvent>,
+) {
+    if growth_reader.iter().next().is_some() {
+        segments.push(spawn_segment(commands, last_tail_position.0.unwrap()));
+    }
+}
 
 fn size_scaling(windows: Res<Windows>, mut q: Query<(&Size, &mut Transform)>) {
     let window = windows.get_primary().unwrap();
@@ -209,13 +242,17 @@ fn main() {
         })
         .add_startup_system(setup_camera)
         .add_startup_system(spawn_snake)
+        .insert_resource(SnakeSegments::default())
+        .insert_resource(LastTailPosition::default())
+        .add_event::<GrowthEvent>()
         .add_system(snake_movement_input.before(snake_movement))
         .add_system_set(
             SystemSet::new()
                 .with_run_criteria(FixedTimestep::step(0.150))
-                .with_system(snake_movement),
+                .with_system(snake_movement)
+                .with_system(snake_eating.after(snake_movement))
+                .with_system(snake_growth.after(snake_eating)),
         )
-        .insert_resource(SnakeSegments::default())
         .add_system_set(
             SystemSet::new()
                 .with_run_criteria(FixedTimestep::step(1.0))