feat: made SlingShot use InitialPosition

lib/game/components/board.dart

@@ -69,11 +69,10 @@ class _BottomGroupSide extends Component {
           );
     final slingShot = SlingShot(
       side: _side,
-      position: _position +
+    )..initialPosition = _position +
         Vector2(
           (Flipper.width) * direction,
           Flipper.height + SlingShot.size.y,
-          ),
         );
 
     await addAll([flipper, baseboard, slingShot]);

lib/game/components/sling_shot.dart

@@ -10,22 +10,17 @@ import 'package:pinball/game/game.dart';
 ///
 /// [SlingShot]s are usually positioned above each [Flipper].
 /// {@endtemplate sling_shot}
-class SlingShot extends BodyComponent {
+class SlingShot extends BodyComponent with InitialPosition {
   /// {@macro sling_shot}
   SlingShot({
-    required Vector2 position,
     required BoardSide side,
-  })  : _position = position,
-        _side = side {
+  }) : _side = side {
     // TODO(alestiago): Use sprite instead of color when provided.
     paint = Paint()
       ..color = const Color(0xFF00FF00)
       ..style = PaintingStyle.fill;
   }
 
-  /// The initial position of the [SlingShot] body.
-  final Vector2 _position;
-
   /// Whether the [SlingShot] is on the left or right side of the board.
   ///
   /// A [SlingShot] with [BoardSide.left] propels the [Ball] to the right,
@@ -88,7 +83,7 @@ class SlingShot extends BodyComponent {
 
   @override
   Body createBody() {
-    final bodyDef = BodyDef()..position = _position;
+    final bodyDef = BodyDef()..position = initialPosition;
     final body = world.createBody(bodyDef);
     _createFixtureDefs().forEach(body.createFixture);
 

test/game/components/sling_shot_test.dart

@@ -13,9 +13,8 @@ void main() {
       'loads correctly',
       (game) async {
         final slingShot = SlingShot(
-          position: Vector2.zero(),
           side: BoardSide.left,
-        );
+        )..initialPosition = Vector2(0, 0);
         await game.ensureAdd(slingShot);
 
         expect(game.contains(slingShot), isTrue);
@@ -28,9 +27,8 @@ void main() {
         (game) async {
           final position = Vector2.all(10);
           final slingShot = SlingShot(
-            position: position,
             side: BoardSide.left,
-          );
+          )..initialPosition = position;
           await game.ensureAdd(slingShot);
 
           expect(slingShot.body.position, equals(position));
@@ -41,9 +39,8 @@ void main() {
         'is static',
         (game) async {
           final slingShot = SlingShot(
-            position: Vector2.zero(),
             side: BoardSide.left,
-          );
+          )..initialPosition = Vector2(0, 0);
           await game.ensureAdd(slingShot);
 
           expect(slingShot.body.bodyType, equals(BodyType.static));
@@ -56,9 +53,8 @@ void main() {
         'exists',
         (game) async {
           final slingShot = SlingShot(
-            position: Vector2.zero(),
             side: BoardSide.left,
-          );
+          )..initialPosition = Vector2(0, 0);
           await game.ensureAdd(slingShot);
 
           expect(slingShot.body.fixtures[0], isA<Fixture>());
@@ -69,9 +65,8 @@ void main() {
         'shape is triangular',
         (game) async {
           final slingShot = SlingShot(
-            position: Vector2.zero(),
             side: BoardSide.left,
-          );
+          )..initialPosition = Vector2(0, 0);
           await game.ensureAdd(slingShot);
 
           final fixture = slingShot.body.fixtures[0];
@@ -85,13 +80,11 @@ void main() {
         'when side is left or right',
         (game) async {
           final leftSlingShot = SlingShot(
-            position: Vector2.zero(),
             side: BoardSide.left,
-          );
+          )..initialPosition = Vector2(0, 0);
           final rightSlingShot = SlingShot(
-            position: Vector2.zero(),
             side: BoardSide.right,
-          );
+          )..initialPosition = Vector2(0, 0);
 
           await game.ensureAdd(leftSlingShot);
           await game.ensureAdd(rightSlingShot);
@@ -109,9 +102,8 @@ void main() {
         'has no friction',
         (game) async {
           final slingShot = SlingShot(
-            position: Vector2.zero(),
             side: BoardSide.left,
-          );
+          )..initialPosition = Vector2(0, 0);
           await game.ensureAdd(slingShot);
 
           final fixture = slingShot.body.fixtures[0];
@@ -125,9 +117,8 @@ void main() {
         'exists',
         (game) async {
           final slingShot = SlingShot(
-            position: Vector2.zero(),
             side: BoardSide.left,
-          );
+          )..initialPosition = Vector2(0, 0);
           await game.ensureAdd(slingShot);
 
           expect(slingShot.body.fixtures[1], isA<Fixture>());
@@ -138,9 +129,8 @@ void main() {
         'shape is edge',
         (game) async {
           final slingShot = SlingShot(
-            position: Vector2.zero(),
             side: BoardSide.left,
-          );
+          )..initialPosition = Vector2(0, 0);
           await game.ensureAdd(slingShot);
 
           final fixture = slingShot.body.fixtures[1];
@@ -152,9 +142,8 @@ void main() {
         'has restitution',
         (game) async {
           final slingShot = SlingShot(
-            position: Vector2.zero(),
             side: BoardSide.left,
-          );
+          )..initialPosition = Vector2(0, 0);
           await game.ensureAdd(slingShot);
 
           final fixture = slingShot.body.fixtures[1];
@@ -166,9 +155,8 @@ void main() {
         'has no friction',
         (game) async {
           final slingShot = SlingShot(
-            position: Vector2.zero(),
             side: BoardSide.left,
-          );
+          )..initialPosition = Vector2(0, 0);
           await game.ensureAdd(slingShot);
 
           final fixture = slingShot.body.fixtures[1];