feat: made Ball use InitialPosition

lib/game/components/ball.dart

@@ -6,14 +6,9 @@ import 'package:pinball/game/game.dart';
 /// A solid, [BodyType.dynamic] sphere that rolls and bounces along the
 /// [PinballGame].
 /// {@endtemplate}
-class Ball extends BodyComponent<PinballGame> {
+class Ball extends BodyComponent<PinballGame> with InitialPosition {
   /// {@macro ball}
-  Ball({
+  Ball();
-    required Vector2 position,
-  }) : _position = position;
-
-  /// The initial position of the [Ball] body.
-  final Vector2 _position;
 
   /// The size of the [Ball]
   final Vector2 size = Vector2.all(2);
@@ -45,7 +40,7 @@ class Ball extends BodyComponent<PinballGame> {
 
     final bodyDef = BodyDef()
       ..userData = this
-      ..position = Vector2(_position.x, _position.y + size.y)
+      ..position = initialPosition
       ..type = BodyType.dynamic;
 
     return world.createBody(bodyDef)..createFixture(fixtureDef);

lib/game/pinball_game.dart

@@ -78,7 +78,11 @@ class PinballGame extends Forge2DGame
   }
 
   void spawnBall() {
-    add(Ball(position: plunger.body.position));
+    final ball = Ball();
+    add(
+      ball
+        ..initialPosition = plunger.body.position + Vector2(0, ball.size.y / 2),
+    );
   }
 
   void _addContactCallbacks() {
@@ -112,6 +116,8 @@ class DebugPinballGame extends PinballGame with TapDetector {
 
   @override
   void onTapUp(TapUpInfo info) {
-    add(Ball(position: info.eventPosition.game));
+    add(
+      Ball()..initialPosition = info.eventPosition.game,
+    );
   }
 }

test/game/components/ball_test.dart

@@ -17,7 +17,7 @@ void main() {
     flameTester.test(
       'loads correctly',
       (game) async {
-        final ball = Ball(position: Vector2.zero());
+        final ball = Ball()..initialPosition = Vector2.all(0);
         await game.ready();
         await game.ensureAdd(ball);
 
@@ -30,22 +30,18 @@ void main() {
         'positions correctly',
         (game) async {
           final position = Vector2.all(10);
-          final ball = Ball(position: position);
+          final ball = Ball()..initialPosition = position;
           await game.ensureAdd(ball);
           game.contains(ball);
 
-          final expectedPosition = Vector2(
+          expect(ball.body.position, equals(position));
-            position.x,
-            position.y + ball.size.y,
-          );
-          expect(ball.body.position, equals(expectedPosition));
         },
       );
 
       flameTester.test(
         'is dynamic',
         (game) async {
-          final ball = Ball(position: Vector2.zero());
+          final ball = Ball()..initialPosition = Vector2.all(0);
           await game.ensureAdd(ball);
 
           expect(ball.body.bodyType, equals(BodyType.dynamic));
@@ -57,7 +53,7 @@ void main() {
       flameTester.test(
         'exists',
         (game) async {
-          final ball = Ball(position: Vector2.zero());
+          final ball = Ball()..initialPosition = Vector2.all(0);
           await game.ensureAdd(ball);
 
           expect(ball.body.fixtures[0], isA<Fixture>());
@@ -67,7 +63,7 @@ void main() {
       flameTester.test(
         'is dense',
         (game) async {
-          final ball = Ball(position: Vector2.zero());
+          final ball = Ball()..initialPosition = Vector2.all(0);
           await game.ensureAdd(ball);
 
           final fixture = ball.body.fixtures[0];
@@ -78,7 +74,7 @@ void main() {
       flameTester.test(
         'shape is circular',
         (game) async {
-          final ball = Ball(position: Vector2.zero());
+          final ball = Ball()..initialPosition = Vector2.all(0);
           await game.ensureAdd(ball);
 
           final fixture = ball.body.fixtures[0];

test/game/components/flipper_test.dart

@@ -94,7 +94,7 @@ void main() {
             final flipper = Flipper.fromSide(
               side: BoardSide.left,
             )..initialPosition = Vector2.zero();
-            final ball = Ball(position: Vector2.zero());
+            final ball = Ball()..initialPosition = Vector2.zero();
 
             await game.ready();
             await game.ensureAddAll([flipper, ball]);