feat: made JointAnchor use InitialPosition

lib/game/components/flipper.dart

@@ -238,14 +238,14 @@ class FlipperAnchor extends JointAnchor {
   /// {@macro flipper_anchor}
   FlipperAnchor({
     required Flipper flipper,
-  }) : super(
-          position: Vector2(
-            flipper.side.isLeft
-                ? flipper.body.position.x - Flipper.width / 2
-                : flipper.body.position.x + Flipper.width / 2,
-            flipper.body.position.y,
-          ),
-        );
+  }) {
+    initialPosition = Vector2(
+      flipper.side.isLeft
+          ? flipper.body.position.x - Flipper.width / 2
+          : flipper.body.position.x + Flipper.width / 2,
+      flipper.body.position.y,
+    );
+  }
 }
 
 /// {@template flipper_anchor_revolute_joint_def}

lib/game/components/joint_anchor.dart

@@ -1,11 +1,12 @@
 import 'package:flame_forge2d/flame_forge2d.dart';
+import 'package:pinball/game/game.dart';
 
 /// {@template joint_anchor}
 /// Non visual [BodyComponent] used to hold a [BodyType.dynamic] in [Joint]s
 /// with this [BodyType.static].
 ///
-/// It is recommended to [_position] the anchor first and then use the body
-/// position as the anchor point when initializing a [JointDef].
+/// It is recommended to use [JointAnchor.body.position] to position the anchor
+/// point when initializing a [JointDef].
 ///
 /// ```dart
 /// initialize(
@@ -15,17 +16,13 @@ import 'package:flame_forge2d/flame_forge2d.dart';
 /// );
 /// ```
 /// {@endtemplate}
-class JointAnchor extends BodyComponent {
+class JointAnchor extends BodyComponent with InitialPosition {
   /// {@macro joint_anchor}
-  JointAnchor({
-    required Vector2 position,
-  }) : _position = position;
-
-  final Vector2 _position;
+  JointAnchor();
 
   @override
   Body createBody() {
-    final bodyDef = BodyDef()..position = _position;
+    final bodyDef = BodyDef()..position = initialPosition;
 
     return world.createBody(bodyDef);
   }

lib/game/components/plunger.dart

@@ -98,12 +98,12 @@ class PlungerAnchor extends JointAnchor {
   /// {@macro plunger_anchor}
   PlungerAnchor({
     required Plunger plunger,
-  }) : super(
-          position: Vector2(
-            plunger.body.position.x,
-            plunger.body.position.y - plunger.compressionDistance,
-          ),
-        );
+  }) {
+    initialPosition = Vector2(
+      plunger.body.position.x,
+      plunger.body.position.y - plunger.compressionDistance,
+    );
+  }
 }
 
 /// {@template plunger_anchor_prismatic_joint_def}

test/game/components/joint_anchor_test.dart

@@ -13,7 +13,7 @@ void main() {
     flameTester.test(
       'loads correctly',
       (game) async {
-        final anchor = JointAnchor(position: Vector2.zero());
+        final anchor = JointAnchor()..initialPosition = Vector2.zero();
         await game.ready();
         await game.ensureAdd(anchor);
 
@@ -27,7 +27,7 @@ void main() {
         (game) async {
           await game.ready();
           final position = Vector2.all(10);
-          final anchor = JointAnchor(position: position);
+          final anchor = JointAnchor()..initialPosition = position;
           await game.ensureAdd(anchor);
           game.contains(anchor);
 
@@ -39,7 +39,7 @@ void main() {
         'is static',
         (game) async {
           await game.ready();
-          final anchor = JointAnchor(position: Vector2.zero());
+          final anchor = JointAnchor()..initialPosition = Vector2.zero();
           await game.ensureAdd(anchor);
 
           expect(anchor.body.bodyType, equals(BodyType.static));
@@ -51,7 +51,7 @@ void main() {
       flameTester.test(
         'has none',
         (game) async {
-          final anchor = JointAnchor(position: Vector2.zero());
+          final anchor = JointAnchor()..initialPosition = Vector2.zero();
           await game.ensureAdd(anchor);
 
           expect(anchor.body.fixtures, isEmpty);