feat: defined InitialPosition mixin

lib/game/components/components.dart

@@ -2,6 +2,7 @@ export 'anchor.dart';
 export 'ball.dart';
 export 'board_side.dart';
 export 'flipper.dart';
+export 'initial_position.dart';
 export 'pathway.dart';
 export 'plunger.dart';
 export 'score_points.dart';

lib/game/components/initial_position.dart

@@ -0,0 +1,14 @@
+import 'package:flame_forge2d/flame_forge2d.dart';
+
+mixin InitialPosition on BodyComponent {
+  late final Vector2 initialPosition;
+
+  @override
+  void onMount() {
+    super.onMount();
+    assert(
+      body.position == initialPosition,
+      'Body position is not equal to initial position.',
+    );
+  }
+}

test/game/components/initial_position_test.dart

@@ -0,0 +1,68 @@
+// ignore_for_file: cascade_invocations
+
+import 'package:flame_forge2d/flame_forge2d.dart';
+import 'package:flame_test/flame_test.dart';
+import 'package:flutter_test/flutter_test.dart';
+import 'package:pinball/game/game.dart';
+
+class EmptyGame extends Forge2DGame {}
+
+class TestBodyComponent extends BodyComponent with InitialPosition {
+  @override
+  Body createBody() {
+    return world.createBody(BodyDef());
+  }
+}
+
+class TestPositionBodyComponent extends BodyComponent with InitialPosition {
+  @override
+  Body createBody() {
+    return world.createBody(BodyDef()..position = initialPosition);
+  }
+}
+
+void main() {
+  final flameTester = FlameTester(EmptyGame.new);
+  group('InitialPosition', () {
+    test('correctly sets and gets', () {
+      final component = TestBodyComponent()..initialPosition = Vector2(1, 2);
+      expect(component.initialPosition, Vector2(1, 2));
+    });
+
+    test('can only be set once', () {
+      final component = TestBodyComponent()..initialPosition = Vector2(1, 2);
+      expect(
+        () => component.initialPosition = Vector2(3, 4),
+        throwsA(isA<Error>()),
+      );
+    });
+
+    flameTester.test(
+      'returns normally '
+      'when the body sets the position to initial position',
+      (game) async {
+        final component = TestPositionBodyComponent()
+          ..initialPosition = Vector2.zero();
+
+        await expectLater(
+          () async => game.ensureAdd(component),
+          returnsNormally,
+        );
+      },
+    );
+
+    flameTester.test(
+      'throws AssertionError '
+      'when not setting initialPosition to body',
+      (game) async {
+        final component = TestBodyComponent()..initialPosition = Vector2.zero();
+        await game.ensureAdd(component);
+
+        await expectLater(
+          () async => game.ensureAdd(component),
+          throwsAssertionError,
+        );
+      },
+    );
+  });
+}