feat: included default value

lib/game/components/initial_position.dart

@@ -2,9 +2,24 @@ import 'package:flame_forge2d/flame_forge2d.dart';
 
 /// Forces a given [BodyComponent] to position their [body] to an
 /// [initialPosition].
+///
+/// Note: If the [initialPosition] is set after the [BodyComponent] has been
+/// loaded it will have no effect; defaulting to [Vector2.zero].
 mixin InitialPosition<T extends Forge2DGame> on BodyComponent<T> {
+  final Vector2 _initialPosition = Vector2.zero();
+
+  set initialPosition(Vector2 value) {
+    assert(
+      !isLoaded,
+      'Cannot set initialPosition after component has been loaded.',
+    );
+    if (value == initialPosition) return;
+
+    _initialPosition.setFrom(value);
+  }
+
   /// The initial position of the [body].
-  late final Vector2 initialPosition;
+  Vector2 get initialPosition => _initialPosition;
 
   @override
   Future<void> onLoad() async {

test/game/components/initial_position_test.dart

@@ -28,11 +28,39 @@ void main() {
       );
     });
 
-    flameTester.test('positions correctly', (game) async {
-      final position = Vector2.all(10);
-      final component = TestBodyComponent()..initialPosition = position;
-      await game.ensureAdd(component);
-      expect(component.body.position, equals(position));
-    });
+    flameTester.test(
+      'positions correctly',
+      (game) async {
+        final position = Vector2.all(10);
+        final component = TestBodyComponent()..initialPosition = position;
+        await game.ensureAdd(component);
+        expect(component.body.position, equals(position));
+      },
+    );
+
+    flameTester.test(
+      'deafaults to zero '
+      'when no initialPosition is given',
+      (game) async {
+        final component = TestBodyComponent();
+        await game.ensureAdd(component);
+        expect(component.body.position, equals(Vector2.zero()));
+      },
+    );
+
+    flameTester.test(
+      'setting throws AssertionError '
+      'when component has loaded',
+      (game) async {
+        final component = TestBodyComponent();
+        await game.ensureAdd(component);
+
+        expect(component.isLoaded, isTrue);
+        expect(
+          () => component.initialPosition = Vector2.all(4),
+          throwsAssertionError,
+        );
+      },
+    );
   });
 }