feat: anchor component (#12)

* feat: implemented Anchor component

* docs: included initialize example

* docs: improved Anchor doc comment

* chore: fixed white space difference

lib/game/components/anchor.dart

@@ -0,0 +1,32 @@
+import 'package:flame_forge2d/flame_forge2d.dart';
+
+/// {@template 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].
+///
+/// ```dart
+/// initialize(
+///   dynamicBody.body,
+///   anchor.body,
+///   anchor.body.position,
+/// );
+/// ```
+/// {@endtemplate}
+class Anchor extends BodyComponent {
+  /// {@macro anchor}
+  Anchor({
+    required Vector2 position,
+  }) : _position = position;
+
+  final Vector2 _position;
+
+  @override
+  Body createBody() {
+    final bodyDef = BodyDef()..position = _position;
+
+    return world.createBody(bodyDef);
+  }
+}

lib/game/components/components.dart

@@ -1,3 +1,4 @@
+export 'anchor.dart';
 export 'ball.dart';
 export 'score_points.dart';
 export 'wall.dart';

test/game/components/anchor_test.dart

@@ -0,0 +1,60 @@
+// 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';
+
+void main() {
+  TestWidgetsFlutterBinding.ensureInitialized();
+
+  group('Anchor', () {
+    final flameTester = FlameTester(PinballGame.new);
+
+    flameTester.test(
+      'loads correctly',
+      (game) async {
+        final anchor = Anchor(position: Vector2.zero());
+        await game.ensureAdd(anchor);
+
+        expect(game.contains(anchor), isTrue);
+      },
+    );
+
+    group('body', () {
+      flameTester.test(
+        'positions correctly',
+        (game) async {
+          final position = Vector2.all(10);
+          final anchor = Anchor(position: position);
+          await game.ensureAdd(anchor);
+          game.contains(anchor);
+
+          expect(anchor.body.position, position);
+        },
+      );
+
+      flameTester.test(
+        'is static',
+        (game) async {
+          final anchor = Anchor(position: Vector2.zero());
+          await game.ensureAdd(anchor);
+
+          expect(anchor.body.bodyType, equals(BodyType.static));
+        },
+      );
+    });
+
+    group('fixtures', () {
+      flameTester.test(
+        'has none',
+        (game) async {
+          final anchor = Anchor(position: Vector2.zero());
+          await game.ensureAdd(anchor);
+
+          expect(anchor.body.fixtures, isEmpty);
+        },
+      );
+    });
+  });
+}