refactor: moved gravity logic to `BallGravitatingBehavior` (#314)

packages/pinball_components/lib/src/components/ball/ball.dart

@@ -5,6 +5,7 @@ import 'package:flame/components.dart';
 import 'package:flame_forge2d/flame_forge2d.dart';
 import 'package:flutter/widgets.dart';
 import 'package:pinball_components/pinball_components.dart';
+import 'package:pinball_components/src/components/ball/behaviors/ball_gravitating_behavior.dart';
 import 'package:pinball_components/src/components/ball/behaviors/ball_scaling_behavior.dart';
 import 'package:pinball_flame/pinball_flame.dart';
 
@@ -21,6 +22,7 @@ class Ball<T extends Forge2DGame> extends BodyComponent<T>
           children: [
             _BallSpriteComponent()..tint(baseColor.withOpacity(0.5)),
             BallScalingBehavior(),
+            BallGravitatingBehavior(),
           ],
         ) {
     // TODO(ruimiguel): while developing Ball can be launched by clicking mouse,
@@ -86,31 +88,6 @@ class Ball<T extends Forge2DGame> extends BodyComponent<T>
     body.linearVelocity = impulse;
     await add(_TurboChargeSpriteAnimationComponent());
   }
-
-  @override
-  void update(double dt) {
-    super.update(dt);
-
-    _setPositionalGravity();
-  }
-
-  void _setPositionalGravity() {
-    final defaultGravity = gameRef.world.gravity.y;
-    final maxXDeviationFromCenter = BoardDimensions.bounds.width / 2;
-    const maxXGravityPercentage =
-        (1 - BoardDimensions.perspectiveShrinkFactor) / 2;
-    final xDeviationFromCenter = body.position.x;
-
-    final positionalXForce = ((xDeviationFromCenter / maxXDeviationFromCenter) *
-            maxXGravityPercentage) *
-        defaultGravity;
-
-    final positionalYForce = math.sqrt(
-      math.pow(defaultGravity, 2) - math.pow(positionalXForce, 2),
-    );
-
-    body.gravityOverride = Vector2(positionalXForce, positionalYForce);
-  }
 }
 
 class _BallSpriteComponent extends SpriteComponent with HasGameRef {

packages/pinball_components/lib/src/components/ball/behaviors/ball_gravitating_behavior.dart

@@ -0,0 +1,35 @@
+import 'dart:math' as math;
+
+import 'package:flame/components.dart';
+import 'package:flame_forge2d/flame_forge2d.dart';
+import 'package:pinball_components/pinball_components.dart';
+import 'package:pinball_flame/pinball_flame.dart';
+
+/// Scales the ball's gravity according to its position on the board.
+class BallGravitatingBehavior extends Component
+    with ParentIsA<Ball>, HasGameRef<Forge2DGame> {
+  @override
+  void update(double dt) {
+    super.update(dt);
+    final defaultGravity = gameRef.world.gravity.y;
+
+    final maxXDeviationFromCenter = BoardDimensions.bounds.width / 2;
+    const maxXGravityPercentage =
+        (1 - BoardDimensions.perspectiveShrinkFactor) / 2;
+    final xDeviationFromCenter = parent.body.position.x;
+
+    final positionalXForce = ((xDeviationFromCenter / maxXDeviationFromCenter) *
+            maxXGravityPercentage) *
+        defaultGravity;
+    final positionalYForce = math.sqrt(
+      math.pow(defaultGravity, 2) - math.pow(positionalXForce, 2),
+    );
+
+    final gravityOverride = parent.body.gravityOverride;
+    if (gravityOverride != null) {
+      gravityOverride.setValues(positionalXForce, positionalYForce);
+    } else {
+      parent.body.gravityOverride = Vector2(positionalXForce, positionalYForce);
+    }
+  }
+}

packages/pinball_components/lib/src/components/ball/behaviors/behaviors.dart

@@ -1 +1,2 @@
+export 'ball_gravitating_behavior.dart';
 export 'ball_scaling_behavior.dart';

packages/pinball_components/test/src/components/ball/ball_test.dart

@@ -36,7 +36,8 @@ void main() {
       },
     );
 
-    flameTester.test('add a BallScalingBehavior', (game) async {
+    group('adds', () {
+      flameTester.test('a BallScalingBehavior', (game) async {
         final ball = Ball(baseColor: baseColor);
         await game.ensureAdd(ball);
         expect(
@@ -45,6 +46,16 @@ void main() {
         );
       });
 
+      flameTester.test('a BallGravitatingBehavior', (game) async {
+        final ball = Ball(baseColor: baseColor);
+        await game.ensureAdd(ball);
+        expect(
+          ball.descendants().whereType<BallGravitatingBehavior>().length,
+          equals(1),
+        );
+      });
+    });
+
     group('body', () {
       flameTester.test(
         'is dynamic',

packages/pinball_components/test/src/components/ball/behaviors/ball_gravitating_behavior_test.dart

@@ -0,0 +1,63 @@
+// ignore_for_file: cascade_invocations
+
+import 'dart:ui';
+
+import 'package:flame/components.dart';
+import 'package:flame_test/flame_test.dart';
+import 'package:flutter_test/flutter_test.dart';
+import 'package:pinball_components/pinball_components.dart';
+import 'package:pinball_components/src/components/ball/behaviors/behaviors.dart';
+
+import '../../../../helpers/helpers.dart';
+
+void main() {
+  TestWidgetsFlutterBinding.ensureInitialized();
+  final asset = Assets.images.ball.ball.keyName;
+  final flameTester = FlameTester(() => TestGame([asset]));
+
+  group('BallGravitatingBehavior', () {
+    const baseColor = Color(0xFFFFFFFF);
+    test('can be instantiated', () {
+      expect(
+        BallGravitatingBehavior(),
+        isA<BallGravitatingBehavior>(),
+      );
+    });
+
+    flameTester.test('can be loaded', (game) async {
+      final ball = Ball.test(baseColor: baseColor);
+      final behavior = BallGravitatingBehavior();
+      await ball.add(behavior);
+      await game.ensureAdd(ball);
+      expect(
+        ball.firstChild<BallGravitatingBehavior>(),
+        equals(behavior),
+      );
+    });
+
+    flameTester.test(
+      "overrides the body's horizontal gravity symmetrically",
+      (game) async {
+        final ball1 = Ball.test(baseColor: baseColor)
+          ..initialPosition = Vector2(10, 0);
+        await ball1.add(BallGravitatingBehavior());
+
+        final ball2 = Ball.test(baseColor: baseColor)
+          ..initialPosition = Vector2(-10, 0);
+        await ball2.add(BallGravitatingBehavior());
+
+        await game.ensureAddAll([ball1, ball2]);
+        game.update(1);
+
+        expect(
+          ball1.body.gravityOverride!.x,
+          equals(-ball2.body.gravityOverride!.x),
+        );
+        expect(
+          ball1.body.gravityOverride!.y,
+          equals(ball2.body.gravityOverride!.y),
+        );
+      },
+    );
+  });
+}

packages/pinball_components/test/src/components/ball/behaviors/ball_scaling_behavior_test.dart

@@ -35,17 +35,6 @@ void main() {
       );
     });
 
-    flameTester.test('can be loaded', (game) async {
-      final ball = Ball.test(baseColor: baseColor);
-      final behavior = BallScalingBehavior();
-      await ball.add(behavior);
-      await game.ensureAdd(ball);
-      expect(
-        ball.firstChild<BallScalingBehavior>(),
-        equals(behavior),
-      );
-    });
-
     flameTester.test('scales the shape radius', (game) async {
       final ball1 = Ball.test(baseColor: baseColor)
         ..initialPosition = Vector2(0, 10);
@@ -66,9 +55,9 @@ void main() {
       );
     });
 
-    flameTester.testGameWidget(
+    flameTester.test(
       'scales the sprite',
-      setUp: (game, tester) async {
+      (game) async {
         final ball1 = Ball.test(baseColor: baseColor)
           ..initialPosition = Vector2(0, 10);
         await ball1.add(BallScalingBehavior());
@@ -80,9 +69,6 @@ void main() {
         await game.ensureAddAll([ball1, ball2]);
         game.update(1);
 
-        await tester.pump();
-        await game.ready();
-
         final sprite1 = ball1.firstChild<SpriteComponent>()!;
         final sprite2 = ball2.firstChild<SpriteComponent>()!;
         expect(