feat: defined `BumpingBehavior` (#248)

* feat: defined BumpingBehavior

* refactor: removed unused import

* docs: fixed typo

Co-authored-by: Allison Ryan <77211884+allisonryan0002@users.noreply.github.com>

* feat: made strength private

Co-authored-by: Allison Ryan <77211884+allisonryan0002@users.noreply.github.com>

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

@@ -0,0 +1,25 @@
+import 'package:flame_forge2d/flame_forge2d.dart';
+import 'package:pinball_flame/pinball_flame.dart';
+
+/// {@template bumping_behavior}
+/// Makes any [BodyComponent] that contacts with [parent] bounce off.
+/// {@endtemplate}
+class BumpingBehavior extends ContactBehavior {
+  /// {@macro bumping_behavior}
+  BumpingBehavior({required double strength}) : _strength = strength;
+
+  /// Determines how strong the bump is.
+  final double _strength;
+
+  @override
+  void postSolve(Object other, Contact contact, ContactImpulse impulse) {
+    super.postSolve(other, contact, impulse);
+    if (other is! BodyComponent) return;
+
+    other.body.applyLinearImpulse(
+      contact.manifold.localPoint
+        ..normalize()
+        ..multiply(Vector2.all(other.body.mass * _strength)),
+    );
+  }
+}

packages/pinball_components/test/src/components/bumping_behavior_test.dart

@@ -0,0 +1,84 @@
+// 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:mocktail/mocktail.dart';
+import 'package:pinball_components/src/components/bumping_behavior.dart';
+
+import '../../helpers/helpers.dart';
+import 'layer_test.dart';
+
+class MockContactImpulse extends Mock implements ContactImpulse {}
+
+class MockManifold extends Mock implements Manifold {}
+
+class TestHeavyBodyComponent extends BodyComponent {
+  @override
+  Body createBody() {
+    final shape = CircleShape();
+    return world.createBody(
+      BodyDef(
+        type: BodyType.dynamic,
+      ),
+    )..createFixtureFromShape(shape, 20);
+  }
+}
+
+void main() {
+  TestWidgetsFlutterBinding.ensureInitialized();
+  final flameTester = FlameTester(TestGame.new);
+
+  group('BumpingBehavior', () {
+    flameTester.test('can be added', (game) async {
+      final behavior = BumpingBehavior(strength: 0);
+      final component = TestBodyComponent();
+      await component.add(behavior);
+      await game.ensureAdd(component);
+    });
+
+    flameTester.testGameWidget(
+      'the bump is greater when the strengh is greater',
+      setUp: (game, tester) async {
+        final component1 = TestBodyComponent();
+        final behavior1 = BumpingBehavior(strength: 1);
+        await component1.add(behavior1);
+
+        final component2 = TestBodyComponent();
+        final behavior2 = BumpingBehavior(strength: 2);
+        await component2.add(behavior2);
+
+        final dummy1 = TestHeavyBodyComponent();
+        final dummy2 = TestHeavyBodyComponent();
+
+        await game.ensureAddAll([
+          component1,
+          component2,
+          dummy1,
+          dummy2,
+        ]);
+
+        expect(dummy1.body.inverseMass, greaterThan(0));
+        expect(dummy2.body.inverseMass, greaterThan(0));
+
+        final contact = MockContact();
+        final manifold = MockManifold();
+        final contactImpulse = MockContactImpulse();
+        when(() => manifold.localPoint).thenReturn(Vector2.all(1));
+        when(() => contact.manifold).thenReturn(manifold);
+
+        behavior1.postSolve(dummy1, contact, contactImpulse);
+        behavior2.postSolve(dummy2, contact, contactImpulse);
+
+        expect(
+          dummy2.body.linearVelocity.x,
+          greaterThan(dummy1.body.linearVelocity.x),
+        );
+        expect(
+          dummy2.body.linearVelocity.y,
+          greaterThan(dummy1.body.linearVelocity.y),
+        );
+      },
+    );
+  });
+}