feat: allow `Ball` to be stopped and resumed (#58)

* feat: implemented stop and reusme methods

* feat: included tests

* docs: improved doc comment

* feat: improved tests

* feat: removed gravity from tests

* refactor: renamed tests

lib/game/components/ball.dart

@@ -68,4 +68,19 @@ class Ball extends BodyComponent<PinballGame> with InitialPosition, Layered {
       gameRef.spawnBall();
     }
   }
+
+  /// Immediatly and completly [stop]s the ball.
+  ///
+  /// The [Ball] will no longer be affected by any forces, including it's
+  /// weight and those emitted from collisions.
+  void stop() {
+    body.setType(BodyType.static);
+  }
+
+  /// Allows the [Ball] to be affected by forces.
+  ///
+  /// If previously [stop]ed, the previous ball's velocity is not kept.
+  void resume() {
+    body.setType(BodyType.dynamic);
+  }
 }

test/game/components/ball_test.dart

@@ -35,6 +35,26 @@ void main() {
           expect(ball.body.bodyType, equals(BodyType.dynamic));
         },
       );
+
+      group('can be moved', () {
+        flameTester.test('by its weight', (game) async {
+          final ball = Ball();
+          await game.ensureAdd(ball);
+
+          game.update(1);
+          expect(ball.body.position, isNot(equals(ball.initialPosition)));
+        });
+
+        flameTester.test('by applying velocity', (game) async {
+          final ball = Ball();
+          await game.ensureAdd(ball);
+
+          ball.body.gravityScale = 0;
+          ball.body.linearVelocity.setValues(10, 10);
+          game.update(1);
+          expect(ball.body.position, isNot(equals(ball.initialPosition)));
+        });
+      });
     });
 
     group('fixture', () {
@@ -151,5 +171,60 @@ void main() {
         },
       );
     });
+
+    group('stop', () {
+      group("can't be moved", () {
+        flameTester.test('by its weight', (game) async {
+          final ball = Ball();
+          await game.ensureAdd(ball);
+          ball.stop();
+
+          game.update(1);
+          expect(ball.body.position, equals(ball.initialPosition));
+        });
+      });
+
+      flameTester.test('by applying velocity', (game) async {
+        final ball = Ball();
+        await game.ensureAdd(ball);
+        ball.stop();
+
+        ball.body.linearVelocity.setValues(10, 10);
+        game.update(1);
+        expect(ball.body.position, equals(ball.initialPosition));
+      });
+    });
+
+    group('resume', () {
+      group('can move', () {
+        flameTester.test(
+          'by its weight when previously stopped',
+          (game) async {
+            final ball = Ball();
+            await game.ensureAdd(ball);
+            ball.stop();
+            ball.resume();
+
+            game.update(1);
+            expect(ball.body.position, isNot(equals(ball.initialPosition)));
+          },
+        );
+
+        flameTester.test(
+          'by applying velocity when previously stopped',
+          (game) async {
+            final ball = Ball();
+            await game.ensureAdd(ball);
+            ball.stop();
+            ball.resume();
+
+            ball.body.gravityScale = 0;
+            ball.body.linearVelocity.setValues(10, 10);
+            game.update(1);
+            expect(ball.body.position, isNot(equals(ball.initialPosition)));
+          },
+        );
+      });
+    });
   });
 }