refactor: suggestions

lib/game/components/plunger.dart

@@ -1,15 +1,15 @@
 import 'package:flame_forge2d/flame_forge2d.dart';
+import 'package:pinball/game/game.dart';
 
 /// {@template plunger}
-/// Plunger body component to be pulled and released by the player to launch
-/// the pinball.
+/// [Plunger] serves as a spring, that shoots the ball on the right side of the
+/// playfield.
 ///
-/// The plunger body ignores gravity so the player can control its downward
-/// pull.
+/// [Plunger] ignores gravity so the player controls its downward [pull].
 /// {@endtemplate}
 class Plunger extends BodyComponent {
   /// {@macro plunger}
-  Plunger(this._position);
+  Plunger({required Vector2 position}) : _position = position;
 
   final Vector2 _position;
 
@@ -28,13 +28,15 @@ class Plunger extends BodyComponent {
     return world.createBody(bodyDef)..createFixture(fixtureDef);
   }
 
-  /// Set a constant downward velocity on the plunger body.
+  /// Set a constant downward velocity on the [Plunger].
   void pull() {
     body.linearVelocity = Vector2(0, -7);
   }
 
-  /// Set an upward velocity on the plunger body. The velocity's magnitude
-  /// depends on how far the plunger has been pulled from its original position.
+  /// Set an upward velocity on the [Plunger].
+  ///
+  /// The velocity's magnitude depends on how far the [Plunger] has been pulled
+  /// from its original [_position].
   void release() {
     final velocity = (_position.y - body.position.y) * 9;
     body.linearVelocity = Vector2(0, velocity);
@@ -42,19 +44,21 @@ class Plunger extends BodyComponent {
 }
 
 /// {@template plunger_anchor_prismatic_joint_def}
-/// Prismatic joint def between a [Plunger] and an anchor body given motion on
+/// [PrismaticJointDef] between a [Plunger] and an [Anchor] with motion on
 /// the vertical axis.
 ///
-/// The [Plunger] is constrained to vertical motion between its starting
-/// position and the anchor body. The anchor needs to be below the plunger for
-/// this joint to function properly.
+/// The [Plunger] is constrained to vertically between its starting position and
+/// the [Anchor]. The [Anchor] must be below the [Plunger].
 /// {@endtemplate}
 class PlungerAnchorPrismaticJointDef extends PrismaticJointDef {
   /// {@macro plunger_anchor_prismatic_joint_def}
   PlungerAnchorPrismaticJointDef({
     required Plunger plunger,
-    required BodyComponent anchor,
-  }) {
+    required Anchor anchor,
+  }) : assert(
+          anchor.body.position.y < plunger.body.position.y,
+          "Anchor can't be positioned above the Plunger",
+        ) {
     initialize(
       plunger.body,
       anchor.body,

test/game/components/plunger_test.dart

@@ -1,5 +1,6 @@
 // ignore_for_file: cascade_invocations
 
+import 'package:flame/extensions.dart';
 import 'package:flame_forge2d/flame_forge2d.dart';
 import 'package:flame_test/flame_test.dart';
 import 'package:flutter_test/flutter_test.dart';
@@ -13,7 +14,7 @@ void main() {
     flameTester.test(
       'loads correctly',
       (game) async {
-        final plunger = Plunger(Vector2.zero());
+        final plunger = Plunger(position: Vector2.zero());
         await game.ensureAdd(plunger);
 
         expect(game.contains(plunger), isTrue);
@@ -25,7 +26,7 @@ void main() {
         'positions correctly',
         (game) async {
           final position = Vector2.all(10);
-          final plunger = Plunger(position);
+          final plunger = Plunger(position: position);
           await game.ensureAdd(plunger);
           game.contains(plunger);
 
@@ -36,7 +37,7 @@ void main() {
       flameTester.test(
         'is dynamic',
         (game) async {
-          final plunger = Plunger(Vector2.zero());
+          final plunger = Plunger(position: Vector2.zero());
           await game.ensureAdd(plunger);
 
           expect(plunger.body.bodyType, equals(BodyType.dynamic));
@@ -46,7 +47,7 @@ void main() {
       flameTester.test(
         'ignores gravity',
         (game) async {
-          final plunger = Plunger(Vector2.zero());
+          final plunger = Plunger(position: Vector2.zero());
           await game.ensureAdd(plunger);
 
           expect(plunger.body.gravityScale, isZero);
@@ -58,7 +59,7 @@ void main() {
       flameTester.test(
         'exists',
         (game) async {
-          final plunger = Plunger(Vector2.zero());
+          final plunger = Plunger(position: Vector2.zero());
           await game.ensureAdd(plunger);
 
           expect(plunger.body.fixtures[0], isA<Fixture>());
@@ -68,7 +69,7 @@ void main() {
       flameTester.test(
         'shape is a polygon',
         (game) async {
-          final plunger = Plunger(Vector2.zero());
+          final plunger = Plunger(position: Vector2.zero());
           await game.ensureAdd(plunger);
 
           final fixture = plunger.body.fixtures[0];
@@ -80,12 +81,13 @@ void main() {
     flameTester.test(
       'pull sets a negative linear velocity',
       (game) async {
-        final plunger = Plunger(Vector2.zero());
+        final plunger = Plunger(position: Vector2.zero());
         await game.ensureAdd(plunger);
 
         plunger.pull();
 
         expect(plunger.body.linearVelocity.y, isNegative);
+        expect(plunger.body.linearVelocity.x, isZero);
       },
     );
 
@@ -94,12 +96,13 @@ void main() {
         'does not set a linear velocity '
         'when plunger is in starting position',
         (game) async {
-          final plunger = Plunger(Vector2.zero());
+          final plunger = Plunger(position: Vector2.zero());
           await game.ensureAdd(plunger);
 
           plunger.release();
 
           expect(plunger.body.linearVelocity.y, isZero);
+          expect(plunger.body.linearVelocity.x, isZero);
         },
       );
 
@@ -107,13 +110,14 @@ void main() {
         'sets a positive linear velocity '
         'when plunger is below starting position',
         (game) async {
-          final plunger = Plunger(Vector2.zero());
+          final plunger = Plunger(position: Vector2.zero());
           await game.ensureAdd(plunger);
 
           plunger.body.setTransform(Vector2(0, -1), 0);
           plunger.release();
 
           expect(plunger.body.linearVelocity.y, isPositive);
+          expect(plunger.body.linearVelocity.x, isZero);
         },
       );
     });
@@ -121,13 +125,30 @@ void main() {
 
   group('PlungerAnchorPrismaticJointDef', () {
     late Plunger plunger;
-    late Plunger anchor;
+    late Anchor anchor;
 
     setUp(() {
-      plunger = Plunger(Vector2.zero());
-      anchor = Plunger(Vector2(0, -1));
+      plunger = Plunger(position: Vector2.zero());
+      anchor = Anchor(position: Vector2(0, -1));
     });
 
+    flameTester.test(
+      'throws AssertionError '
+      'when anchor is above plunger',
+      (game) async {
+        final anchor = Anchor(position: Vector2(0, 1));
+        await game.ensureAddAll([plunger, anchor]);
+
+        expect(
+          () => PlungerAnchorPrismaticJointDef(
+            plunger: plunger,
+            anchor: anchor,
+          ),
+          throwsAssertionError,
+        );
+      },
+    );
+
     group('initializes with', () {
       flameTester.test(
         'plunger body as bodyA',