refactor: defined FlipperJoint

lib/game/components/flipper.dart

@@ -113,16 +113,12 @@ class Flipper extends BodyComponent with KeyboardHandler, InitialPosition {
       flipper: this,
       anchor: anchor,
     );
-    // TODO(alestiago): Remove casting once the following is closed:
+    final joint = _FlipperJoint(jointDef)..create(world);
-    // https://github.com/flame-engine/forge2d/issues/36
-    final joint = world.createJoint(jointDef) as RevoluteJoint;
 
     // FIXME(erickzanardo): when mounted the initial position is not fully
     // reached.
     unawaited(
-      mounted.whenComplete(
+      mounted.whenComplete(joint.unlock),
-        () => FlipperAnchorRevoluteJointDef.unlock(joint, side),
-      ),
     );
   }
 
@@ -211,21 +207,28 @@ class Flipper extends BodyComponent with KeyboardHandler, InitialPosition {
   }
 }
 
-/// {@template flipper_anchor}
+class _FlipperJoint extends RevoluteJoint {
-/// [JointAnchor] positioned at the end of a [Flipper].
+  _FlipperJoint(FlipperAnchorRevoluteJointDef def)
+      : side = def.side,
+        super(def);
+
+  final BoardSide side;
+
+  // TODO(alestiago): Remove once Forge2D supports custom joints.
+  void create(World world) {
+    world.joints.add(this);
+    bodyA.joints.add(this);
+    bodyB.joints.add(this);
+  }
+
+  /// Unlocks the [Flipper] from its resting position.
   ///
-/// The end of a [Flipper] depends on its [Flipper.side].
+  /// The [Flipper] is locked when initialized in order to force it to be at
-/// {@endtemplate}
+  /// its resting position.
-class FlipperAnchor extends JointAnchor {
+  void unlock() {
-  /// {@macro flipper_anchor}
+    setLimits(
-  FlipperAnchor({
+      lowerLimit * side.direction,
-    required Flipper flipper,
+      -upperLimit * side.direction,
-  }) {
-    initialPosition = Vector2(
-      flipper.side.isLeft
-          ? flipper.body.position.x - Flipper.size.x / 2
-          : flipper.body.position.x + Flipper.size.x / 2,
-      flipper.body.position.y,
     );
   }
 }
@@ -238,39 +241,39 @@ class FlipperAnchorRevoluteJointDef extends RevoluteJointDef {
   FlipperAnchorRevoluteJointDef({
     required Flipper flipper,
     required FlipperAnchor anchor,
-  }) {
+  }) : side = flipper.side {
     initialize(
       flipper.body,
       anchor.body,
       anchor.body.position,
     );
-    enableLimit = true;
 
+    enableLimit = true;
     final angle = (flipper.side.isLeft ? _sweepingAngle : -_sweepingAngle) / 2;
     lowerAngle = upperAngle = angle;
   }
 
+  final BoardSide side;
+
   /// The total angle of the arc motion.
   static const _sweepingAngle = math.pi / 3.5;
-
-  /// Unlocks the [Flipper] from its resting position.
-  ///
-  /// The [Flipper] is locked when initialized in order to force it to be at
-  /// its resting position.
-  // TODO(alestiago): consider refactor once the issue is solved:
-  // https://github.com/flame-engine/forge2d/issues/36
-  static void unlock(RevoluteJoint joint, BoardSide side) {
-    late final double upperLimit, lowerLimit;
-    switch (side) {
-      case BoardSide.left:
-        lowerLimit = -joint.lowerLimit;
-        upperLimit = joint.upperLimit;
-        break;
-      case BoardSide.right:
-        lowerLimit = joint.lowerLimit;
-        upperLimit = -joint.upperLimit;
 }
 
-    joint.setLimits(lowerLimit, upperLimit);
+/// {@template flipper_anchor}
+/// [JointAnchor] positioned at the end of a [Flipper].
+///
+/// The end of a [Flipper] depends on its [Flipper.side].
+/// {@endtemplate}
+class FlipperAnchor extends JointAnchor {
+  /// {@macro flipper_anchor}
+  FlipperAnchor({
+    required Flipper flipper,
+  }) {
+    initialPosition = Vector2(
+      flipper.side.isLeft
+          ? flipper.body.position.x - Flipper.size.x / 2
+          : flipper.body.position.x + Flipper.size.x / 2,
+      flipper.body.position.y,
+    );
   }
 }