feat: moved triangle to centroid

lib/game/components/sling_shot.dart

@@ -24,12 +24,19 @@ class SlingShot extends BodyComponent {
 
   List<FixtureDef> _createFixtureDefs() {
     final fixtures = <FixtureDef>[];
+    // TODO(alestiago): use size from [PositionBodyComponent].
+    final size = Vector2(10, 10);
 
     final triangleVertices = [
       Vector2(0, 0),
-      Vector2(0, -10),
+      Vector2(0, -size.y),
-      Vector2(10, -10),
+      Vector2(size.x, -size.y),
     ];
+    final triangleCentroid = centroid(triangleVertices);
+    for (final vertex in triangleVertices) {
+      vertex.setFrom(vertex - triangleCentroid);
+    }
+
     final triangle = PolygonShape()..set(triangleVertices);
     fixtures.add(FixtureDef(triangle));
 
@@ -44,10 +51,19 @@ class SlingShot extends BodyComponent {
     return fixtures;
   }
 
+  /// https://en.wikipedia.org/wiki/Centroid
+  // TODO(alestiago): use from geometry package.
+  static Vector2 centroid(List<Vector2> vertices) {
+    final centroid = Vector2.zero();
+    for (final vertex in vertices) {
+      centroid.add(vertex);
+    }
+    return centroid / vertices.length.toDouble();
+  }
+
   @override
   Body createBody() {
     final bodyDef = BodyDef()..position = _position;
-
     final body = world.createBody(bodyDef);
     _createFixtureDefs().forEach(body.createFixture);