class CustomListener {

​

  //collision event functions!

  BeginContact(contact) {

    //get both fixtures

    let f1 = contact.GetFixtureA();

    let f2 = contact.GetFixtureB();

    //get both bodies

    let b1 = f1.GetBody();

    let b2 = f2.GetBody();

​

    //get our objects that reference these bodies

    let o1 = b1.GetUserData();

    let o2 = b2.GetUserData();

​

    if (o1 instanceof Particle && o2 instanceof Particle) {

      o1.change();

      o2.change();

    }

  }

​

  //objects stop touching each other

  EndContact(contact) {};

​

  PreSolve(contact, manifold) {};

​

  PostSolve(contact, manifold) {};

}