code: public void registerBall(final BallModel ballModel) {
          if (!this.ballMap.containsKey(ballModel.getEventModel().target())) {
              this.ballMap.put(ballModel.getEventModel().target(), ballModel);
              view().getPanel().getChildren().add(ballModel.node());
          }
      }
docs: Register a ball model.
  
  @param ballModel the ball model to register