Component Lifecycle Callbacks

@Tag("div")
public class ShoppingCartSummaryLabel extends Component {
  private final Consumer<EventObject> eventHandler = this::onCartSummaryUpdate;
  @Override
  protected void onAttach(AttachEvent attachEvent) {
    ShopEventBus eventBus = attachEvent.getSession().getAttribute(ShopEventBus.class);
  }
  @Override
  protected void onDetach(DetachEvent detachEvent) {
    ShopEventBus eventBus = detachEvent.getSession().getAttribute(ShopEventBus.class);
    // after detaching don't need any updates
    eventBus.unregister(eventHandler);
  }
    // update cart summary ...
  }
}
  void register(Consumer<EventObject> eventHandler);
  void unregister(Consumer<EventObject> eventHandler);
}