code: public static void onAttach(HTMLElement element, ObserverCallback callback) {
          if (element != null) {
              BodyObserver.addAttachObserver(element, callback);
          }
      }
docs: Registers a callback when an element is appended to the document body. Note that the callback will be called
  only once, if the element is appended more than once a new callback should be registered.
  
  @param element  the HTML element which is going to be added to the body
  @param callback {@link ObserverCallback}