code: public void apply() {
          if (element != null) {
              String uid = DOM.createUniqueId();
              element.setId(uid);
              JsScrollfire.apply("#" + uid, offset, callback::call);
          } else {
              GWT.log("You must set the element before applying the scrollfire", new IllegalStateException());
          }
      }
docs: Executes callback method depending on how far into the page you've scrolled