code: private void doNotifyListeners(R oldValue, R newValue) {
          List<ValueChangeListener<R>> listenersCopy = new ArrayList<ValueChangeListener<R>>(listeners);
          notifyingListeners = true;
          for (ValueChangeListener<R> listener : listenersCopy) {
              listener.valueChanged(this, oldValue, newValue);
          }
          notifyingListeners = false;
      }
docs: Notifies the listeners that the property value has changed, unconditionally.
  
  @param oldValue Previous value.
  @param newValue New value.