@FXML
  void dragOver(DragEvent event) {

    /* data is dragged over the target */

    /* accept it only if it is  not dragged from the same node
     * and if it has a string data */
    if (event.getGestureSource() != imageView && event.getDragboard().hasString()) {
      /* allow for both copying and moving, whatever user chooses */
      event.acceptTransferModes(TransferMode.COPY_OR_MOVE);
    }

    event.consume();
  }
  @FXML
  void dragEntered(DragEvent event) {

    /* the drag-and-drop gesture entered the target */

    /* show to the user that it is an actual gesture target */
    if (event.getGestureSource() != imageView && event.getDragboard().hasString()) {
      ColorAdjust colorAdjust = new ColorAdjust();
      colorAdjust.setBrightness(0.5);
      imageView.setEffect(colorAdjust);
    }

    event.consume();
  }