Adding Event listener to mainScene in Javafx using fxml

Question

I have an fxml that describes my gui. I want to change text of the gui and start a task on any key press anywhere.

FXML

<Text fx:id="barcodeText"/>

Controller

   @FXML
   Text barcodeText;

   public void start(Stage primaryStage) throws IOException {
        this.primaryStage=primaryStage;
        Scene mainScene =new Scene(root);
        primaryStage.setScene(mainScene);
        primaryStage.setResizable(false);
        primaryStage.show();
        Parent root = FXMLLoader.load(getClass().getResource("/view/foo.fxml"));
        mainScene.addEventHandler(KeyEvent.KEY_PRESSED,new KeyboardEventHandler(){
                @Override
                public void handle(KeyEvent event) {
                   barcodeText.setText("foo");
                }
        });

This gives me a NullPointerException(inside JavaFX Application Thread) for the barcodeText pointer when I fire the event.

Am I doing something wrong?

The examples I looked at were using this approach without fxml, do I have to use an annotation to define the handler? where would I put "onAction" for the scene in the fxml?

The FXML loader initializes `@FXML`-annotated fields in the *controller*, not in the instance of the `Application` class on which `start` is called. So `barcodeText` (or `textBox`, or whatever you really called it) will be null in the `Application` instance. — James_D, Feb 28 '17 at 15:25
@James_D Ok, so that is the root of the problem. How do I solve it? for a button i can do `@FXML public void onClick(Event event) ...` how do I do this for the scene? — Flying Swissman, Feb 28 '17 at 15:37

score 4 · Accepted Answer · edited May 23 '17 at 12:31

(Aside: it looks like you are trying to use the same class for the controller, and for the application. Don't do that.)

Define a method in the controller class for setting the barcode text:

public void setBarcodeText(String barcode) {
    barcodeText.setText(barcode);
}

Then call that method from your handler:

FXMLLoader loader = new FXMLLoader(getClass().getResource("/view/foo.fxml"));
Parent root = loader.load();

MyControllerClass controller = loader.getController();

Scene mainScene = new Scene(root);
mainScene.addEventHandler(KeyEvent.KEY_PRESSED, new KeyboardEventHandler(){
        @Override
        public void handle(KeyEvent event) {
            controller.setBarcodeText("foo");
        }
});

Obviously, replace MyControllerClass with the actual name of the controller class.

Thanks, fixed my problem. I will also take the hint and separate controller from the application. — Flying Swissman, Feb 28 '17 at 15:50
What is the difference between the handler and the controller? Also where is loader.getController defined? — Angus, Jan 20 '20 at 21:59

Adding Event listener to mainScene in Javafx using fxml

1 Answers1