JavaFx State change Listener - events

Question

I am working on an application where I want to implement a menu. I have a GameState and a MainMenu class. Both extends Group. I can't figure out how to write a change listener to the Main.state, so when it changes from .MENU to .GAME the scenes will switch.
Here's is a part of the MainMenu class:
public class MainMenu extends Group {
private final Image background;
private final Rectangle bgRect;
private final int buttonNo = 3;
private MenuButton[] buttons;
private final double xStart = -200;
private final double yStart = 100;
private Group root;
private Scene scene;
public MainMenu () {
background = new Image(getClass().getResource("mainmenuBg.png").toString());
bgRect = new Rectangle(660,660);
bgRect.setFill(new ImagePattern(background));
root = new Group();
scene = new Scene(root, 650, 650);
scene.setCamera(new PerspectiveCamera());
root.getChildren().add(bgRect);
initButtons(root);
//Start game
buttons[0].setOnMouseClicked(new EventHandler<Event>() {
#Override
public void handle(Event t) {
Main.state = STATE.GAME;
}
});
//Options
buttons[1].setOnMouseClicked(new EventHandler<Event>() {
#Override
public void handle(Event t) {
//options menu will come here
}
});
//Exit
buttons[2].setOnMouseClicked(new EventHandler<Event>() {
#Override
public void handle(Event t) {
Platform.exit();
}
});
}
//...
}
The main class:
public class Main extends Application {
public int difficulty = 1;
GameState gameState;
MainMenu mainMenu;
public enum STATE {
MENU,
GAME
}
public static STATE state = STATE.MENU;
#Override
public void start(Stage stage) {
stage.resizableProperty().setValue(false);
stage.setTitle("Main");
Scene scene = new Scene(new StackPane(), 650, 650);
stage.setScene(scene);
stage.show();
if(Main.state == STATE.MENU)
enterMainMenu(stage);
if(Main.state == STATE.GAME)
enterGameState(stage);
}
//...
}
Any help would be appreciated.

Answer 1

I have succeeded to find a good solution.
I have removed the scene field from these classes, and added the super method in the constructors, than added the elements to the class (this.getChildren().addAll(..)).
Finally, here's my main controller:
public class Main extends Application {
public int difficulty = 1;
public GameState gameState = new GameState(difficulty);
public MainMenu mainMenu = new MainMenu();;
StackPane stackPane = new StackPane();
#Override
public void start(final Stage stage) {
stage.resizableProperty().setValue(false);
stage.setTitle("Main");
Scene scene = new Scene(stackPane, 650, 650);
scene.setCamera(new PerspectiveCamera());
stage.setScene(scene);
stage.show();
stackPane.getChildren().add(mainMenu);
mainMenu.getStartButton().setOnAction(new EventHandler<ActionEvent>() {
#Override
public void handle(ActionEvent t) {
changeScene(gameState);
try {
gameState.startGame();
} catch (InterruptedException ex) {
Logger.getLogger(Main.class.getName()).log(Level.SEVERE, null, ex);
}
}
});
}
public void changeScene(Parent newPage) {
stackPane.getChildren().add(newPage);
EventHandler<ActionEvent> finished = new EventHandler<ActionEvent>() {
#Override
public void handle(ActionEvent event) {
stackPane.getChildren().remove(0);
}
};
final Timeline switchPage = new Timeline(
new KeyFrame(Duration.seconds(0), new KeyValue(stackPane.getChildren().get(1).opacityProperty(), 0.0), new KeyValue(stackPane.getChildren().get(0).opacityProperty(), 1.0)),
new KeyFrame(Duration.seconds(3), finished, new KeyValue(stackPane.getChildren().get(1).opacityProperty(), 1.0), new KeyValue(stackPane.getChildren().get(0).opacityProperty(), 0.0))
);
switchPage.play();
}
}