JavaFX - 如何聽場景焦點所有者更改事件

//Code revised from Learn JavaFX 8
//http://www.apress.com/9781484211434

import javafx.application.Application;
import javafx.geometry.Point2D;
import javafx.scene.Node;
import javafx.scene.Scene;
import javafx.scene.control.Label;
import javafx.scene.control.TextField;
import javafx.scene.layout.HBox;
import javafx.scene.layout.VBox;
import javafx.scene.paint.Color;
import javafx.scene.shape.Circle;
import javafx.stage.Stage;

public class Main extends Application {
  Circle marker;

  public static void main(String[] args) {
    Application.launch(args);
  }

  @Override
  public void start(Stage stage) {
    TextField fName = new TextField();
    TextField lName = new TextField();
    TextField salary = new TextField();

    marker = new Circle(5);
    marker.setManaged(false);
    marker.setFill(Color.RED);
    marker.setMouseTransparent(true);

    HBox hb1 = new HBox();
    HBox hb2 = new HBox();
    HBox hb3 = new HBox();
    hb1.getChildren().addAll(new Label("First Name:"), fName);
    hb2.getChildren().addAll(new Label("Last Name:"), lName);
    hb3.getChildren().addAll(new Label("Salary:"), salary);

    VBox root = new VBox();
    root.getChildren().addAll(hb1, hb2, hb3, marker);

    Scene scene = new Scene(root);

    scene.focusOwnerProperty().addListener(
        (prop, oldNode, newNode) -> placeMarker(newNode));

    stage.setScene(scene);
    stage.setTitle("");
    stage.show();
  }

  public void placeMarker(Node newNode) {
    double nodeMinX = newNode.getLayoutBounds().getMinX();
    double nodeMinY = newNode.getLayoutBounds().getMinY();
    Point2D nodeInScene = newNode.localToScene(nodeMinX, nodeMinY);
    Point2D nodeInMarkerLocal = marker.sceneToLocal(nodeInScene);
    Point2D nodeInMarkerParent = marker.localToParent(nodeInMarkerLocal);

    marker.relocate(nodeInMarkerParent.getX()
        + marker.getLayoutBounds().getMinX(), nodeInMarkerParent.getY()
        + marker.getLayoutBounds().getMinY());
  }
}