JavaFX - 如何重定位文本以创建帮助消息

/**
 *
 * @author Kishori Sharan (www.jdojo.com)
 * @version 1.0
 *
 * <b>@copyright</b> You can use, modify, copy-paste the code for any purpose, in any way you see fit. No restrictions apply.
 */


import javafx.application.Application;
import javafx.application.Platform;
import javafx.beans.value.ObservableValue;
import javafx.geometry.VPos;
import javafx.scene.Node;
import javafx.scene.Scene;
import javafx.scene.control.Button;
import javafx.scene.control.Label;
import javafx.scene.control.TextField;
import javafx.scene.layout.GridPane;
import javafx.scene.paint.Color;
import javafx.scene.text.Font;
import javafx.scene.text.Text;
import javafx.stage.Stage;

public class Main extends Application {
    // An instance variable to store the Text node reference
    private Text helpText = new Text();

    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();

        Button closeBtn = new Button("Close");
        closeBtn.setOnAction(e -> Platform.exit());

        fName.getProperties().put("microHelpText", "Enter the first name");
        lName.getProperties().put("microHelpText", "Enter the last name");
        salary.getProperties().put("microHelpText", 
                                   "Enter a salary greater than $2000.00.");

        // The help text node is unmanaged
        helpText.setManaged(false);
        helpText.setTextOrigin(VPos.TOP);
        helpText.setFill(Color.RED);
        helpText.setFont(Font.font(null, 9));
        helpText.setMouseTransparent(true);
        
        // Add all nodes to a GridPane
        GridPane root = new GridPane();

        root.add(new Label("First Name:"), 1, 1);
        root.add(fName, 2, 1);
        root.add(new Label("Last Name:"), 1, 2);
        root.add(lName, 2, 2);

        root.add(new Label("Salary:"), 1, 3);
        root.add(salary, 2, 3);
        root.add(closeBtn, 3, 3);
        root.add(helpText, 4, 3);

        Scene scene = new Scene(root, 300, 100);

        // Add a change listener to the scene, so we know when
        // the focus owner changes and display the micro help   
        scene.focusOwnerProperty().addListener(
            (ObservableValue<? extends Node> value, Node oldNode, Node newNode) 
                -> focusChanged(value, oldNode, newNode));
        stage.setScene(scene);
        stage.setTitle("Showing Micro Help");
        stage.show();
    }

    public void focusChanged(ObservableValue<? extends Node> value, 
                             Node oldNode, Node newNode) {
        // Focus has changed to a new node
        String microHelpText = (String)newNode.getProperties().get("microHelpText");
        
        if (microHelpText != null && microHelpText.trim().length() > 0)  {
            helpText.setText(microHelpText);
            helpText.setVisible(true);

            // Position the help text node
            double x = newNode.getLayoutX() + 
                       newNode.getLayoutBounds().getMinX() - 
                       helpText.getLayoutBounds().getMinX();
            double y = newNode.getLayoutY() + 
                       newNode.getLayoutBounds().getMinY() + 
                       newNode.getLayoutBounds().getHeight() - 
                       helpText.getLayoutBounds().getMinX();

            helpText.setLayoutX(x);
            helpText.setLayoutY(y);
            helpText.setWrappingWidth(newNode.getLayoutBounds().getWidth());
        }
        else {
            helpText.setVisible(false);
        }
    }
}