什么是侦听器?如何在JavaFX TextField上实现侦听器？

import javafx.application.Application;
import javafx.beans.value.ChangeListener;
import javafx.beans.value.ObservableValue;
import javafx.scene.Scene;
import javafx.scene.control.TextField;
import javafx.scene.layout.StackPane;
import javafx.stage.Stage;

public class TextFieldListenerExample extends Application {
    @Override
    public void start(Stage primaryStage) {
        TextField textField = new TextField();
        textField.textProperty().addListener(new ChangeListener<String>() {
            @Override
            public void changed(ObservableValue<? extends String> observable, String oldValue, String newValue) {
                System.out.println("Old Value: " + oldValue);
                System.out.println("New Value: " + newValue);
            }
        });

        StackPane root = new StackPane();
        root.getChildren().add(textField);

        Scene scene = new Scene(root, 300, 250);

        primaryStage.setTitle("TextField Listener Example");
        primaryStage.setScene(scene);
        primaryStage.show();
    }

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