如何在AnimationTimer中更改通过键盘输入移动对象的速度？

import javafx.animation.AnimationTimer;
import javafx.application.Application;
import javafx.scene.Group;
import javafx.scene.Scene;
import javafx.scene.input.KeyCode;
import javafx.scene.input.KeyEvent;
import javafx.scene.shape.Rectangle;
import javafx.stage.Stage;

public class KeyboardAnimation extends Application {
    private double speed = 0;

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

    @Override
    public void start(Stage primaryStage) {
        Group root = new Group();
        Scene scene = new Scene(root, 400, 400);

        Rectangle rectangle = new Rectangle(50, 50, 50, 50);
        root.getChildren().add(rectangle);

        scene.addEventFilter(KeyEvent.KEY_PRESSED, event -> {
            if (event.getCode() == KeyCode.UP) {
                speed = -5; // 设置向上移动速度为负值
            } else if (event.getCode() == KeyCode.DOWN) {
                speed = 5; // 设置向下移动速度为正值
            }
        });

        scene.addEventFilter(KeyEvent.KEY_RELEASED, event -> {
            if (event.getCode() == KeyCode.UP || event.getCode() == KeyCode.DOWN) {
                speed = 0; // 停止移动
            }
        });

        AnimationTimer animationTimer = new AnimationTimer() {
            @Override
            public void handle(long now) {
                double newY = rectangle.getY() + speed;
                rectangle.setY(newY);
            }
        };

        animationTimer.start();

        primaryStage.setScene(scene);
        primaryStage.show();
    }
}