请用javafx写一段贪吃蛇代码，要求能暂停、重启和退出，并通过按钮调整游戏难度

以下是一份简单的贪吃蛇游戏代码，使用JavaFX实现，支持暂停、重启和退出，并可通过按钮调整游戏难度。

import javafx.animation.AnimationTimer;
import javafx.application.Application;
import javafx.scene.Scene;
import javafx.scene.canvas.Canvas;
import javafx.scene.canvas.GraphicsContext;
import javafx.scene.control.Button;
import javafx.scene.layout.BorderPane;
import javafx.scene.paint.Color;
import javafx.stage.Stage;

import java.util.ArrayList;
import java.util.List;
import java.util.Random;

public class SnakeGame extends Application {

    private static final int WIDTH = 600;
    private static final int HEIGHT = 600;
    private static final int CELL_SIZE = 20;
    private static final int SNAKE_INIT_LENGTH = 3;
    private static final int FOOD_COUNT = 2;
    private static final int[] DELTAS = {-1, 0, 1, 0, -1};

    private Canvas canvas;
    private GraphicsContext gc;
    private List<Point> snake;
    private List<Point> food;
    private AnimationTimer timer;
    private boolean isPaused;
    private boolean isOver;
    private int direction;
    private int score;
    private int speed;

    @Override
    public void start(Stage primaryStage) throws Exception {
        canvas = new Canvas(WIDTH, HEIGHT);
        gc = canvas.getGraphicsContext2D();
        snake = new ArrayList<>();
        food = new ArrayList<>();
        isPaused = false;
        isOver = false;
        direction = 0;
        score = 0;
        speed = 6;

        BorderPane root = new BorderPane(canvas);
        Button pauseButton = new Button("Pause");
        Button restartButton = new Button("Restart");
        Button quitButton = new Button("Quit");
        Button speedButton = new Button("Speed: " + speed);
        pauseButton.setOnAction(event -> isPaused = !isPaused);
        restartButton.setOnAction(event -> restart());
        quitButton.setOnAction(event -> System.exit(0));
        speedButton.setOnAction(event -> {
            speed = speed % 10 + 1;
            speedButton.setText("Speed: " + speed);
        });
        root.setTop(pauseButton);
        root.setLeft(restartButton);
        root.setRight(quitButton);
        root.setBottom(speedButton);

        Scene scene = new Scene(root);
        scene.setOnKeyPressed(event -> {
            switch (event.getCode()) {
                case UP:
                    if (direction != 2) direction = 0;
                    break;
                case RIGHT:
                    if (direction != 3) direction = 1;
                    break;
                case DOWN:
                    if (direction != 0) direction = 2;
                    break;
                case LEFT:
                    if (direction != 1) direction = 3;
                    break;
                case P:
                    isPaused = !isPaused;
                    break;
                case R:
                    restart();
                    break;
                case Q:
                    System.exit(0);
                    break;
                case S:
                    speed = speed % 10 + 1;
                    speedButton.setText("Speed: " + speed);
                    break;
                default:
                    break;
            }
        });

        primaryStage.setScene(scene);
        primaryStage.setTitle("Snake Game");
        primaryStage.show();

        restart();
        timer = new AnimationTimer() {
            private long lastTime = 0;

            @Override
            public void handle(long now) {
                if (isPaused || isOver) return;
                if (now - lastTime < 1000000000 / speed) return;
                lastTime = now;
                update();
                draw();
            }
        };
        timer.start();
    }

    private void restart() {
        snake.clear();
        food.clear();
        for (int i = 0; i < SNAKE_INIT_LENGTH; i++) {
            snake.add(new Point(WIDTH / 2, HEIGHT / 2 + i * CELL_SIZE));
        }
        for (int i = 0; i < FOOD_COUNT; i++) {
            spawnFood();
        }
        direction = 0;
        score = 0;
        isPaused = false;
        isOver = false;
    }

    private void spawnFood() {
        Random random = new Random();
        int x = random.nextInt(WIDTH / CELL_SIZE) * CELL_SIZE;
        int y = random.nextInt(HEIGHT / CELL_SIZE) * CELL_SIZE;
        Point p = new Point(x, y);
        if (snake.contains(p) || food.contains(p)) {
            spawnFood();
        } else {
            food.add(p);
        }
    }

    private void update() {
        Point head = snake.get(0);
        Point newHead = new Point(head.x + DELTAS[direction] * CELL_SIZE,
                head.y + DELTAS[direction + 1] * CELL_SIZE);
        if (newHead.x < 0 || newHead.x >= WIDTH || newHead.y < 0 || newHead.y >= HEIGHT
                || snake.contains(newHead)) {
            isOver = true;
            return;
        }
        snake.add(0, newHead);
        if (food.contains(newHead)) {
            food.remove(newHead);
            spawnFood();
            score++;
        } else {
            snake.remove(snake.size() - 1);
        }
    }

    private void draw() {
        gc.setFill(Color.WHITE);
        gc.fillRect(0, 0, WIDTH, HEIGHT);
        gc.setFill(Color.BLACK);
        gc.fillText("Score: " + score, 20, 20);
        gc.setStroke(Color.BLACK);
        gc.strokeRect(0, 0, WIDTH, HEIGHT);
        gc.setFill(Color.GREEN);
        for (Point p : snake) {
            gc.fillRect(p.x, p.y, CELL_SIZE, CELL_SIZE);
        }
        gc.setFill(Color.RED);
        for (Point p : food) {
            gc.fillOval(p.x + CELL_SIZE / 4, p.y + CELL_SIZE / 4,
                    CELL_SIZE / 2, CELL_SIZE / 2);
        }
        if (isPaused) {
            gc.setFill(Color.BLACK);
            gc.fillText("PAUSED", WIDTH / 2 - 30, HEIGHT / 2);
        }
        if (isOver) {
            gc.setFill(Color.BLACK);
            gc.fillText("GAME OVER", WIDTH / 2 - 40, HEIGHT / 2);
        }
    }

    private static class Point {
        public int x;
        public int y;

        public Point(int x, int y) {
            this.x = x;
            this.y = y;
        }

        @Override
        public boolean equals(Object obj) {
            if (obj == null || !(obj instanceof Point)) return false;
            Point p = (Point) obj;
            return x == p.x && y == p.y;
        }
    }

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

该代码使用JavaFX的Canvas和GraphicsContext实现绘制，通过AnimationTimer实现游戏循环。游戏逻辑包括蛇的移动、食物的生成和吃掉、游戏结束等。同时，通过按钮和键盘事件处理实现暂停、重启、退出和调整游戏难度等功能。