在Pygame中如何让球停留在屏幕上

import pygame
pygame.init()

screen = pygame.display.set_mode((width, height))

ball_x = 200
ball_y = 200
ball_speed_x = 2
ball_speed_y = 2

running = True
while running:
    for event in pygame.event.get():
        if event.type == pygame.QUIT:
            running = False

    # 更新球的位置
    ball_x += ball_speed_x
    ball_y += ball_speed_y

    # 碰撞检测，使球在碰到屏幕边缘时反弹
    if ball_x < 0 or ball_x > width:
        ball_speed_x *= -1
    if ball_y < 0 or ball_y > height:
        ball_speed_y *= -1

    # 填充屏幕背景色
    screen.fill((0, 0, 0))

    # 绘制球
    pygame.draw.circle(screen, (255, 255, 255), (ball_x, ball_y), 10)

    # 更新屏幕显示
    pygame.display.flip()

# 退出游戏
pygame.quit()