如何在pygame中按键连续移动一个表面？

import pygame
import sys

pygame.init()
screen = pygame.display.set_mode((800, 600))

surface = pygame.Surface((50, 50))
surface.fill((255, 0, 0))  # 设置表面的颜色
surface_rect = surface.get_rect()
surface_rect.center = (400, 300)  # 设置表面的初始位置

speed = 5  # 移动速度
direction = None  # 移动方向

while True:
    for event in pygame.event.get():
        if event.type == pygame.QUIT:
            pygame.quit()
            sys.exit()
        elif event.type == pygame.KEYDOWN:
            if event.key == pygame.K_UP:
                direction = "up"
            elif event.key == pygame.K_DOWN:
                direction = "down"
            elif event.key == pygame.K_LEFT:
                direction = "left"
            elif event.key == pygame.K_RIGHT:
                direction = "right"
        elif event.type == pygame.KEYUP:
            if event.key == pygame.K_UP or event.key == pygame.K_DOWN or event.key == pygame.K_LEFT or event.key == pygame.K_RIGHT:
                direction = None

    if direction == "up":
        surface_rect.y -= speed
    elif direction == "down":
        surface_rect.y += speed
    elif direction == "left":
        surface_rect.x -= speed
    elif direction == "right":
        surface_rect.x += speed

    screen.fill((255, 255, 255))  # 清空屏幕
    screen.blit(surface, surface_rect)  # 绘制表面
    pygame.display.flip()  # 更新屏幕显示