pygame循环通过一个类实例的移动

import pygame
from pygame.locals import *

class Player(pygame.sprite.Sprite):
    def __init__(self):
        super().__init__()
        self.image = pygame.Surface((50, 50))
        self.image.fill((255, 0, 0))
        self.rect = self.image.get_rect()
        self.rect.center = (200, 200)
        self.speed = 5

    def update(self):
        keys = pygame.key.get_pressed()
        if keys[K_LEFT]:
            self.rect.x -= self.speed
        if keys[K_RIGHT]:
            self.rect.x += self.speed
        if keys[K_UP]:
            self.rect.y -= self.speed
        if keys[K_DOWN]:
            self.rect.y += self.speed

pygame.init()
screen = pygame.display.set_mode((400, 400))
clock = pygame.time.Clock()

player = Player()
all_sprites = pygame.sprite.Group()
all_sprites.add(player)

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

    all_sprites.update()

    screen.fill((255, 255, 255))
    all_sprites.draw(screen)

    pygame.display.flip()
    clock.tick(60)

pygame.quit()