在pygame中，当我与某物发生碰撞时，如何使其停留？

import pygame

# 初始化pygame
pygame.init()

# 创建窗口
screen = pygame.display.set_mode((800, 600))

# 创建精灵类
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 = (400, 300)
        self.velocity = pygame.Vector2(0, 0)

    def update(self):
        # 更新位置
        self.rect.move_ip(self.velocity)

# 创建精灵组
all_sprites = pygame.sprite.Group()
player = Player()
all_sprites.add(player)

# 游戏主循环
running = True
while running:
    for event in pygame.event.get():
        if event.type == pygame.QUIT:
            running = False

    # 检测碰撞
    collisions = pygame.sprite.spritecollide(player, all_sprites, False)
    if collisions:
        # 发生碰撞时停止移动
        player.velocity = pygame.Vector2(0, 0)
        # 或者重置位置
        # player.rect.center = player.previous_position

    # 更新精灵
    all_sprites.update()

    # 绘制精灵
    screen.fill((255, 255, 255))
    all_sprites.draw(screen)

    # 更新屏幕
    pygame.display.flip()

# 退出游戏
pygame.quit()