我一直在尝试用PyGame做一个游戏，但是碰撞检测器不工作

import pygame
from pygame.locals import *

pygame.init()

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

# 创建两个矩形
rect1 = pygame.Rect(100, 100, 200, 200)
rect2 = pygame.Rect(300, 300, 200, 200)

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

    # 清屏
    screen.fill((255, 255, 255))

    # 绘制矩形
    pygame.draw.rect(screen, (255, 0, 0), rect1)
    pygame.draw.rect(screen, (0, 255, 0), rect2)

    # 碰撞检测
    if rect1.colliderect(rect2):
        print("碰撞发生！")

    # 刷新屏幕
    pygame.display.flip()

pygame.quit()