很简单的pygame游戏有一步我没懂

def花

1. import pygame,sys
   
2. 
   
3. pygame.init()
   
4. size = width,height = 600,400
   
5. speed = [1,1]
   
6. BLACK = 0,0,0
   
7. screen = pygame.display.set_mode(size）
   
8. pygame.display.set_caption("pygame壁球碰撞")
   
9. ball = pygame.image.load('PYG02-ball.gif')
   
10. ball_ract = ball.get_rect()
    
11. fps = 300
    
12. clock = pygame.time.Clock()
    
13. still = False
    
14. 
    
15. while True:
    
16.     for event in pygame.event.get():
    
17.         if event.type == pygame.QUIT:
    
18.             sys.exit()
    
19.         elif event.type == pygame.MOUSEBUTTONDOWN:
    
20.             if event.button == 1:
    
21.                 still = True
    
22.         elif event.type == pygame.MOUSEBUTTONUP:
    
23.             still = False
    
24.             if event.button == 1:
    
25.                 ball_ract = ball_ract.move(event.pos[0] - ball_ract.left,event.pos[1]-ball_ract.top)#就是这里，是为了防止鼠标点击到窗口旁边，卡在旁边不动，没懂为什么这么判断
    
26.         elif event.type == pygame.MOUSEMOTION:
    
27.             if event.buttons[0] == 1:
    
28.                 ball_ract = ball_ract.move(event.pos[0]-ball_ract.left,event.pos[1]-ball_ract.top)
    
29.     if pygame.display.get_active() and not still:
    
30.         ball_ract = ball_ract.move(speed[0],speed[1])
    
31.     if ball_ract.right > width or ball_ract.left < 0:
    
32.         speed[0] = -speed[0]
    
33.         if ball_ract.right > width and ball_ract.right + speed[0] > ball_ract.right:
    
34.             speed[0] = -speed[0]
    
35.     if ball_ract.bottom > height or ball_ract.top <0:
    
36.         speed[1] = -speed[1]
    
37.         if ball_ract.bottom > height and ball_ract.bottom + speed[1] > ball_ract.bottom:
    
38.             speed[1] = -speed[1]
    
39. 
    
40.     screen.fill(BLACK)
    
41.     screen.blit(ball,ball_ract)
    
42.     pygame.display.update()
    
43.     clock.tick(fps)

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