所以我正在尝试为沿着屏幕向下移动的虚线设置动画效果……就像白色虚线在路上经过您时一样。我很容易画出虚线：

    import pygame
    GREY = (211,211,211)
    WHITE = (255,255,255)
    pygame.init()
    screen = pygame.display.set_mode(800,600)
    pygame.display.set_caption("Line Test")
    clock = pygame.time.Clock()

    running = True
    while running:
        clock.tick(60)
        for event in pygame.event,get():
            if event.type == pygame.QUIT:
                running = False

        all_sprites.update()
        screen.fill(GREY)
        dash = 0
        for i in range(30):
            pygame.draw.line(screen,WHITE,[400,1 + dash],dash + 10],6)
            dash += 20
        all_sprites.draw(screen)
        pygame.display.flip()

    pygame.quit()

但是我正在尝试对Class中的那一行进行动画处理，因此它似乎一直在向下移动。到目前为止没有运气，只能通过以下方式在屏幕上向下移动一个破折号：

    class Line(pygame.sprite.Sprite):
        def __init__(self):
            pygame.sprite.Sprite.__init__(self)
            self.image = pygame.Surface((6,10))
            self.image.fill(WHITE)
            self.rect = self.image.get_rect()
            self.last_uodate = pygame.time.get_ticks()
            self.rect.x = 400
            dash = 0
            for i in range(30):
                self.rect.y = 0 + dash
                dash += 20
            self.speedy = 5

        def update(self):
            self.rect.y += self.speedy
            if self.rect.y > 600:
                now = pygame.time.get_ticks()
                if now - self.last_update > 0:
                    self.last_update = now
                    self.rect.x = 400
                    self.rect.y = 0
                    self.speedy = 5

    all_sprites = pygame.sprite.Group()
    line = pygame.sprite.Group()
    for i in range(30):
        ln = Line()
        all_sprites.add(ln)
        line.add(ln)

有什么建议或告诉我我哪里出错了吗？这只会在屏幕上产生单个破折号，而不会产生从上到下的完整虚线不断移动的情况。