如何擦拭不重复的精灵

Question

我正在学习在pygame中编码。我面临的问题是，我无法擦除以前的精灵。我的意思是图像只是沿着屏幕延伸。如何移动精灵而不是扩展精灵？

def check_edges(self):
    screen_rect = self.screen.get_rect()
    if self.rect.right >= screen_rect.right: #if alien has caught up the right edge of the screen
        return True
    elif self.rect.left <= 0: #if alien has caught up the left edge of the screen
        return True

def update_aliens(custom_settings, aliens):
    check_fleet_edges(custom_settings, aliens)
    aliens.update()

def check_fleet_edges(custom_settings, aliens):
    for alien in aliens.sprites():
       if alien.check_edges():
           change_fleet_direction(custom_settings, aliens)
           break

def change_fleet_direction(custom_settings, aliens):
   for alien in aliens.sprites():
       alien.rect.y += custom_settings.fleet_drop_speed #fleet_drop_speed is the value of whole fleet's speed
   custom_settings.fleet_direction *= -1 #change direction of fleet's moving

我的绘图代码就在这里：

def update_screen(custom_settings, screen, spaceship, aliens, bullets, rockets, stars):
   screen.fill(custom_settings.bg_color)
   for bullet in bullets.sprites():#возвращает список спрайтов в группе
       bullet.draw_bullet() 
   for rocket in rockets.sprites():
       rocket.draw_rocket()
   stars.draw(screen)
   aliens.draw(screen)
   spaceship.blitme()
   pygame.display.flip()

这是我的主循环：

while True:
    gf.check_events(custom_settings, screen, spaceship, bullets, rockets)
    spaceship.update()
    bullets.update()
    rockets.update()
    gf.update_ammunition(bullets, rockets)
    gf.update_aliens(custom_settings, aliens, screen)
    gf.create_fleet(custom_settings, screen, spaceship, aliens)
    gf.starsky(custom_settings, screen, spaceship, stars)
    gf.update_screen(custom_settings, screen, spaceship, aliens, bullets, rockets, 
        stars)
    clock.tick(FPS)
    print(clock.get_fps())

This is how they move

Answer 1

问题在于精灵会无限生成。我应该限制外星人的数量

def create_fleet(custom_settings, screen, spaceship, aliens):
   alien = Alien(custom_settings, screen)
   number_aliens_x = get_number_aliens_x(custom_settings, alien.rect.width)
   number_rows = get_number_rows(custom_settings, spaceship.rect.height,    alien.rect.height)
   for row_number in range(number_rows):
       for alien_number in range(number_aliens_x):
           if len(aliens) <= 48:
               create_alien(custom_settings, screen, aliens, alien_number, row_number)
           else:
               aliens.remove(alien)