import pygame

class UpDownBox(pygame.sprite.Sprite):
    def __init__(self, imagesList, initial_position):
        pygame.sprite.Sprite.__init__(self)
        self.images = imagesList
        self.listLen = len(imagesList)
        self.listPos = 0
        self.image = imagesList[self.listPos]
        self.rect = self.image.get_rect()
        self.rect.topleft = initial_position
        self.going_down = True # Start going downwards
        self.next_update_time = 0 # update() hasn't been called yet.

    def update(self, current_time, bottom):
        # Update every 10 milliseconds = 1/100th of a second.
        if self.next_update_time < current_time:

            # If we're at the top or bottom of the screen, switch directions.
            if self.rect.bottom == bottom - 1: self.going_down = False
            elif self.rect.top == 0: self.going_down = True
     
            # Move our position up or down by one pixel
            if self.going_down: self.rect.top += 1
            else: self.rect.top -= 1

            if self.listPos < self.listLen - 1:
               self.listPos += 1
            else:
               self.listPos = 0
            self.image = self.images[self.listPos]
            self.next_update_time = current_time