屏幕上下滚动

        :params incrment: 1 向下滚动
                          -1 向上滚动

def updown(self, increment):
        """
        屏幕上下滚动

        :params incrment: 1 向下滚动
                          -1 向上滚动
        """
        scroll_line_num = self.screen_height - 4
        # paging
        if increment == -1 and self.markline == 0 and self.topline != 0:
            self.topline -= 1
        elif increment == 1 and self.markline + self.topline != len(self._lines) - 1 and self.markline == scroll_line_num:
            self.topline += 1
        # scroll
        if increment == -1 and self.markline != 0:
            self.markline -= 1
        elif increment == 1 and self.markline != scroll_line_num and self.markline < len(self._lines) - 1:
            self.markline += 1