python 图形开发kivy小问题

用kivy 做个2048的小游戏，运行环境：python3.10, kivy 2.1

on_pos = resize
on_size = resize
这两行代码没看懂！怎么运行的呢？是赋值吗？后面也没有再引用啊！ 如果去掉，背景就会缩小到左下角！
从网上查了半天也没找到相关的内容！请赐教！

以下是全部代码

from kivy.app import App
from kivy.core.window import Window
from kivy.graphics import BorderImage
from kivy.uix.widget import Widget
from kivy.utils import get_color_from_hex

spacing = 15


class Board(Widget):
    def __init__(self, **kwargs):
        super().__init__(**kwargs)
        self.cell_size = None
        self.resize()

    def resize(self, *args):
        self.cell_size = (0.25 * (self.width - 5 * spacing),) * 2

        # redraw background
        self.canvas.before.clear()
        with self.canvas.before:
            BorderImage(pos=self.pos, size=self.size, source='board.png')

    on_pos = resize
    on_size = resize


class GameApp(App):
    def build(self):
        # 通过id获取到FloatLayout布局中的Board
        board = self.root.ids.board


if __name__ == '__main__':
    Window.clearcolor = get_color_from_hex('faf8ef')
    GameApp().run()

kv language game.kv代码如下：

FloatLayout:
    Board:
        id: board
        pos_hint: {'center_x': 0.5, 'center_y': 0.5}
        size_hint: (None, None)
        center: root.center
        size: [min(root.width, root.height) - 2 * padding] * 2

on_pos表示窗口长
on_size表示窗口宽