Kivy -下拉列表-如何从所选按钮获取当前文本？

from kivy.app import App
from kivy.uix.button import Button
from kivy.uix.dropdown import DropDown

class MyApp(App):
    def build(self):
        # 创建下拉列表
        dropdown = DropDown()

        # 创建选项按钮
        for i in range(5):
            btn = Button(text='Option %d' % i, size_hint_y=None, height=44)

            # 绑定按钮点击事件
            btn.bind(on_release=lambda btn: dropdown.select(btn.text))

            # 将按钮添加到下拉列表中
            dropdown.add_widget(btn)

        # 创建主按钮
        main_button = Button(text='Select an option', size_hint=(None, None))

        # 绑定主按钮点击事件
        main_button.bind(on_release=dropdown.open)

        # 更新主按钮文本
        dropdown.bind(on_select=lambda instance, x: setattr(main_button, 'text', x))

        return main_button

if __name__ == '__main__':
    MyApp().run()