如何创建函数来更改变量并使用更新后的变量刷新Kivy应用

from kivy.app import App
from kivy.uix.button import Button
from kivy.uix.boxlayout import BoxLayout

class MyApp(App):
    def build(self):
        layout = BoxLayout(orientation='vertical')
        
        # 创建一个标签和一个按钮
        self.label = Label(text='初始值')
        button = Button(text='点击更新', on_press=self.update_label)
        
        # 将标签和按钮添加到布局中
        layout.add_widget(self.label)
        layout.add_widget(button)
        
        return layout
    
    def update_label(self, instance):
        # 在这个函数中更改变量并刷新应用
        new_value = '更新后的值'
        self.label.text = new_value
        self.root_window.update()
        
if __name__ == '__main__':
    MyApp().run()