Kivy:如何在另一个屏幕中更改属性的值，如当前屏幕中的标签文本

from kivy.app import App
from kivy.uix.screenmanager import ScreenManager, Screen
from kivy.uix.label import Label
from kivy.uix.button import Button

class Screen1(Screen):
    def __init__(self, **kwargs):
        super(Screen1, self).__init__(**kwargs)
        self.label = Label(text="Hello World")
        self.add_widget(self.label)

class Screen2(Screen):
    def __init__(self, **kwargs):
        super(Screen2, self).__init__(**kwargs)
        self.button = Button(text="Change Text")
        self.button.bind(on_release=self.change_text)
        self.add_widget(self.button)

    def change_text(self, instance):
        app = App.get_running_app()
        screen1 = app.root.get_screen('screen1')
        screen1.label.text = "New Text"

class MyApp(App):
    def build(self):
        sm = ScreenManager()
        sm.add_widget(Screen1(name='screen1'))
        sm.add_widget(Screen2(name='screen2'))
        return sm

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