如何从不同的类访问屏幕管理器？(Kivy)

from kivy.app import App
from kivy.uix.screenmanager import ScreenManager, Screen

class MyScreen(Screen):
    pass

class MyScreenManager(ScreenManager):
    pass

class MyApp(App):
    def build(self):
        screen_manager = MyScreenManager()
        screen_manager.add_widget(MyScreen(name='screen1'))
        screen_manager.add_widget(MyScreen(name='screen2'))
        return screen_manager

from kivy.app import App

class AnotherClass:
    def switch_screen(self, screen_name):
        app = App.get_running_app()
        screen_manager = app.root
        screen_manager.current = screen_name