在kivy中未切换屏幕

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


class Screen1(Screen):
    def __init__(self, **kwargs):
        super(Screen1, self).__init__(**kwargs)
        self.add_widget(Label(text="Screen 1"))
        self.add_widget(Button(text="Go to Screen 2", on_press=self.switch_screen))

    def switch_screen(self, instance):
        self.manager.current = 'screen2'


class Screen2(Screen):
    def __init__(self, **kwargs):
        super(Screen2, self).__init__(**kwargs)
        self.add_widget(Label(text="Screen 2"))
        self.add_widget(Button(text="Go to Screen 1", on_press=self.switch_screen))

    def switch_screen(self, instance):
        self.manager.current = 'screen1'


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


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