无法在Kivy中切换屏幕

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

class Screen1(Screen):
    pass

class Screen2(Screen):
    pass

# 可以根据需要创建更多的屏幕类

class MyApp(App):
    def build(self):
        sm = ScreenManager()
        sm.add_widget(Screen1(name='screen1'))
        sm.add_widget(Screen2(name='screen2'))
        # 添加更多的屏幕

        return sm

# 在Screen1类中定义布局和逻辑
class Screen1(Screen):
    def on_button_press(self):
        # 处理按钮按下事件
        self.manager.current = 'screen2'  # 切换到Screen2

# 在Screen2类中定义布局和逻辑
class Screen2(Screen):
    def on_button_press(self):
        # 处理按钮按下事件
        self.manager.current = 'screen1'  # 切换到Screen1