如何在Calllback上切换Kivy布局？

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

class MainLayout(BoxLayout):
    def __init__(self, **kwargs):
        super(MainLayout, self).__init__(**kwargs)
        self.orientation = 'vertical'
        
        # 创建按钮并绑定回调函数
        btn1 = Button(text='布局1', on_press=self.switch_layout1)
        btn2 = Button(text='布局2', on_press=self.switch_layout2)
        self.add_widget(btn1)
        self.add_widget(btn2)
        
        # 默认显示布局1
        self.switch_layout1(None)
        
    def switch_layout1(self, instance):
        self.clear_widgets()
        
        # 创建布局1的组件
        label = Label(text='这是布局1')
        back_btn = Button(text='返回', on_press=self.switch_layout2)
        
        # 将布局1的组件添加到主布局中
        self.add_widget(label)
        self.add_widget(back_btn)
        
    def switch_layout2(self, instance):
        self.clear_widgets()
        
        # 创建布局2的组件
        label = Label(text='这是布局2')
        back_btn = Button(text='返回', on_press=self.switch_layout1)
        
        # 将布局2的组件添加到主布局中
        self.add_widget(label)
        self.add_widget(back_btn)

class MyApp(App):
    def build(self):
        return MainLayout()

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