如何使用 KivyMD 在 NavigationDrawer 内的屏幕之间切换

我是使用 kivymd 的初学者，并尝试使用 KivyMD 在 NavigationDrawer 内的屏幕之间切换，我正在阅读一些文档并最终得到了这段代码，但它对我不起作用：

from kivy.lang import Builder
from kivy.core.window import Window
from kivy.uix.boxlayout import BoxLayout
from kivy.properties import ObjectProperty
from kivymd.theming import ThemableBehavior
from kivymd.uix.list import MDList
from kivymd.app import MDApp



Window.size = (300, 500)
navigation_helper = """
Screen:
    MDToolbar:
        id: toolbar
        pos_hint: {"top": 1}
        elevation: 10
        title: "MDNavigationDrawer"
        left_action_items: [["menu", lambda x: nav_drawer.set_state("open")]]
        
    NavigationLayout:
        x: toolbar.height

        ScreenManager:
            id: screen_manager

            Screen:
                name: "scr 1"

                MDLabel:
                    text: "Go to Hell"
                    halign: "center"

            Screen:
                name: "scr 2"

                MDLabel:
                    text: "Hell"
                    halign: "center"

        MDNavigationDrawer:
            id: nav_drawer

            ContentNavigationDrawer:
                screen_manager: screen_manager
                nav_drawer: nav_drawer
           
                ScrollView:
                    MDList:

                        OneLineListItem:
                            text: "Screen 1"
                            on_press:
                                root.nav_drawer.set_state("close")
                                root.screen_manager.current = "scr 1"
            
                        OneLineListItem:
                            text: "Screen 2"
                            on_press:
                                root.nav_drawer.set_state("close")
                                root.screen_manager.current = "scr 2"


"""



class DemoApp(MDApp):
    class ContentNavigationDrawer(BoxLayout):
        screen_manager = ObjectProperty()
        nav_drawer = ObjectProperty()

    class DrawerList(ThemableBehavior, MDList):
        pass

    def build(self):
        screen = Builder.load_string(navigation_helper)
        return screen

    def on_start(self):
        pass


DemoApp().run()

但它给了我这个错误“ AttributeError：'Screen'对象没有属性'nav_drawer'”，我不确定我做错了什么。

当你使用一个对象来引用一个对象时id在同一规则内id已定义，您不需要在其前面添加root:

                    OneLineListItem:
                        text: "Screen 1"
                        on_press:
                            nav_drawer.set_state("close")
                            screen_manager.current = "scr 1"

                    OneLineListItem:
                        text: "Screen 2"
                        on_press:
                            nav_drawer.set_state("close")
                            screen_manager.current = "scr 2"