KivyMD中ScreenManager配置NavigationDrawer报AssertionError问题求助

错误原因 #

• 核心触发AssertionError的原因是KV规则出现递归自引用：你在<ContentNavigationDrawer>的规则定义内部，又实例化了ContentNavigationDrawer组件，导致Kivy的Builder重复加载同一条规则，触发了规则不允许重复的断言校验。
• 同时存在多处逻辑错误：
  1. ContentNavigationDrawer是抽屉的内容组件，不需要继承Screen，也不需要作为屏幕加到外层ScreenManager中
  2. 你在Python代码中手动创建的sm ScreenManager完全没有被使用，属于冗余代码
  3. ContentNavigationDrawer的KV规则里同时写了抽屉列表和主屏幕布局，结构完全混乱，导航相关的布局应该放在外层，而不是抽屉内容组件内部

修复后完整可运行代码 #

from kivymd.app import MDApp
from kivy.lang.builder import Builder
from kivy.uix.screenmanager import ScreenManager, Screen
from kivy.properties import ObjectProperty
from kivy.uix.boxlayout import BoxLayout

screen_helper = """
NavigationLayout:
    ScreenManager:
        id: main_sm
        MenuScreen:
        ProfileScreen:
        UploadScreen:
    
    MDNavigationDrawer:
        id: nav_drawer
        ContentNavigationDrawer:
            screen_manager: main_sm
            nav_drawer: nav_drawer

<MenuScreen>:
    name: 'menu'
    MDToolbar:
        id: toolbar
        pos_hint: {"top": 1}
        elevation: 10
        title: "菜单页"
        left_action_items: [["menu", lambda x: nav_drawer.set_state("open")]]
    MDRectangleFlatButton:
        text: 'Profile'
        pos_hint: {'center_x':0.5,'center_y':0.6}
        on_press: root.manager.current = 'profile'
    MDRectangleFlatButton:
        text: 'Upload'
        pos_hint: {'center_x':0.5,'center_y':0.5}
        on_press: root.manager.current = 'upload'
    
<ProfileScreen>:
    name: 'profile'
    MDToolbar:
        id: toolbar
        pos_hint: {"top": 1}
        elevation: 10
        title: "资料页"
        left_action_items: [["menu", lambda x: nav_drawer.set_state("open")]]
    MDLabel:
        text: 'Profile'
        halign: 'center'
    MDRectangleFlatButton:
        text: 'Back'
        pos_hint: {'center_x':0.5,'center_y':0.1}
        on_press: root.manager.current = 'menu'

<UploadScreen>:
    name: 'upload'
    MDToolbar:
        id: toolbar
        pos_hint: {"top": 1}
        elevation: 10
        title: "上传页"
        left_action_items: [["menu", lambda x: nav_drawer.set_state("open")]]
    ScreenManager:
        id: upload_sm
        Screen:
            name: "scr 1"
            MDLabel:
                text: "Screen 1"
                halign: "center"
        Screen:
            name: "scr 2"
            MDLabel:
                text: "Screen 2"
                halign: "center"

<ContentNavigationDrawer>:
    orientation: "vertical"
    padding: "8dp"
    spacing: "8dp"

    ScrollView:
        MDList:
            OneLineListItem:
                text: "Screen 1"
                on_press:
                    root.nav_drawer.set_state("close")
                    if root.screen_manager.current == "upload": root.screen_manager.current_screen.ids.upload_sm.current = "scr 1"
                    else: root.screen_manager.current = "upload"; root.screen_manager.current_screen.ids.upload_sm.current = "scr 1"

            OneLineListItem:
                text: "Screen 2"
                on_press:
                    root.nav_drawer.set_state("close")
                    if root.screen_manager.current == "upload": root.screen_manager.current_screen.ids.upload_sm.current = "scr 2"
                    else: root.screen_manager.current = "upload"; root.screen_manager.current_screen.ids.upload_sm.current = "scr 2"
"""


class MenuScreen(Screen):
    pass


class ProfileScreen(Screen):
    pass


class UploadScreen(Screen):
    pass


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


class DemoApp(MDApp):
    def build(self):
        return Builder.load_string(screen_helper)


DemoApp().run()

内容的提问来源于stack exchange，提问作者سید عبدل علی