如何修复 AttributeError：“NoneType”对象没有属性“theme_cls”

Question

我正在尝试 Kivymd，但不断收到错误消息：AttributeError: 'NoneType' object has no attribute 'theme_cls'。

我在 App 类中添加了 theme_cls = ThemeManager() 但仍然收到错误消息。请帮忙，因为没有更多的在线解决方案。

from kivy.lang import Builder
from kivy.uix.screenmanager import ScreenManager, Screen, FadeTransition, WipeTransition

from kivy.app import App
from kivy.uix.button import Button
from kivymd.navigationdrawer import MDNavigationDrawer
from kivymd.theming import ThemeManager

class BeginScreen(Screen):
    pass

class MyScreenManager(ScreenManager):
    pass

Login = Builder.load_string('''
#: import Toolbar kivymd.toolbar.Toolbar


MyScreenManager:
    Screen:


<Screen>:

    Toolbar:
        title: "Simple toolbar"
        pos_hint: {'center_x': 0.5, 'center_y': 0.75}
        md_bg_color: get_color_from_hex(colors['Teal']['500'])
        background_palette: 'Teal'
        background_hue: '500'
    Toolbar:
        title: "Toolbar with right buttons"
        pos_hint: {'center_x': 0.5, 'center_y': 0.5}
        md_bg_color: get_color_from_hex(colors['Amber']['700'])
        background_palette: 'Amber'
        background_hue: '700'
        right_action_items: [['content-copy', lambda x: None]]
    Toolbar:
        title: "Toolbar with left and right buttons"
        pos_hint: {'center_x': 0.5, 'center_y': 0.25}
        md_bg_color: get_color_from_hex(colors['DeepPurple']['A400'])
        background_palette: 'DeepPurple'
        background_hue: 'A400'
        left_action_items: [['arrow-left', lambda x: None]]
        right_action_items: [['lock', lambda x: None], \
            ['camera', lambda x: None], \
            ['play', lambda x: None]]


''')

class MyApp(App):
    theme_cls = ThemeManager()

    def build(self):
        return Login

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

Answer 1

根本原因

当 Kivy 处理 kv 文件/字符串时，它在实例化 theme_cls 之前实例化了 根规则，MyScreenManager:。当MyScreenManager: 被实例化时，它会实例化它的孩子Screen:。当Screen: 被实例化时，它会实例化其子代Toolbar:，它引用theme_cls。但theme_cls 尚未实例化。因此，它抛出了以下错误，

 self._shadow = App.get_running_app().theme_cls.quad_shadow

AttributeError: 'NoneType' 对象没有属性 'theme_cls'

解决方案

kv 字符串

• 将根规则MyScreenManager: 替换为类规则<MyScreenManager>:

片段

Login = Builder.load_string('''
#:import Toolbar kivymd.toolbar.Toolbar


<MyScreenManager>:
    Screen:

Python 脚本

• 将return Login 替换为return MyScreenManager()

片段

class MyApp(App):
    theme_cls = ThemeManager()

    def build(self):
        return MyScreenManager()

输出