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

Question

我正在尝试Kivymd，但始终收到错误：AttributeError：'NoneType'对象没有属性'theme_cls'。

我在App类中添加了theme_cls = ThemeManager（），但仍然出现错误。由于没有更多的在线解决方案，请提供帮助。

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MyScreenManager:之前实例化了根规则 theme_cls。实例化MyScreenManager:时，实例化其子节点Screen:。实例化Screen:时，实例化了引用Toolbar:的子节点theme_cls。但是theme_cls尚未实例化。因此，它引发了以下错误，

 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

• 用类规则MyScreenManager:替换根规则<MyScreenManager>:

摘要

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


<MyScreenManager>:
    Screen:

Python脚本

• 将return Login替换为return MyScreenManager()

摘要

class MyApp(App):
    theme_cls = ThemeManager()

    def build(self):
        return MyScreenManager()

输出