python - 错误代码：“KivyMD：应用程序对象必须在加载前初始化”

Question

当我尝试添加 kivymd 按钮 (MDRaisedButton) 时，我收到错误消息：“KivyMD: App object must be initialized before loading”。

我尝试调试，问题是当我在 DateScreen 下添加 BoxLayout 时，我只是找不到任何有关如何修复它的信息。

在加载之前应该如何初始化一个应用程序对象？

import kivy
kivy.require("1.11.1")
from kivy.app import App
from kivy.lang import Builder
from kivy.clock import Clock
from kivy.uix.screenmanager import Screen
from kivymd.theming import ThemeManager


class WelcomeScreen(Screen):  # Defines WelcomeScreen instance as a screen widget.
    pass

class DateScreen(Screen):  # Defines DateScreen instance as a screen widget.
    pass

class ResultScreen(Screen):  # Defines ResultScreen instance as a screen widget.
    pass

root_widget = Builder.load_string("""
ScreenManager:
    WelcomeScreen:
    DateScreen:
    ResultScreen:
<WelcomeScreen>:
    _welcome_screen_text_: welcome_screen
    name: 'my_welcome_screen'
    Label:
        id: welcome_screen
        Image: 
            source: 'Cheers.jpg'
            size: 200, 200
            center: self.parent.center

<DateScreen>:
    _date_screen_text_: date_screen
    name: 'my_date_screen'
    Label:
        id: date_screen
        text: "This is the date selection screen"
    BoxLayout:
        size_hint: 0.1, 0.5
        pos_hint: {'center_x': 0.5, 'center_y': 0.5}
        MDRaisedButton:
            text: 'MDButton1'
<ResultScreen>:
    _result_screen_text_: result_screen
    name: 'my_result_screen'
    Label:
        id: result_screen
        text: "This is where the result will be displayed"
""")

class MainApp(App):

    theme_cls = ThemeManager()

    def build(self):
        self.theme_cls.theme_style = "Dark"
        Clock.schedule_once(self.screen_switch_one, 36)  # clock callback for the first screen
        Clock.schedule_once(self.screen_switch_two, 4)  # clock callback for the second screen
        return root_widget


    def screen_switch_one(a, b):
        root_widget.current = 'my_welcome_screen'  

    def screen_switch_two(a, b):
        root_widget.current = 'my_date_screen' 

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

请帮忙！

标签： pythonandroidkivykivy-language

Answer 1

我相信您需要在初始化Builder后调用App（如您的错误中所述）。为此，您可以将您的保存kv在一个字符串中，然后在方法Builder内部调用build()。像这样：

import kivy
from kivymd.app import MDApp

kivy.require("1.11.1")
from kivy.lang import Builder
from kivy.clock import Clock
from kivy.uix.screenmanager import Screen


class WelcomeScreen(Screen):  # Defines WelcomeScreen instance as a screen widget.
    pass

class DateScreen(Screen):  # Defines DateScreen instance as a screen widget.
    pass

class ResultScreen(Screen):  # Defines ResultScreen instance as a screen widget.
    pass

kv = """
ScreenManager:
    WelcomeScreen:
    DateScreen:
    ResultScreen:
<WelcomeScreen>:
    _welcome_screen_text_: welcome_screen
    name: 'my_welcome_screen'
    Label:
        id: welcome_screen
        Image: 
            source: 'Cheers.jpg'
            size: 200, 200
            center: self.parent.center

<DateScreen>:
    _date_screen_text_: date_screen
    name: 'my_date_screen'
    Label:
        id: date_screen
        text: "This is the date selection screen"
    BoxLayout:
        size_hint: 0.1, 0.5
        pos_hint: {'center_x': 0.5, 'center_y': 0.5}
        MDRaisedButton:
            text: 'MDButton1'
<ResultScreen>:
    _result_screen_text_: result_screen
    name: 'my_result_screen'
    Label:
        id: result_screen
        text: "This is where the result will be displayed"
"""


class MainApp(MDApp):

    def build(self):
        self.theme_cls.theme_style = "Dark"
        self.root_widget = Builder.load_string(kv)
        Clock.schedule_once(self.screen_switch_one, 36)  # clock callback for the first screen
        Clock.schedule_once(self.screen_switch_two, 4)  # clock callback for the second screen
        return self.root_widget


    def screen_switch_one(self, dt):
        self.root_widget.current = 'my_welcome_screen'

    def screen_switch_two(self, dt):
        self.root_widget.current = 'my_date_screen'

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