错误代码 : "KivyMD：加载前必须初始化App对象"

Question

当我试图添加一个kivymd按钮(MDRaisedButton)时，我收到了错误："KivyMD: App对象必须在加载前初始化"。

我试着调试了一下，问题是在DateScreen下添加BoxLayout时，我没有找到任何关于如何解决这个问题的信息。

在加载前应该如何初始化app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()

请帮助我

Answer 1

我相信你需要调用 Builder 之后 App 是初始化的（如您的错误中提到的）。要做到这一点，您可以保存您的 kv 的字符串中，然后调用 Builder 里面 build() 方法。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()