screenmanager kivy定义回调函数以切换屏幕

Question

I want to switch screens in python file with kivy. I want this to be via the callback function gonext . How do have to define this?. When I run the code it shows mainwidgetApp but the button doesnt work. Here is what I tried: changing the sm to global (propably not a good idea). changing the kivy file for mainwidget to: on_press: root.manager.current = 'settings'

here is my code:

Builder.load_string("""
<MenuScreen>:
 mainwidgetApp






<SettingsScreen>:
 BoxLayout:
    Button:
        text: 'My settings button'
    Button:
        text: 'Back to menu'
        on_press: root.manager.current = 'settings'
""")


 class mainwidget(Widget):




def __init__(self, **kwargs):

    btnnext = Button(text='go to next', pos=(200, 400))
    btnnext.bind(on_press=self.gonext)
    self.add_widget(btnnext)


# def savecard(self, btn_instance):




def gonext(self ,btn_inst):

  ScreenManager().current = "SettingsScreen"

# Create the screen manager
sm = ScreenManager()
sm.add_widget(MenuScreen(name='menu'))
sm.add_widget(SettingsScreen(name='settings'))



class mainwidgetApp(App):

 def build(self):   
    Window.clearcolor = (0,0,0.3,1)
    return mainwidget()



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


class TestApp(App):

 def build(self):
    return sm

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

Answer 1

1. Replace ScreenManager().current = "SettingsScreen" with sm.current = "settings"
2. Rename class name from mainwidget to MainWidget ( CapWords convention)

Example

main.py

from kivy.app import App
from kivy.uix.screenmanager import ScreenManager, Screen
from kivy.uix.widget import Widget
from kivy.uix.button import Button
from kivy.core.window import Window
from kivy.lang import Builder


Builder.load_string("""
#:kivy 1.11.0

<MenuScreen>:
    MainWidget:


<SettingsScreen>:
    BoxLayout:
        Button:
            text: 'My settings button'
        Button:
            text: 'Back to menu'
            on_press: root.manager.current = 'settings'
""")


class MainWidget(Widget):

    def __init__(self, **kwargs):
        super(MainWidget, self).__init__(**kwargs)
        print("\nmainwidget:")
        btnnext = Button(text='go to next', pos=(200, 400))
        btnnext.bind(on_press=self.gonext)
        self.add_widget(btnnext)

    # def savecard(self, btn_instance):

    def gonext(self ,btn_inst):
        sm.current = "settings"


class MenuScreen(Screen):
    pass


class SettingsScreen(Screen):
    pass


class ScreenManager(ScreenManager):
    pass

# Create the screen manager
sm = ScreenManager()
sm.add_widget(MenuScreen(name='menu'))
sm.add_widget(SettingsScreen(name='settings'))


class TestApp(App):

    def build(self):
        Window.clearcolor = (0,0,0.3,1)
        return sm


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

Output