使用Python Kivy制作多个屏幕时遇到困难

Question

I am just trying to get code working where I have two screens in a Python Kivy app that can switch back and forth, without using the .kv file stuff.

On this page: https://kivy.org/docs/api-kivy.uix.screenmanager.html , the second block of code from the top is what I am trying to accomplish, except I want to do it without the 'Builder.load_string("""' section, and instead just instantiate buttons normally.

Here is my attempt at doing so, except I can't get it to work:

from kivy.app import App
from kivy.lang import Builder
from kivy.uix.screenmanager import ScreenManager, Screen
from kivy.uix.floatlayout import FloatLayout
from kivy.uix.label import Label
from kivy.uix.button import Button

class MenuScreen(Screen):
    def build(self):

        def switchScreen():
            root.manager.current = 'settings'

        f = FloatLayout()

        button1 = Button(text = "My settings button")
        button2 = Button(text = "Back to menu", on_press = switchScreen)

        f.add_widget(button1)
        f.add_widget(button2)

class SettingsScreen(Screen):
    def build(self):

        def switchScreen():
            root.manager.current = 'menu'

        f = FloatLayout()

        button1 = Button(text = "My settings button")
        button2 = Button(text = "Back to menu", on_press = switchScreen)

        f.add_widget(button1)
        f.add_widget(button2)

sm = ScreenManager()
sm.add_widget(MenuScreen(name='menu'))
sm.add_widget(SettingsScreen(name='settings'))

class MainApp(App):
    def build(self):
        return sm

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

Running this code just creates a blank page that produces no errors.

Is there a way to designate it to draw a certain screen to begin with that I am missing? I'm not really sure where my issue is.

Answer 1

What you did wrong:

• If you want to create Widget content from Python code you should place it inside Widget __init__ method, not build
• You're creating a layout and then discarding it. You need to use self.add_widget(f) to actually use it after its creation
• You're binding to your switchScreen method, so it needs to accept caller widget as an argument. Or you can simply use *args and not worry about it.
• You're not in kv anymore, so there's no root. Use self instead.

Putting this all together:

from kivy.app import App
from kivy.lang import Builder
from kivy.uix.screenmanager import ScreenManager, Screen
from kivy.uix.floatlayout import FloatLayout
from kivy.uix.label import Label
from kivy.uix.button import Button

class MenuScreen(Screen):
    def __init__(self, **kwargs):
        super(MenuScreen, self).__init__(**kwargs)

        def switchScreen(*args):
            self.manager.current = 'settings'

        f = FloatLayout()

        button1 = Button(text = "My settings button")
        button2 = Button(text = "Back to menu", on_press = switchScreen)

        f.add_widget(button1)
        f.add_widget(button2)

        self.add_widget(f)

class SettingsScreen(Screen):
    def __init__(self, **kwargs):
        super(SettingsScreen, self).__init__(**kwargs)

        def switchScreen(*args):
            self.manager.current = 'menu'

        f = FloatLayout()

        button1 = Button(text = "My settings button")
        button2 = Button(text = "Back to menu", on_press = switchScreen)

        f.add_widget(button1)
        f.add_widget(button2)
        self.add_widget(f)

sm = ScreenManager()
sm.add_widget(MenuScreen(name='menu'))
sm.add_widget(SettingsScreen(name='settings'))

class MainApp(App):
    def build(self):
        return sm

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