python - Kivy kv 文件的行为与 Builder.load_string 不同

Question

我正在尝试在 kivy 上实现一个 2 屏幕应用程序。我遵循了他们的示例，如果在 main.py 中调用“Builder.load_string”，它就会起作用。

但是，如果我将其注释掉并创建一个 .kv 文件，我只会得到一个空白屏幕。我做错了什么？

from kivy.app import App
from kivy.lang import Builder
from kivy.uix.screenmanager import ScreenManager, Screen

#Builder.load_string("""
#<MainScreen>:
    #BoxLayout:
        #Button:
            #text: 'Goto settings'
            #on_press: root.manager.current = 'settings'
        #Button:
            #text: 'Quit'

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

# Declare both screens
class MainScreen(Screen):
    pass

class SettingsScreen(Screen):
    pass

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

class TestApp(App):

    def build(self):
        return sm

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

这是 testapp.kv

<MainScreen>:
    BoxLayout:
        Button:
            text: 'Goto settings'
            on_press: root.manager.current = 'settings'
        Button:
            text: 'Quit'

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

Answer 1

kv 文件由 App 类在调用 run 时加载，但在 build 方法之前。您在此之前实例化您的小部件(在 App 类声明之上)，因此 kv 规则尚未加载且不适用。要修复此问题，请将实例化移至构建方法。

编辑:更改:

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

class TestApp(App):

    def build(self):
        return sm

至

class TestApp(App):

    def build(self):
        # Create the screen manager
        sm = ScreenManager()
        sm.add_widget(MainScreen(name='main'))
        sm.add_widget(SettingsScreen(name='settings'))
        return sm