Kivy Label.text属性在UI上不更新

Question

我遇到了一个问题，就是我更新的标签文本在界面上显示不正确。我检查了文本属性，确认它已经被设置为新的值，但在标签渲染时，这个值并没有显示出来。

这个标签在一个不同的屏幕上，我是通过一个屏幕管理器来切换到那个屏幕的。这个屏幕的子组件和结构是在不同的.kv文件中构建的（在这个例子中，我使用了load_string()构建方法，但结果是一样的）。

下面是一个简化的例子，说明发生了什么：

from kivy.config import Config
Config.set('graphics', 'width', '300')
Config.set('graphics', 'height', '200')
Config.set('graphics', 'resizable', '0')

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

Builder.load_string("""
<screen1>:
    Button:
        id: button
        text: 'Press me to Update Screen2'
        on_press:
            root.update_screen2()

<screen2>:
    Label:
        id: mainLabel
        text: 'I have not been updated'
""")

class screen1(Screen):

    def __init__(self, **kwargs):
        """
            Set super and store the id of the button
        """
        super(screen1, self).__init__(**kwargs)
        self.button = self.ids.button.__self__
        return

    def update_screen2(self):
        """
            send text 1 and text 2 to the screen2 object 
            for it to set on it's label
        """
        text1 = 'Hi There'
        text2 = 'Does this work'
        screen2.update_label(screen2(),text1, text2)
        return


class screen2(Screen):

    def __init__(self, **kwargs):
        """
            set super ond store the id of the label
        """
        super(screen2, self).__init__(**kwargs)
        self.mainLabel = self.ids.mainLabel.__self__
        return

    def update_label(self, text1, text2):
        """
            using text1 and text2, update the text property of the label
            the output of each print() shows the text property is being updated
            it just doesn't display on the UI
        """
        print(self.mainLabel.text)
        self.mainLabel.text = text1 + '\n\n' + text2
        print(self.mainLabel.text)
        screenManager.current = 'screen2'
        return

screenManager = ScreenManager()


class MainApp(App):
    def build(self):
        #root widget is abox layout
        self.root = BoxLayout()

        #instantiate the screen objects
        self.screen1 = screen1(name='screen1')
        self.screen2 = screen2(name='screen2')

        #add the screens to the screenManager
        screenManager.add_widget(self.screen1)
        screenManager.add_widget(self.screen2)

        #set the current screen
        screenManager.current = 'screen1'

        #add the screenManager to the root widget and return it
        self.root.add_widget(screenManager)
        return self.root


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

我尝试了各种方法，比如在Python中更改我引用标签组件的方式，要求画布更新，或者创建一个'screen2()'对象来从screen1中调用方法，但都没有成功。我到底遗漏了什么呢？

方法调用 kivy label UI更新 kv文件屏幕管理器组件渲染画布更新

Kivy Label.text属性在UI上不更新

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