Kivy 透明画布/背景

Question

我想让我的画布/背景透明。这意味着如果我将任何按钮/标签或任何其他对象放在画布上，只有它们（标签/按钮...）将可见，而背景不可见。最好的例子是使背景不可见，上面有标签和按钮，而背景是不可见/透明的...

Window.clearcolor 不起作用的示例：

from kivy.app import App
from kivy.lang import Builder
from kivy.uix.boxlayout import BoxLayout
from kivy.uix.screenmanager import ScreenManager, Screen
from kivy.uix.dropdown import DropDown
from kivy.uix.button import Button
from kivy.core.window import Window

Builder.load_string('''

<ToggleButton>:
    on_state: app.root.on_state(self)

<MainScreen>:

    BoxLayout:

        ToggleButtons:


''')

class MainScreen(BoxLayout):
    Window.clearcolor = (0, 0, 0, 0.5)

    def on_state(self, togglebutton):
        tb = togglebutton
        print(tb,tb.state,tb.text)


class ToggleButtons(Screen):

    def __init__(self,**kwargs):
        super(ToggleButtons,self).__init__(**kwargs)
        ResolutionsArray = ["1024x768", "1280x720", "1360x768", "1440x900", "1600x900", "1920x1080"]

        self.dropdown = DropDown()
        for index in ResolutionsArray:

            self.btn = Button(text=index, size_hint_y=None, height=44)

            self.btn.bind(on_release=lambda btn: self.dropdown.select(btn.text))

            self.dropdown.add_widget(self.btn)

        mainbutton = Button(text='1024x768',
                            border=(0, 0, 0, 0),
                            size_hint=(0.25, 0.15),
                            pos_hint={'x': .01, 'y': 0.1})

        mainbutton.bind(on_release=self.dropdown.open)
        
        self.dropdown.bind(on_select=lambda instance, x: setattr(mainbutton, 'text', x))

        self.add_widget(mainbutton)

class MyApp(App):

    def build(self):
        return MainScreen()


MyApp().run()