如何在kivy代码中使用keyboard_on_key_down函数？

from kivy.app import App
from kivy.uix.boxlayout import BoxLayout
from kivy.uix.label import Label
from kivy.core.window import Window

class MyBoxLayout(BoxLayout):
    def __init__(self, **kwargs):
        super(MyBoxLayout, self).__init__(**kwargs)
        Window.bind(on_key_down=self.keyboard_on_key_down)

    def keyboard_on_key_down(self, instance, keyboard, keycode, text, modifiers):
        # 在这里处理键盘按键按下事件的逻辑
        if keycode[1] == 'a':
            self.add_widget(Label(text='You pressed the "a" key!'))

class MyApp(App):
    def build(self):
        return MyBoxLayout()

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