有没有办法在kivy BoxLayouts周围设置一个边界？

from kivy.app import App
from kivy.uix.boxlayout import BoxLayout
from kivy.lang import Builder

Builder.load_string('''
<MyBoxLayout>:
    canvas.before:
        Color:
            rgba: 1, 0, 0, 1  # 设置边界颜色为红色
        Line:
            rectangle: self.x, self.y, self.width, self.height
''')

class MyBoxLayout(BoxLayout):
    pass

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

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