Kivy:如何在kivy中创建一个圆角的TextInput？

from kivy.app import App
from kivy.uix.textinput import TextInput
from kivy.lang import Builder
from kivy.properties import ObjectProperty

class RoundedTextInput(TextInput):
    pass

Builder.load_string('''
<RoundedTextInput>:
    background_color: 0, 0, 0, 0
    background_normal: ''
    background_active: ''
    canvas.before:
        Color:
            rgba: 0.5, 0.5, 0.5, 1
        RoundedRectangle:
            pos: self.pos
            size: self.size
            radius: [10,]
''')

class MyApp(App):
    text_input = ObjectProperty()

    def build(self):
        self.text_input = RoundedTextInput()
        return self.text_input

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