如何使用Kivy将矩形与父对象的中心对齐

from kivy.app import App
from kivy.uix.boxlayout import BoxLayout
from kivy.uix.widget import Widget
from kivy.graphics import Rectangle

class MyRectangle(Widget):
    def __init__(self, **kwargs):
        super(MyRectangle, self).__init__(**kwargs)
        self.size_hint = (None, None)  # 禁用自动调整大小
        self.size = (200, 200)  # 设置矩形的大小
        self.pos_hint = {'center_x': 0.5, 'center_y': 0.5}  # 设置矩形的位置为父对象的中心
        with self.canvas:
            Rectangle(pos=self.pos, size=self.size)  # 绘制矩形

class MyApp(App):
    def build(self):
        layout = BoxLayout()
        rectangle = MyRectangle()
        layout.add_widget(rectangle)
        return layout

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