如何使用kivy对齐矩形？

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

class AlignRectApp(App):
    def build(self):
        layout = BoxLayout(orientation='vertical')
        
        # 创建一个矩形
        rect = Widget()
        with rect.canvas:
            Rectangle(pos=(0, 0), size=(100, 100))
        
        # 将矩形添加到布局中
        layout.add_widget(rect)
        
        return layout

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