Kivy:如何使标签大小和画布大小相等？

from kivy.app import App
from kivy.uix.label import Label
from kivy.uix.widget import Widget
from kivy.uix.boxlayout import BoxLayout

class MyWidget(Widget):
    def __init__(self, **kwargs):
        super(MyWidget, self).__init__(**kwargs)
        
        layout = BoxLayout(orientation='vertical')
        
        label = Label(text='Hello, Kivy!')
        layout.add_widget(label)
        
        canvas = Widget()
        layout.add_widget(canvas)
        
        self.add_widget(layout)

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

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