Kivy异步问题-画布在滚动视图后未更新

from kivy.app import App
from kivy.uix.boxlayout import BoxLayout
from kivy.uix.scrollview import ScrollView
from kivy.uix.label import Label
from kivy.graphics import Color, Rectangle
from kivy.clock import Clock

class MyScrollView(ScrollView):
    def __init__(self, **kwargs):
        super(MyScrollView, self).__init__(**kwargs)
        self.layout = BoxLayout(orientation='vertical', size_hint=(1, None))
        self.layout.bind(minimum_height=self.layout.setter('height'))
        self.add_widget(self.layout)

        # 在滚动视图中添加一个标签
        self.label = Label(text='Hello World!', size_hint=(1, None), height=100)
        self.layout.add_widget(self.label)

        # 使用Clock模块安排在下一个事件循环迭代中更新画布的函数
        Clock.schedule_once(self.update_canvas)

    def update_canvas(self, dt):
        # 更新画布的操作
        with self.label.canvas:
            self.label.canvas.clear()
            Color(1, 0, 0, 1)  # 设置画布颜色为红色
            Rectangle(pos=self.label.pos, size=self.label.size)  # 绘制一个矩形

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

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