Kivy canvas.clear() not clearing the canvas

问题

I have followed the widget tutorial for Kivy, but I cannot get the canvas to clear after painting (from their 'a simple paint app' tutorial), unless I resize the window after pressing the clear button. Please help. I am using kivy v1.10.1 and python v3.7.0. on windows 10 64-bit OS. Further, a widget in scatter mode leaves a trace when you drag to a new position, this also clears when resizing the window. See code below:

from random import random
from kivy.app import App
from kivy.uix.widget import Widget
from kivy.uix.button import Button
from kivy.graphics import Color, Ellipse, Line


class MyPaintWidget(Widget):
    def on_touch_down(self, touch):
        color = (random(), 1, 1)
        with self.canvas:
            Color(*color, mode='hsv')
            d = 30.
            Ellipse(pos=(touch.x - d / 2, touch.y - d / 2), size=(d, d))
            touch.ud['line'] = Line(points=(touch.x, touch.y))

    def on_touch_move(self, touch):
        touch.ud['line'].points += [touch.x, touch.y]


class MyPaintApp(App):
    def build(self):
        parent = Widget()
        self.painter = MyPaintWidget()
        clearbtn = Button(text='Clear')
        clearbtn.bind(on_release=self.clear_canvas)
        parent.add_widget(self.painter)
        parent.add_widget(clearbtn)
        return parent

    def clear_canvas(self, obj):
        self.painter.canvas.clear()


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

来源：https://stackoverflow.com/questions/52561692/kivy-canvas-clear-not-clearing-the-canvas