Kivy:如何通过拖动来切换按钮

from kivy.app import App
from kivy.uix.button import Button
from kivy.uix.boxlayout import BoxLayout

class MyBoxLayout(BoxLayout):
    def on_button_press(self, button):
        if button.text == '按钮1':
            button.text = '按钮2'
        else:
            button.text = '按钮1'

class MyApp(App):
    def build(self):
        layout = MyBoxLayout()
        button = Button(text='按钮1')
        button.bind(on_press=layout.on_button_press)
        layout.add_widget(button)
        return layout

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