没有 .kv 文件的 Kivy ScreenManager

Question

i'm trying to make a screen transition without a .kv file. Is it possible to do it like this without .kv? Here is the code

class ScreenManagement(ScreenManager):
    def __init__(self, **kwargs):
        super(ScreenManagement, self).__init__(**kwargs)


class testW(Screen):
    def __init__(self, **kwargs):
        super(testW, self).__init__()

class emotionRecog(Screen):
    def __init__(self, **kwargs):
        super(emotionRecog, self).__init__(**kwargs)
        self.button1 = Button(text="Next", size_hint=(1, .1))
        self.button1.bind(on_press=self.screenTransition())
        layout = BoxLayout(orientation="vertical")
        layout.add_widget(self.button1)
        return layout

    def screenTransition(self, *args):
        self.manager.current = 'test'


class CamApp(App):
    def build(self):
        sm = ScreenManagement(transition=FadeTransition())
        sm.add_widget(emotionRecog(name='emotion'))
        sm.add_widget(testW(name='test'))
        return sm


if __name__ == "__main__":
    CamApp().run()

After running it i get these error

     Traceback (most recent call last):
   File "C:\Study\KivyMultiScreen\main.py", line 53, in <module>
     CamApp().run()
   File "C:\Study\KivyMultiScreen\main.py", line 47, in build
     sm.add_widget(emotionRecog(name='emotion'))
   File "C:\Study\KivyMultiScreen\main.py", line 35, in __init__
     self.button1.bind(on_press=self.screenTransition())
   File "C:\Study\KivyMultiScreen\main.py", line 41, in screenTransition
     self.manager.current = 'test'
 AttributeError: 'NoneType' object has no attribute 'current'

Process finished with exit code 1

Can you please help me out?

Answer 1

You are supposed to bind the callback itself to the on_press action. This way, Kivy can call that function later.

Change

        self.button1.bind(on_press=self.screenTransition())

to

        self.button1.bind(on_press=self.screenTransition)

The reason your script raised that specific error is because Screen.manager is not yet set at the point you called self.screenTransition in that line.

You can also check out the first several examples in the Kivy documentation .