Kivy 屏幕管理器问题

Question

I'm trying to create a mobile app with multiple screen (one of these with camara object embedded), using kivy, but the code below, produces only a black window, without showing nothing.Can you please help me figuring out why this isn't working?

    import kivy, ctypes
    from kivy.app import App
    from kivy.uix.label import Label
    from kivy.uix.screenmanager import ScreenManager, Screen
    from kivy.uix.camera import Camera
    from kivy.uix.button import Button
    from kivy.uix.boxlayout import BoxLayout

    class camera(Screen):
        def build(self):
            #camera obj
            self.camera_obj = Camera()
            self.camera_obj.resolution = (800,800)

            #button obj
            button_go_back = Button(text = "go back")
            button_go_back.size_hint = (.5,.2)
            button_go_back.pos_hint = {"x": .25, "y": .25}
    
            #layout
            layout = BoxLayout()
            layout.add_widget(self.camera_obj)
            layout.add_widget(button_go_back)

            return layout

    class MainWindow(Screen):
        def build(self):
            button_take_picture = Button(text = "take a picture")
            button_take_picture.size_hint = (.5,.2)
            button_take_picture.pos_hint = {"x": .25, "y": .25}

            layout = BoxLayout()
            lab = Label(text="some text")
            layout.add_widget(button_take_picture)
            layout.add_widget(lab)

            return layout


    screen_manager=ScreenManager()
    screen_manager.add_widget(MainWindow(name='main'))
    screen_manager.add_widget(camera(name='camera'))

    class NMS(App):
        def build(self):
            return screen_manager
    

    if __name__ == "__main__":
        ctypes.windll.shcore.SetProcessDpiAwareness(1)
        NMS().run()

Answer 1

Since you are initializing the Screen s in python you might need the __init__ constructor not build (as far as I'm aware there is no default method named build for Screen widget).

Thus you may change MainWindow to something like this,

class MainWindow(Screen):

    def __init__(self,**kwargs):
        super().__init__(**kwargs)
        button_take_picture = Button(text = "take a picture")
        button_take_picture.size_hint = (.5,.2)
        button_take_picture.pos_hint = {"x": .25, "y": .25}

        layout = BoxLayout()
        lab = Label(text="some text")
        layout.add_widget(button_take_picture)
        layout.add_widget(lab)
        self.add_widget(layout) # Add all those in this screen.

Similar rule applies for other screen(s).