為什么我的屏幕管理器不能單獨使用 kivy 語言並且需要 python？

Question

為什么這可以完美運行，但我必須明確地從 python 添加屏幕小部件

from kivy.uix.screenmanager import Screen, ScreenManager
from kivy.lang import Builder
from kivy.app import App
class start(Screen):
  pass
class filexp(Screen):
  pass
class shuff(ScreenManager):
  pass
  
sty='''
#:import FadeTransition kivy.uix.screenmanager.FadeTransition
<shuff>:
    transition:FadeTransition()
    start:
    filexp:
<start>:
    name:'main'
    Button:
        text:'what'
        on_release:app.root.current='exp'
<filexp>:
    name:'exp'
    GridLayout:
        cols:1
        Button:
            on_release:app.root.current='main'
          
'''
Builder.load_string(sty)
class a(App):
  def build(self):
    x=shuff()
    x.add_widget(start())
    x.add_widget(filexp())
    return(x)
a().run()

但是，盡管在 kivy 語言中向屏幕管理器添加了屏幕，但這只顯示黑屏

from kivy.uix.screenmanager import Screen, ScreenManager
from kivy.lang import Builder
from kivy.app import App
class start(Screen):
  pass
class filexp(Screen):
  pass
class shuff(ScreenManager):
  pass
  
sty='''
#:import FadeTransition kivy.uix.screenmanager.FadeTransition
<shuff>:
    transition:FadeTransition()
    start:
    filexp:
<start>:
    name:'main'
    Button:
        text:'what'
        on_release:app.root.current='exp'
<filexp>:
    name:'exp'
    GridLayout:
        cols:1
        Button:
            on_release:app.root.current='main'
          
'''
Builder.load_string(sty)
class a(App):
  def build(self):
    return(shuff())
a().run()

是我做錯了什么，還是我在使用 pydroid 應用程序在移動設備上運行解釋器時出現了一些小故障

Answer 1

使所有類名都以大寫字母開頭，效果很好

from kivy.uix.screenmanager import Screen, ScreenManager
from kivy.lang import Builder
from kivy.app import App
class Start(Screen):
  pass
class Filexp(Screen):
  pass
class Shuff(ScreenManager):
  pass
  
sty='''
#:import FadeTransition kivy.uix.screenmanager.FadeTransition
<Shuff>:
    transition:FadeTransition()
    Start:
    Filexp:
<Start>:
    name:'main'
    Button:
        text:'what'
        on_release:app.root.current='exp'
<Filexp>:
    name:'exp'
    GridLayout:
        cols:1
        Button:
            on_release:app.root.current='main'
          
'''
Builder.load_string(sty)
class a(App):
  def build(self):
    return(Shuff())
a().run()