new_contents: '''
  App: base class for creating a Kivy application
  
  Example of very simple application with button ::
  
      from kivy.app import App
      from kivy.uix.button import Button
  
      class MyApp(App):
          def init(self):
              self.root = Button(label='hello world'))
  
      MyApp().run()
  '''
  
  from kivy.base import EventLoop, runTouchApp
  from kivy.event import EventDispatcher
  
  class App(EventDispatcher):
      def __init__(self, **kwargs):
          super(App, self).__init__()
          self.register_event_type('on_start')
          self.register_event_type('on_stop')
          self.use_default_uxl = kwargs.get('use_default_uxl', True)
          self.is_build = False
          self.root = None
  
      def build(self):
          '''Initialization of the application, will be called only once.
          If the build() return a widget, it will be used as the root widget.
          '''
          pass
  
      def run(self):
          '''Launch the app in standalone mode
          '''
          if not self.is_build:
              self.root = self.build()
          self.dispatch('on_start')
          if self.root:
              runTouchApp(self.root)
          else:
              runTouchApp()
          self.dispatch('on_stop')
  
      def on_start(self):
          '''Event called after initialization, and before running the
          application
          '''
          pass
  
      def on_stop(self):
          '''Event called after running, before closing the application
          '''
          pass
  
  
subject: add app classe for building application or test