title: kivy: changing label text of dynamically generated label
body: <p>In the end, the labels.text should display sensor data. The data is generated within a separate process and is transferred to kivy via a pipe. Currently I am trying to implement a very simple minimal example. However, I do not get my head around the update concept for dynamically generated labels:</p>
  
  <pre><code>from math import sin
  
  """
  Activate the touch keyboard. It is important that this part is on top
  because the global config should be initiated first.
  """
  from kivy.config import Config
  Config.set('kivy', 'keyboard_mode', 'multi')
  
  from kivy.app import App
  
  from kivy.clock import Clock
  # The Builder is used to define the main interface.
  from kivy.lang import Builder
  
  from kivy.properties import StringProperty, ObjectProperty, NumericProperty
  from kivy.uix.label import Label
  from kivy.uix.screenmanager import ScreenManager, Screen
  from kivy.uix.stacklayout import StackLayout
  
  
  import numpy as np
  
  class MainScreen(Screen):
      pass
  
  
  class DataScreen(Screen):
  
      def __init__(self, **kwargs):
          layout = StackLayout()
          super(DataScreen, self).__init__(**kwargs)
          self.n_probes = 8
          self.label_text = []
          for i in range(self.n_probes):
              self.label_text.append(StringProperty())
              self.label_text[i] = str(i)
  
          for i in range(self.n_probes):
              l = Label(id='l_{}'.format(i),
                        name='l_{}'.format(i),
                        text=self.label_text[i],
                        font_size='60sp',
                        height=20,
                        width=20,
                        size_hint=(0.5, 0.2))
              self.ids.stack.add_widget(l)
  
      def change_text(self):
          for i in range(self.n_probes):
              self.label_text[i] = str(2)
  
  
  
  Builder.load_file('phapp.kv')
  
  class MyApp(App):
      """
      The settings App is the main app of the pHBot application.
      It is initiated by kivy and contains the functions defining the main interface.
      """
  
  
      def build(self):
          """
          This function initializes the app interface and has to be called "build(self)".
          It returns the user interface defined by the Builder.
          """
  
          sm = ScreenManager()
          sm.add_widget(MainScreen())
          sm.add_widget(DataScreen())
          # returns the user interface defined by the Builder
          return sm
  
  
  if __name__ == '__main__':
      MyApp().run()
  </code></pre>
  
  <p>The .kv file:</p>
  
  <pre><code>&lt;MainScreen&gt;:
      name: 'main'
      BoxLayout:
          orientation: 'vertical'
          Button:
              text: 'Go to data'
              font_size: 40
              on_release: app.root.current = 'data'
          Button:
              text: 'Exit'
              font_size: 40
              on_release: app.stop()
  
  &lt;DataScreen&gt;:
      name: 'data'
      StackLayout:
          id: stack
          orientation: 'lr-tb'
      BoxLayout:
          Button:
              size_hint: (0.5, 0.1)
              text: 'Update'
              font_size: 30
              on_release: root.change_text()
          Button:
              size_hint: (0.5, 0.1)
              text: 'Back to main menu'
              font_size: 30
              on_release: app.root.current = 'main'
  </code></pre>
  
  <p>The labels are not updating with this version. Any ideas why ?</p>