title: Kivy update label after type
body: <p>Hellow im newbie on this. Im trying to send value to my screen class. I think im sending correctly but its not updating itself.I tought if i write things in .kv file , it does update for me. But im not sure how can i update new value to my label.</p>
  <pre><code>from kivy.app import App
  from kivy.uix.screenmanager import Screen,ScreenManager
  from kivy.lang import Builder
  from kivy.properties import StringProperty
  from kivy.uix.popup import Popup
  Builder.load_string('''
  &lt;SM&gt;:
      Page1:
          name: 'page1name'
  &lt;TestPop&gt;:
      title: 'test pop title'
      BoxLayout:
          orientation: 'vertical'
          TextInput:
              id: test_textinput
          Button:
              text: 'Confirm'
              on_release: root.dismiss()
  &lt;Page1&gt;:
      BoxLayout:
          orientation: 'vertical'
          Button:
              text: 'Open Pop'
              on_release: root.func1()
          Label:
              text: root.my_label_string
  ''')
  class Page1(Screen):
      my_label_string = StringProperty('Default')
      def func1(self):
          a = TestPop()
          a.open()
  class TestPop(Popup):
      my_tinput_string = StringProperty('')
      def on_dismiss(self):
          new_text = self.ids.test_textinput.text
          Page1.my_label_string = new_text
  class SM(ScreenManager):
      pass
  class MainApp(App):
      def build(self):
          return SM()
  if __name__ in ('__main__'):
      MainApp().run()
  </code></pre>