title: Kivy: Self-updating label text
body: <p>Let's say I have 3 classes: a "woking class" where stuff takes place, a label class and a class to contain them. 
  For example the label class could be a status bar showing the status of something going on the working class. I wish I could find a way to make the label self-update the value to show, since this value is a value of the working class being changed inside the latter.</p>
  
  <p>Here I have an example code</p>
  
  <pre><code>Builder.load_string('''
  &lt;CustomLabel&gt;
      text: 'Value is {}'.format(root.value)
  
  &lt;WorkingClass&gt;:
      orientation: 'vertical'
  
      Button:
          text: 'Update'
          on_release: root.update()
  
  &lt;MainLayout&gt;
      orientation: 'vertical'
  
  ''')
  
  class CustomLabel(Label):
      value = NumericProperty()
  
  class WorkingClass(BoxLayout):
  
      def __init__(self, *args, **kwargs):
  
          super(WorkingClass, self).__init__(*args, **kwargs)
  
          self.a = 5
  
      def update(self):
          self.a += 1
          print(self.a)
  
  class MainLayout(BoxLayout):
  
      def __init__(self, *args, **kwargs):
  
          super(MainLayout, self).__init__(*args, **kwargs)
  
          self.workingClass = WorkingClass()
          self.customLabel = CustomLabel(value=self.workingClass.a)
  
          self.add_widget(self.customLabel)
          self.add_widget(self.workingClass)
  
  
  
  
  
  class MyApp(App):
      def build(self):
          return MainLayout()
  
  if __name__ == "__main__":
      MyApp().run()
  </code></pre>
  
  <p>Is there a way of doing it with properties or whatever? Becouse I don't want to need to manually update (sommehow) the label each time I change the value. Anyway to achieve this?</p>