title: Action in another Screen with kivy
body: <p>I am trying to make a app that I can execute some function in a screen, but the function acts in other screen. So, I have an app with screenmanager, this app has two screen, and each screen has two buttons. A button do something in other screen, and the other button change screen. But I can't execute any action upon other screen. I want to create a Numeric value, and I want this value may be show and manipulate, by any screen. My actual's code are:
  .py</p>
  
  <pre><code>class PrimeiroScreen(Screen):
      def __init__(self, **kwargs):
          self.name = 'um'
          super(Screen,self).__init__(**kwargs)
  
      def fc1(self):
          value = self.manager.get_screen('dois')
          self.ids.lb5.value += 5
          #self.ids.lb6.value += 5
  </code></pre>
  
  <p>I don't know how I can change this value in others screens, and how to show the same value on this two labels</p>
  
  <pre><code>class SegundoScreen(Screen):
      def __init__(self, **kwargs):
          self.name = 'dois'
          super(Screen,self).__init__(**kwargs)
  
      def fc1(self):
          self.ids.lb6.value += 5
          #self.ids.lb5.value += 5
  </code></pre>
  
  <p>and .kv</p>
  
  <pre><code>&lt;PrimeiroScreen&gt;:
      GridLayout:
          cols: 1     
          size_hint: (.2, .4)
          pos_hint:{'x': .35, 'y': .7}
          #padding: 30, 0
          spacing: 0, 10
          MyLabel:
              text:"Valor {}".format(self.value)
              font_size: '20dp'
              text_size: self.width, self.height
              id: lb5
          Button:
              text: "Voltar!"
              font_size: '20dp'
              text_size: self.width - 20, None
              size_hint: (.1,.1)
              pos_hint:{'x':.45, 'y':.05}
              on_release: root.manager.current = 'dois'
          Button:
              text: "Somar!"
              font_size: '20dp'
              text_size: self.width - 20, None
              size_hint: (.1,.1)
              pos_hint:{'x':.45, 'y':.3}
              on_press: root.fc1()
  
  &lt;SegundoScreen&gt;:
      GridLayout:
          cols: 1
          size_hint: (.2, .4)
          pos_hint:{'x': .35, 'y': .7}
          #padding: 30, 0
          spacing: 0, 10
          MyLabel:
              text:"Valor{}".format(self.value)
              font_size: '20dp'
              text_size: self.width, self.height
              id: lb6
          Button:
              text: "Voltar!"
              font_size: '20dp'
              text_size: self.width - 20, None
              size_hint: (.1,.1)
              pos_hint:{'x':.45, 'y':.05}
              on_release: root.manager.current = 'um'
          Button:
              text: "Somar!"
              font_size: '20dp'
              text_size: self.width - 20, None
              size_hint: (.1,.1)
              pos_hint:{'x':.45, 'y':.3}
              on_press: root.fc1()
  </code></pre>