title: Creating DropDown in kivy with only kv file
body: <p>I wanted to get a simple combo box like widget using the DropDown class. I can do it using python code, but is it possible using just kv language?</p>
  
  <p>I tried the following. Here's my python code:</p>
  
  <pre><code>class CustomDropDown(DropDown):
     pass
  
  class MainForm(BoxLayout):
      pass
  
  class MainApp(App):
      def build(self):
          self.dropdown = CustomDropDown()
          self.mainForm = MainForm()
          return self.mainForm
      def do_something(self):
          self.dropdown.open(self.mainForm)
  
  MainApp().run()
  </code></pre>
  
  <p>And here's the kv file :</p>
  
  <pre><code>&lt;MainForm&gt;:
      Button:
          text: 'Press'
          size_hint: [None,None]
          height: '40dp'
          on_release: app.do_something()
  &lt;CustomDropDown&gt;:
      Button:
          text: 'First Item'
      Label:
          text: 'Disabled item'
      Button:
          text: 'Second Item'
  </code></pre>
  
  <p>But this is not working. Can you please suggest something? Any help is appreciated.</p>