do something on exiting in kivy

i am making an app in kivy so i want to do something on exiting kivy app so, that run that code background not showing it

it have to work even when directly go home page and remove in background like this image and it have to work then converting this all python code to apk

# import kivy module     
import kivy   
       
# base Class of your App inherits from the App class.     
# app:always refers to the instance of your application    
from kivy.app import App  
     
# this restrict the kivy version i.e   
# below this kivy version you cannot   
# use the app or software   
kivy.require('1.9.0')  
     
# Importing Drop-down from the module to use in the program 
from kivy.uix.dropdown import DropDown 
  
# The Button is a Label with associated actions 
# that are triggered when the button is pressed 
# (or released after a click / touch) 
from kivy.uix.button import Button 
  
# another way used to run kivy app  
from kivy.base import runTouchApp 
  
# create a dropdown with 10 buttons 
dropdown = DropDown() 
for index in range(10): 
  
    # Adding button in drop down list 
    btn = Button(text ='Value % d' % index, size_hint_y = None, height = 40) 
  
    # binding the button to show the text when selected 
    btn.bind(on_release = lambda btn: dropdown.select(btn.text)) 
  
    # then add the button inside the dropdown 
    dropdown.add_widget(btn) 
  
# create a big main button 
mainbutton = Button(text ='Hello', size_hint =(None, None), pos =(350, 300)) 
  
# show the dropdown menu when the main button is released 
# note: all the bind() calls pass the instance of the caller  
# (here, the mainbutton instance) as the first argument of the callback 
# (here, dropdown.open.). 
mainbutton.bind(on_release = dropdown.open) 
  
# one last thing, listen for the selection in the  
# dropdown list and assign the data to the button text. 
dropdown.bind(on_select = lambda instance, x: setattr(mainbutton, 'text', x)) 
  
# runtouchApp: 
# If you pass only a widget in runtouchApp(), a Window will 
# be created and your widget will be added to the window 
# as the root widget. 
runTouchApp(mainbutton)