title: Android Contacts List View same like Whatsapp using Firebase database
body: <p>What approach is Whatsapp using to show an "Invite" button for contacts which are not on Whatsapp?</p>
  
  <p>I also want to show a button in Contact list view for only those which don't exist in my app in Firebase database. </p>
  
  <p>I am using Custom BaseAdapter to show contacts in List View. 
  Can you please help me in understanding how Whatsapp Contact is working? </p>
  
  <p>My Question is not duplicate as that is to read contact list only. I want to show 'Invite' button also as per firebase data. </p>
  
  <p>Thanks!</p>