title: RxSwift: How to add simple observer to int value
body: <p>I want to use rxswift to observe hash value of my model.</p>
  
  <p>I have something like registration form and I need to show "save change" only if some data was changed.</p>
  
  <p>I'm not using any binding. </p>
  
  <p>I'll just want to observe hash of data.</p>