title: How does user's whatsapp application detects change in profile picture of its contact?
body: <p>I was going through <strong>whatsapp</strong> application and I noticed that, for the first time it fetches thumbnails from the server, keeps them in cache memory, and thereafter it fetches these from the cache memory instead of server. (Like bitmap <strong>lazyloading</strong>). </p>
  
  <p>Till now I have completed  implementing the bitmap lazyloading part. </p>
  
  <p>However I have a question as follows:</p>
  
  <p>When any contact changes its profile picture how does the user's whatsapp application detects that particular change and how does it update that particular thumbnail only, cause the application is supposed to fetch the thumbnails from cache memory.</p>
  
  <p>I am clueless about this. Any help is appreciated.</p>
  
  <p><strong>EDIT:</strong></p>
  
  <p>Does it use push notification to notify the client applications to update the corresponding profile thumbnail? </p>