title: How does WhatsApp know if the user gets the push notification even if the app is runnng in the background and is not open?
body: <p>I have already asked related question: 
  <a href="https://stackoverflow.com/questions/9731826/any-option-to-know-if-apple-app-get-the-push-notification">any option to know if apple app get the push notification?</a></p>
  
  <p>And I got an answer that it is not possible to know if push notification comes to the iPhone when application is in the background and the user does not have it opened...</p>
  
  <p>But I found something interesting:
  I take 2 iPhones with WhatsApp. I open, in the first iPhone, WhatsApp and exit it (the WhatsApp is in the background now), then with the second iPhone, I send WhatsApp massage to the first iPhone.</p>
  
  <p>When the push notification of WhatsApp reaches first iPhone I can see two "V" (symbols) near to the message (and I don't open the Push notification). After this I close the Internet &amp; 3G in the first iPhone and again send WhatsApp message with the second iPhone to the first one. - and I can see only one "V" near to the massage (not read - not get push notification).</p>
  
  <p>Now after re-enabling the Internet on the first iPhone and after I get the push notification I can see two "V" (again - I don't open the push notification).</p>
  
  <p>How does it work? How does WhatsApp know whether the iPhone gets the push notification or not while the WhatsApp works in the background? </p>