title: NSTimer and server response
body: <p>I'm developing a messaging app and I have to make a call to a server every x seconds to load from server the new messages. 
  I use a NSTimer that calls the server every x sec.
  But for instance x = 1 sec and the response from server arrives after 1,5 sec. In this case I have one new request to the server that comes before that the previous request is satisfied, and this is a big problem for my app's logic. Can you help me to understand how can I check cyclically the messages on the server without have this problem?
  Thank you.</p>