title: Correct way to poll a webservice in an IPhone app
body: <p>I am trying to determine the best strategy to poll a webservice once a minute, parse the xml returned and then update an object stored in a shared instance. This process needs to run in a separate thread, and will continue as long as the app is running. </p>
  
  <p>It seems that I could put all the code to call the webservice and parse the xml into an NSOperation and add that NSOperation to an NSOperationQueue stored in the app delegate as soon as the app launches.</p>
  
  <p>Is it a correct approach to use an NSTimer inside the main method of the NSOperation so that the operation will loop once a minute, indefinitely? In that scenario the NSOperation would never actually return - this seems what I want but I am not sure if this is the right way to think about it. </p>
  
  <p>The problem I am trying to solve is of course extremely common, so I am trying to figure out the correct way to implement it. Any advice greatly appreciated.  </p>