title: Guidelines for viewWillAppear, viewDidAppear, viewWillDisappear, viewDidDisappear
body: <p>Are there any guidelines for using these methods in the right manner? In particular, I would like to know what type of code I could use inside them.</p>
  
  <p>For example, if I have to call a method that retrieves data from a WS, where do I have to call it? Where can I register/unregister a NSNotification? etc.</p>