title: Data Persistence
body: <p>When the app is loaded, I want to know whether I have already saved the data to disk. If it is saved, I just load from disk, if not, I save. 
  for example, I want to save an array(person) using <code>NSUserDefaults.standardUserDefaults()</code></p>
  
  <p>Save Method:</p>
  
  <pre><code>let defaults = NSUserDefaults.standardUserDefaults()
  
   if let savedPeople = defaults.objectForKey("people") as? NSData 
  
       people = NSKeyedUnarchiver.unarchiveObjectWithData(savedPeople) as! [Person] 
  </code></pre>
  
  <p>But how can I know if I have saved the data or not in <code>viewDidLoad()</code>? I don't want to save every time I open the app, and it will also overwrite the data I modified before. </p>