title: Call function when if value in NSUserDefaults.standardUserDefaults() changes
body: <p>I have a variable that become value from NSUserDefaults.standardUserDefaults()</p>
  
  <pre><code>var GiftCount = NSUserDefaults.standardUserDefaults().valueForKey("Gift") as! Int
  </code></pre>
  
  <p>And i have a function named setGiftCount()...</p>
  
  <p>I need call this function when variable GiftCount has changed... How to do it?</p>