title: NSNotificationCenter addObserver in Swift
body: <p>How do you add an observer in Swift to the default notification center?  I'm trying to port this line of code that sends a notification when the battery level changes.</p>
  
  <pre><code>[[NSNotificationCenter defaultCenter] addObserver:self selector:@selector(batteryLevelChanged:) name:UIDeviceBatteryLevelDidChangeNotification object:nil];
  </code></pre>