title: How to use `SELF` in swift playground?
body: <p>How to reference Swift Playground itself?</p>
  
  <p>I am trying to use <code>NSNotificationCenter</code> to observe some variable, but i do not know how to implement it on <code>Playground</code>.</p>
  
  <p>Here is my code, actually i just want to observe the <code>timeout</code> variable, but without the <code>willSet</code> method.</p>
  
  <pre><code>var timeout = false
  var cont: Int8 = 1
  
  NSNotificationCenter.defaultCenter().addObserver(self, forKeyPath: timeout, options: NSKeyValueObservingOptions.New, context: &amp;cont)
  </code></pre>
  
  <p>Is there any way to do it on playground?</p>