title: What is this `[unowned self] in` Swift code doing?
body: <p>I came across this Swift code that I don't understand. What is the navigationCallback being assigned to? What is <code>[unowned self]</code>? I think I would understand this better if I could see the equivalent example in Objective-C.</p>
  
  <pre><code>navigationCallback = { [unowned self] in
      previousNavigationCallback?()
      if self.state != .Canceled {
          callback()
      }
  }
  </code></pre>