title: How to set contactDelegate for physicsWorld in SpriteKit?
body: <p>I have just encountered a problem when assigning contactDelegate to self (GameScene).</p>
  
  <pre><code>override func didMoveToView(view: SKView)
  {
      self.physicsWorld.contactDelegate = self
      /* ... */
  }
  </code></pre>
  
  <p>Unfortunately, I get an error:</p>
  
  <blockquote>
    <p>Cannot assign a value of type 'GameScene' to value of type 'SKPhysicsContactDelegate?'</p>
  </blockquote>
  
  <p>This wouldn't happen in previous versions of Xcode (probably something new in Swift 2.0), so even in SpriteKit documentation on Apple Developer they have the same code I do.</p>
  
  <p>How do I get around this? I have already tried to force casting</p>
  
  <pre><code>self.physicsWorld.contactDelegate = (self as! SKPhysicsContactDelegate)
  </code></pre>
  
  <p>but I got a runtime error (casting couldn't be performed).</p>
  
  <p>Anyone knows how to delegate contacts to my GameScene?</p>