CAAnimation delegate not working (Swift)

CAAnimation provides a way of receiving a callback upon start or completion of an animation.
If you have a ‘pure’ class, that is a class that does not inherit from anything, doing this will have no effect:

  class Petal {
      func startAnimation(duration: Double = 2.0) {
      let pathAnimation = CABasicAnimation(keyPath: "strokeEnd");
      pathAnimation!.delegate = self;
      pathAnimation!.duration = duration;
      pathAnimation!.fromValue = 0.0;
      pathAnimation!.toValue = 1.0;

      self.shapeLayer.addAnimation(pathAnimation!, forKey: "strokeEnd");
    }

    func animationDidStart(anim: CAAnimation) {
      Swift.print("ANIMATION DID START!");
    }

    func animationDidStop(anim: CAAnimation, finished flag: Bool) {
      Swift.print("ANIMATION DID END!");
    }
  }

The reason is because:
You must subclass NSObject, as the delegate methods exist as an informal protocol extension on NSObject

So simply changing it to this is enough:

  class Petal:NSObject { // Subclass NSObject
      func startAnimation(duration: Double = 2.0) {
      ....
    }

    // These are now overrides
    override func animationDidStart(anim: CAAnimation) {
      Swift.print("ANIMATION DID START!");
    }

    override func animationDidStop(anim: CAAnimation, finished flag: Bool) {
      Swift.print("ANIMATION DID END!");
    }
  }