// Swift 3.0 throws an exception if you call DispatchQueue.main.sync when already on the same thread
func executeOnMain(closure: () -> Void) {
    if Thread.isMainThread {
      closure()
    } else {
      DispatchQueue.main.sync {
        closure()
      }
    }
  }



fileprivate func spawn() {

    let shape = CAShapeLayer()
    shape.opacity = 0.0

    // create an inital path at the starting position
    shape.path = UIBezierPath(arcCenter: CGPoint.zero, radius: 1, startAngle: 0, endAngle: 360 * (CGFloat.pi / 180), clockwise: true).cgPath
    shape.position = CGPoint.zero


    // create an inital path at the starting position
    shape.path = UIBezierPath(arcCenter: startingPosition, radius: startRadius, startAngle: BubbleConstants.StartingAngle, endAngle: BubbleConstants.EndAngle, clockwise: true).cgPath
    shape.position = startingPosition

    // set the fill color
    shape.fillColor = UIColor.white.cgColor

    layer.addSublayer(shape)

    shape.opacity = Float(opacity)

    executeOnMain {
      self.layer.addSublayer(shape)
      CATransaction.begin()
    }

    let radiusAnimation = CABasicAnimation(keyPath: "path")
    radiusAnimation.fromValue = shape.path
    radiusAnimation.toValue = UIBezierPath(arcCenter: center, radius: endRadius, startAngle: BubbleConstants.StartingAngle, endAngle: BubbleConstants.EndAngle, clockwise: true).cgPath


    executeOnMain { [unowned self] in
      CATransaction.setCompletionBlock { [unowned self] in

        // remove the shape
        self.executeOnMain {
          shape.removeFromSuperlayer()
          shape.removeAllAnimations()
        }

        let qos = DispatchQoS.QoSClass.background
        let backgroundQueue = DispatchQueue.global(qos: qos)

        backgroundQueue.async {
          // spawn a new shape
          self.spawn()
        }
      }
    }


    let movementAnimation = CABasicAnimation(keyPath: "position")
    movementAnimation.fromValue = NSValue(cgPoint: startingPosition)
    movementAnimation.toValue = NSValue(cgPoint: destination)


    let animationGroup = CustomAnimationGroup()
    animationGroup.animations = [radiusAnimation, movementAnimation]
    animationGroup.fillMode = kCAFillModeForwards
    animationGroup.isRemovedOnCompletion = false
    animationGroup.duration = duration

    shape.add(animationGroup, forKey: "bubble_spawn")

    executeOnMain {
      CATransaction.commit()
    }
  }