Detecting collisions between views.

AnimationViewController.swift

// 1. Conform to UICillisionBehaviorDelegate
AnimationViewController: UIViewController, UICollisionBehaviorDelegate 

// 2. Handling collision
// This function will be called with every and each collision starts between all the views added to our collision behavior.
func collisionBehavior(behavior: UICollisionBehavior, beganContactForItem item1: UIDynamicItem, withItem item2: UIDynamicItem, atPoint p: CGPoint){

  // Wer're only interested in collisions with the black circle (snappingCircle).
  if (item1 as? UIView)?.tag == snappingCirlceTag || (item2 as? UIView)?.tag == snappingCirlceTag {
      displayCollisionMessage(message)
  }
}

// 3. Setting self as the collision bahavior delegate
lazy var squareBehavior: SquareBehavior = {
    let lazySquareBehavior = SquareBehavior(settings: self.animationSettings)
    self.animator.addBehavior(lazySquareBehavior)
    // Here..
    lazySquareBehavior.collider.collisionDelegate = self
    return lazySquareBehavior
}()