Observation5.swift

override func viewDidLoad() {

    // ...

    for (index, face) in faces.enumerated() {
            // ... 

           let panGesture = UIPanGestureRecognizer(target: self, action: #selector(self.handlePan(gesture:)))
           bubbleView.addGestureRecognizer(panGesture)
    }
}

@objc func handlePan(gesture: UIPanGestureRecognizer) {
    switch gesture.state {
    case .ended:
        guard let bubbleView = gesture.view as? FaceView else {
            return
        }

        let velocity = gesture.velocity(in: self.view)
        let itemBehavior = UIDynamicItemBehavior(items: [bubbleView])
        itemBehavior.addLinearVelocity(velocity, for: bubbleView)

        animator.addBehavior(itemBehavior)
    default:
        break
    }
}