UIViewPropertyAnimator.swift

class ViewController: UIViewController {
    
    let targetView = UIView(frame: .init(x: 50, y: 150, width: 50, height: 50))
    let slider = UISlider(frame: .init(x: 0, y: 100, width: 300, height: 30))
    
    var animator: UIViewPropertyAnimator?
    var displayLink: CADisplayLink?
    var token: NSKeyValueObservation?
    
    override func loadView() {
        super.loadView()
        self.targetView.backgroundColor = .orange
        self.view.addSubview(self.targetView)
        self.view.addSubview(self.slider)
    }
    
    override func viewDidLoad() {
        super.viewDidLoad()
        
        self.slider.addTarget(self, action: #selector(self.valueChanged(_:)), for: .valueChanged)
        self.targetView.addGestureRecognizer(UITapGestureRecognizer(target: self, action: #selector(self.tapped(_:))))
        self.setUpAnimator()
        self.setUpDisplayLink()
    }

    func setUpDisplayLink() {
        
        self.displayLink = CADisplayLink(target: self, selector: #selector(displaying(_:)))
        self.displayLink?.add(to: .main, forMode: .common)
        self.displayLink?.isPaused = true
    }
    
    func setUpAnimator() {
        
        let timing = UICubicTimingParameters(animationCurve: .easeOut)
        self.animator = UIViewPropertyAnimator(duration: 2.0, timingParameters: timing)
        
        self.animator?.addAnimations {
            
            self.targetView.center.x = 300
            self.targetView.transform = .init(rotationAngle: CGFloat(Double.pi / 2))
        }
        self.animator?.pausesOnCompletion = true
        
        // It will never be called.
        // because pausesOnCompletion is true
        self.animator?.addCompletion { _ in
            
            self.targetView.backgroundColor = .blue
        }
        self.token = self.animator?.observe(\.isRunning, changeHandler: { (animator, changed) in
            
            self.displayLink?.isPaused = !animator.isRunning
        })
    }
    
    @objc func displaying(_ sender: CADisplayLink) {
        
        self.slider.value = Float(self.animator?.fractionComplete ?? 0)
    }
    
    @objc func valueChanged(_ slider: UISlider) {
        
        guard let animator = self.animator else { return }
        animator.pauseAnimation()
        animator.fractionComplete = CGFloat(slider.value)
    }
    
    @objc func tapped(_ sender: UITapGestureRecognizer) {
        
        guard let animator = self.animator else { return }
        
        if animator.isRunning {
            animator.pauseAnimation()
        } else {
            animator.startAnimation()
        }
    }
}