displayLink-1.swift

var displayLink:CADisplayLink!

func setUpDisplayLink() {
    displayLink = CADisplayLink(target: self, selector: #selector(update))
    displayLink.preferredFramesPerSecond = 60 // (optional) if you do not define , then device uses maxFramesPS
    displayLink.add(to: .main, forMode: RunLoop.Mode.common)
 }

@objc private func update() { 
  print("refresh : \(CACurrentMediaTime())")
}