AdjustImageFitInScrollView.swift

func adjustImageFitInScrollView(){
  guard let img = self.imageView.image else {return}
  let w_scale = scrollView.frame.width / img.size.width
  let h_scale = scrollView.frame.height / img.size.height
  let scale = min(w_scale, h_scale)
  scrollView.minimumZoomScale = scale
  scrollView.setZoomScale(scale, animated: true)
}

@IBAction func showImg1(_ sender: Any) {
  let image0 = UIImage(named: "Image")
  imageView.image = image0
  imageView.frame.size = image0!.size
  scrollView.contentSize = image0!.size
        
  adjustImageFitInScrollView()
}