WholeAdjustImageFit.swift

import UIKit

class ViewController: UIViewController, UIScrollViewDelegate {
    @IBOutlet var scrollView: UIScrollView!
    
    let imageView = UIImageView()
    
    override func viewDidLoad() {
        super.viewDidLoad()
        // Do any additional setup after loading the view.
        scrollView.delegate = self
        scrollView.addSubview(imageView)
        scrollView.alwaysBounceVertical = true
        scrollView.alwaysBounceHorizontal = true
        scrollView.maximumZoomScale = 2
        scrollView.minimumZoomScale = 0.001
    }
    
    @IBAction func showImg1(_ sender: Any) {
        let image0 = UIImage(named: "Image")
      
        // ここ大事
        scrollView.zoomScale = 1
        imageView.image = image0
        imageView.frame.size = image0!.size
        scrollView.contentSize = image0!.size
        
        adjustImageFitInScrollView()
    }
    
    @IBAction func showImg2(_ sender: Any) {
        let image1 = UIImage(named: "Image-1")
      
        // ここ大事
        scrollView.zoomScale = 1
        imageView.image = image1
        imageView.frame.size = image1!.size
        
        adjustImageFitInScrollView()
    }
    
    public 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
        print(scrollView.contentSize)
        scrollView.setZoomScale(scale, animated: true)
    }

    func viewForZooming(in scrollView: UIScrollView) -> UIView? {
        return imageView
    }
}