WebViewで画像のダウンロードボタンを押された時にアルバムに保存する

import UIKit
import WebKit

class ImageDownloadWebViewController: UIViewController {

    var request: URLRequest

    // MARK: - Initializer

    init(url: URL) {
        request = URLRequest(url: url)
        request.httpMethod = "GET"

        super.init(nibName: nil, bundle: nil)
    }

    required init?(coder aDecoder: NSCoder) {
        fatalError("init(coder:) has not been implemented")
    }

    // MARK: - LifeCycle

    override func viewDidLoad() {
        super.viewDidLoad()
        
        //WebViewの中で長押しメニューを出さないようにする
        let disableCalloutScriptString = "document.documentElement.style.webkitTouchCallout='none';"
        let disableCalloutScript = WKUserScript(source: disableCalloutScriptString, injectionTime: .atDocumentEnd, forMainFrameOnly: true)

        //ユーザーコンテントのインスタンスを作成し入れる
        let userContentController = WKUserContentController()
        userContentController.addUserScript(disableCalloutScript)

        let webConfig = WKWebViewConfiguration()
        webConfig.userContentController = userContentController

        let webView = WKWebView(frame: .zero, configuration: webConfig)
        webView.uiDelegate = self
        webView.navigationDelegate = self
        view.addFull(view: webView)

        webView.load(request)
    }
}

// MARK: - WKUIDelegate, WKNavigationDelegate

extension ImageDownloadWebViewController: WKUIDelegate, WKNavigationDelegate {

    func webView(_ webView: WKWebView, decidePolicyFor navigationAction: WKNavigationAction, decisionHandler: @escaping (WKNavigationActionPolicy) -> Void) {

        let urlString: String = String(describing: navigationAction.request.url)
        // 対象の画像かを判定
        if urlString.contains(".jpg") {
            do {
                let data = try Data(contentsOf: navigationAction.request.url!)
                guard let image = UIImage(data: data) else {
                    return
                }
                
                showDialog(image: image)
                
            } catch let err {
                print("Error : \(err.localizedDescription)")
            }
            decisionHandler(.cancel)
            return
        }

        decisionHandler(.allow)
    }

    private func showDialog(image: UIImage) {
        let alert: UIAlertController = UIAlertController(title: "画像を保存しますか？", message: "", preferredStyle: UIAlertController.Style.alert)
        let confirmAction: UIAlertAction = UIAlertAction(title: "確定", style: UIAlertAction.Style.default, handler: { _ in
            UIImageWriteToSavedPhotosAlbum(image, self, #selector(self.showResultOfSaveImage(_:didFinishSavingWithError:contextInfo:)), nil)
        })
        let cancelAction: UIAlertAction = UIAlertAction(title: "キャンセル", style: UIAlertAction.Style.cancel, handler: { _ in
            print("キャンセル")
        })
        alert.addAction(confirmAction)
        alert.addAction(cancelAction)
        
        present(alert, animated: true, completion: nil)
    }

    @objc func showResultOfSaveImage(_ image: UIImage, didFinishSavingWithError error: NSError!, contextInfo: UnsafeMutableRawPointer) {

        if error != nil {
            return
        }
        
        let alert = UIAlertController(title: "アルバムに保存しました", message: "", preferredStyle: .alert)
        alert.addAction(UIAlertAction(title: "OK", style: .default, handler: nil))
        
        present(alert, animated: true, completion: nil)

    }
    
}

extension UIView {
    func addFull(view: WKWebView) {
        view.translatesAutoresizingMaskIntoConstraints = false
        addSubview(view)
        view.topAnchor.constraint(equalTo: topAnchor, constant: 0.0).isActive = true
        view.bottomAnchor.constraint(equalTo: bottomAnchor, constant: 0.0).isActive = true
        view.leadingAnchor.constraint(equalTo: leadingAnchor, constant: 0.0).isActive = true
        view.trailingAnchor.constraint(equalTo: trailingAnchor, constant: 0.0).isActive = true
    }
}

Privacy - Photo Library Additions Usage Description

func webView(_ webView: WKWebView, decidePolicyFor navigationAction: WKNavigationAction, decisionHandler: @escaping (WKNavigationActionPolicy) -> Void)