This Gist Class for Checking App Store version with local version.

AppStoreUpdate.swift

//
//  AppStoreUpdate.swift
//
//  Created by CodeChanger on 03/11/19.
//  Copyright © 2019 CodeChanger. All rights reserved.
//

import UIKit

enum CustomError: Error {
   case jsonReading
   case invalidIdentifires
   case invalidURL
   case invalidVersion
   case invalidAppName
}

class AppStoreUpdate: NSObject {
    
    static let shared = AppStoreUpdate()
    static let iTunesAppBaseUrl = "https://itunes.apple.com/lookup?bundleId="

    func showAppStoreVersionUpdateAlert(isForceUpdate: Bool) {
        
        do {
            //Get Bundle Identifire from Info.plist
            guard let bundleIdentifire = Bundle.main.infoDictionary?["CFBundleIdentifier"] as? String else {
                print("No Bundle Info found.")
                throw CustomError.invalidIdentifires
            }
            
            // Build App Store URL
            guard let url = URL(string:AppStoreUpdate.iTunesAppBaseUrl + bundleIdentifire) else {
                print("Issue with generating URL.")
                throw CustomError.invalidURL
            }
            let request = URLRequest(url: url, cachePolicy: URLRequest.CachePolicy.reloadIgnoringLocalAndRemoteCacheData, timeoutInterval: 60.0)
            
            let serviceTask = URLSession.shared.dataTask(with: request) { (responseData, response, error) in
                
                do {
                    // Check error
                    if let error = error { throw error }
                    //Parse response
                    guard let data = responseData else { throw CustomError.jsonReading }
                    let result = try? JSONSerialization.jsonObject(with: data, options: .allowFragments)
                    let itunes = ItunesAppInfoItunes.init(fromDictionary: result as! [String : Any])
                    if let itunesResult = itunes.results.first {
                        //Get Bundle Version from Info.plist
                        guard let appShortVersion = Bundle.main.infoDictionary?["CFBundleShortVersionString"] as? String else {
                            print("No Short Version Info found.")
                            throw CustomError.invalidVersion
                        }
                        
                        let compareResult = appShortVersion.versionCompare(itunesResult.version)
                        switch compareResult {
                        case .orderedAscending:
                            print("Itune version grater than local version")
                            //Show Update alert
                            var message = ""
                            //Get Bundle Version from Info.plist
                            if let appName = Bundle.main.infoDictionary?["CFBundleName"] as? String {
                                message = "\(appName) has a new version(\(itunesResult.version!)) available on the App Store."
                            } else {
                                message = "This app has a new version(\(itunesResult.version!)) available on the App Store."
                            }
                            
                            //Show Alert on main thread
                            DispatchQueue.main.async {
                                self.showUpdateAlert(message: message, appStoreURL: itunesResult.trackViewUrl, isForceUpdate: isForceUpdate)
                            }
                        case .orderedDescending:
                            print("Itune version less than local version")
                        case .orderedSame:
                            //App Store & Local App Have same Version.
                            print("Current Version is the latest version of the app.")
                        default:
                            print("No Result")
                        }
                    }
                } catch {
                    print(error)
                }
            }
            serviceTask.resume()
        } catch {
            print(error)
        }
    }
    
    func showUpdateAlert(message : String, appStoreURL: String, isForceUpdate: Bool) {
        
        let controller = UIAlertController(title: "New Version", message: message, preferredStyle: .alert)
        
        //Optional Button
        if !isForceUpdate {
            controller.addAction(UIAlertAction(title: "Later", style: .cancel, handler: { (_) in }))
        }
        
        controller.addAction(UIAlertAction(title: "Update", style: .default, handler: { (_) in
            guard let url = URL(string: appStoreURL) else {
                return
            }
            if #available(iOS 10.0, *) {
                UIApplication.shared.open(url, options: [:], completionHandler: nil)
            } else {
                UIApplication.shared.openURL(url)
            }
            
        }))
        
        let applicationDelegate = UIApplication.shared.delegate as? AppDelegate
        applicationDelegate?.window?.rootViewController?.present(controller, animated: true)
        
    }
}

extension String {
    func versionCompare(_ otherVersion: String) -> ComparisonResult {
        return self.compare(otherVersion, options: .numeric)
    }
}

Use of unresolved identifier 'ItunesAppInfoItunes'

Hey iShawnWang,

I used modal classes for reading iTunes App version API.

Please find below links for missing modal files.

1. ItunesAppInfoResult.swift
2. ItunesAppInfoItunes.swift

Add these files to your project and try again.

Thanks,
CodeChanger