AppDelegate.swift

import UIKit
import StoreKit

@UIApplicationMain
class AppDelegate: UIResponder, UIApplicationDelegate {

    var window: UIWindow?

    func application(_ application: UIApplication, didFinishLaunchingWithOptions launchOptions: [UIApplicationLaunchOptionsKey: Any]?) -> Bool {
        
        // if there is an object which is called appLaunchCount
        // means it is not the first launch
        if ((UserDefaults.standard.object(forKey: "appLaunchCount")) != nil) {
            
            var appLaunchCount = UserDefaults.standard.object(forKey: "appLaunchCount") as? Int
            appLaunchCount = appLaunchCount! + 1
            
            // saves appLaunchCount to local
            UserDefaults.standard.set(appLaunchCount!, forKey: "appLaunchCount")
            UserDefaults.standard.synchronize()
            
            if appLaunchCount == 2 {
                
                // works after 20 seconds of the app launch
                DispatchQueue.main.asyncAfter(deadline: .now() + 20, execute: {
                    
                    // shows review pop up
                    SKStoreReviewController.requestReview()
                })
            }
        }
        // if the app opens at the first time
        else {
            UserDefaults.standard.set(1, forKey: "appLaunchCount")
            UserDefaults.standard.synchronize()
        }
        
        return true
    }

    func applicationWillResignActive(_ application: UIApplication) {}

    func applicationDidEnterBackground(_ application: UIApplication) {}

    func applicationWillEnterForeground(_ application: UIApplication) {}

    func applicationDidBecomeActive(_ application: UIApplication) {}

    func applicationWillTerminate(_ application: UIApplication) {}
}