Full Implementation of AppDelegate.swift for Google sign-in integration

Google-Sign-In-Integration-04.swift

import UIKit
import GoogleSignIn

@UIApplicationMain
class AppDelegate: UIResponder, UIApplicationDelegate {
    
    func application(_ application: UIApplication,
                     didFinishLaunchingWithOptions launchOptions: [UIApplication.LaunchOptionsKey: Any]?) -> Bool {
        
        // Initialize Google sign-in
        GIDSignIn.sharedInstance().clientID = "[OAuth_Client_ID]"
        GIDSignIn.sharedInstance().delegate = self

        // If user already sign in, restore sign-in state.
        GIDSignIn.sharedInstance()?.restorePreviousSignIn()
        
        return true
    }
    
    func application(_ app: UIApplication,
                     open url: URL,
                     options: [UIApplication.OpenURLOptionsKey : Any]) -> Bool {
        
        return GIDSignIn.sharedInstance().handle(url)
    }
}

extension AppDelegate: GIDSignInDelegate {
    
    func sign(_ signIn: GIDSignIn!,
              didSignInFor user: GIDGoogleUser!,
              withError error: Error!) {
        
        // Check for sign in error
        if let error = error {
            if (error as NSError).code == GIDSignInErrorCode.hasNoAuthInKeychain.rawValue {
                print("The user has not signed in before or they have since signed out.")
            } else {
                print("\(error.localizedDescription)")
            }
            return
        }

        // Post notification after user successfully sign in
        NotificationCenter.default.post(name: .signInGoogleCompleted, object: nil)
    }
}

// MARK:- Notification names
extension Notification.Name {
    
    /// Notification when user successfully sign in using Google
    static var signInGoogleCompleted: Notification.Name {
        return .init(rawValue: #function)
    }
}