AWS Mobile SDK AuthUI code snippet for main ViewController. This is the same for User Pools, Facebook, and Google sign-in.

AuthUIViewController.swift

import UIKit
import AWSAuthCore
import AWSAuthUI

class ViewController: UIViewController {

    override func viewDidLoad() {
        super.viewDidLoad()
        
        showSignIn()
    }

    func showSignIn() {
        if !AWSSignInManager.sharedInstance().isLoggedIn {
            AWSAuthUIViewController
            .presentViewController(with: self.navigationController!,
               configuration: nil,
               completionHandler: { (provider: AWSSignInProvider, error: Error?) in
                if error != nil {
                    print("Error occurred: \(String(describing: error))")
                } else {
                    // Sign in successful.
                    print("Identity provider: \(provider.identityProviderName)")
                }
            })
        }
    }
}