I was integrating AWS Cognito user pool to an iOS application. The sign in feature is using custom challenge for authentication, but there is a lack of documentation about how to use the iOS sdk. After many trial and errors, I have finally able to sign in success, so I’m going to document the steps as shown below:

Step 1: Create a CognitoUserPool

In AppDelegate, after didFinishLaunchingWithOptions, the user pool is initialised.

func application(_ application: UIApplication, didFinishLaunchingWithOptions launchOptions: [UIApplicationLaunchOptionsKey: Any]?) -> Bool {
  // setup service configuration
  let serviceConfiguration = AWSServiceConfiguration(region: CognitoIdentityUserPoolRegion, credentialsProvider: nil
  // create pool configuration
  let poolConfiguration = AWSCognitoIdentityUserPoolConfiguration(clientId: CognitoIdentityUserPoolAppClientId, clientSecret: nil, poolId: CognitoIdentityUserPoolId)
// initialize user pool client
  AWSCognitoIdentityUserPool.register(with: serviceConfiguration, userPoolConfiguration: poolConfiguration, forKey: AWSCognitoUserPoolsSignInProviderKey)
  // fetch the user pool client we initialized in above step 
  let pool = AWSCognitoIdentityUserPool(forKey: AWSCognitoUserPoolsSignInProviderKey)
  self.storyboard = UIStoryboard(name: “Main”, bundle: nil)
  pool.delegate = self
  return true 
}

Step 2: Implement the protocol delegate

extension AppDelegate: AWSCognitoIdentityCustomAuthentication{
  func didCompleteStepWithError(_ error: Error?) {
  }
  func getCustomChallengeDetails(_ authenticationInput: AWSCognitoIdentityCustomAuthenticationInput, customAuthCompletionSource: AWSTaskCompletionSource<AWSCognitoIdentityCustomChallengeDetails>) {
  }
  func startCustomAuthentication() -> AWSCognitoIdentityCustomAuthentication {
  if (self.navigationController == nil) {
  self.navigationController = self.storyboard?.instantiateViewController(withIdentifier: “signinController”) as? UINavigationController
  }
  if (self.signInViewController == nil) {
  self.signInViewController =   self.navigationController?.viewControllers[0] as? SignInViewController
  }
  DispatchQueue.main.async {
    self.navigationController!.popToRootViewController(animated: true)
    if (!self.navigationController!.isViewLoaded
    || self.navigationController!.view.window == nil) {
self.window?.rootViewController?.present(self.navigationController!,
animated: true,
completion: nil)
     }
    }
  return self.signInViewController! 
  }
}

Step 3: Handle the custom challenge inside the sign in view controller

extension SignInViewController: AWSCognitoIdentityCustomAuthentication {
func getCustomChallengeDetails(_ authenticationInput: AWSCognitoIdentityCustomAuthenticationInput, customAuthCompletionSource: AWSTaskCompletionSource<AWSCognitoIdentityCustomChallengeDetails>) {
let authDetails = AWSCognitoIdentityCustomChallengeDetails(challengeResponses: [“USERNAME”:”YourUserName”, “ANSWER”: “123456”])
customAuthCompletionSource.set(result: authDetails)
}
public func didCompleteStepWithError(_ error: Error?) {
DispatchQueue.main.async {
if let error = error as? NSError {
print(“error”)
} else {
print(“success”)
self.dismiss(animated: true, completion: nil)
}
}
}
}

Step 4: After sign in success, you can get the username and user attribute:

self.user?.getDetails().continueOnSuccessWith { (task) -> AnyObject? in
DispatchQueue.main.async(execute: {
self.response = task.result
// With user details
print(response)
})
return nil
}

Please let me know if you have any questions. I hope AWS could update the documentations and provide sample example code to save us time to understand the sdk through trial and error.