This is a sample AppDelegate with the required three code snippets for adding AWSMobileClient to your project as a credentials manager

AWSMobileClient-AppDelegate.swift

import UIKit
import AWSMobileClient //#1

@UIApplicationMain
class AppDelegate: UIResponder, UIApplicationDelegate {

    var window: UIWindow?

    func application(_ application: UIApplication, didFinishLaunchingWithOptions launchOptions: [UIApplication.LaunchOptionsKey: Any]?) -> Bool {
        
        //#2 Add this return statement for AWSMobileClient
        return AWSMobileClient.sharedInstance().interceptApplication(
            application, didFinishLaunchingWithOptions:
            launchOptions)
    }
    
    // #3 Add this application:open url with AWSMobileClient singleton
    func application(_ application: UIApplication, open url: URL,
                     sourceApplication: String?, annotation: Any) -> Bool {
        
        return AWSMobileClient.sharedInstance().interceptApplication(
            application, open: url,
            sourceApplication: sourceApplication,
            annotation: annotation)
    }

    func applicationWillResignActive(_ application: UIApplication) {}
    func applicationDidEnterBackground(_ application: UIApplication) {}
    func applicationWillEnterForeground(_ application: UIApplication) {}
    func applicationDidBecomeActive(_ application: UIApplication) {}
    func applicationWillTerminate(_ application: UIApplication) {}
}