Rxswift

This is an excellent approach to handling interactions with APIs (or databases, queues, etc.) I follow a similar pattern when creating Services for my applications. Have you considered using RxJS? I think it would compliment the way you handle different states and events nicely. Maybe take a look at Moya (a fantastic library for writing an API layer in Swift) for inspiration.

This blog post will cover how you can create a custom alert in your iOS app using RxSwift.

The following are two screenshots of our application, and the alert (upcoming appointment) we will be going over which you can see at the bottom of the screen: