Sequential Requests With Logic - Swift 3, Xcode 8, Alamofire 4

Question

UPDATE: I a using SwiftEventBus https://github.com/cesarferreira/SwiftEventBus and this seems to work fine. I can simply define event listeners that wait on each step. Still interested in other approaches for multi-step login process.

I need to solve a multipart login process (stuck on how to wrap logic around serial GET/POST requests)

also on github as https://github.com/Alamofire/Alamofire/issues/1746

• step 1 validate user/pw with basic authentication header
• step 2 use returned token (from step 1) in new authentication header and fetch from server a list
• step 3 user selects from returned list (or does another request to
  create new if thing not found on list) - note really important what the list is because I am just interested in the serialization and wrapping logic across steps
• step 4 use guid from selected/created thing to generate auth token to fetch yet another new general use auth token and refresh token

I can do each of these individually (set headers, set parms, validate and parse result, etc), BUT I do not understand how to create the logic flow around the requests. Do I have to nest all of the requests inside each other so that I step 2 happens inside step 1, step 3 happens inside step 2, etc. Or is there a simple example application somewhere that shows how execute multiple requests with logic around the requests - Not interested in simply queueing the requests as I have to run logic around the requests. Event Bus? Managed queue? Other? Looking for guidance and ideally a downloadable sample app that I can play with and learn from.

I originally built this without Alamofire and had the request timing/logic problem and thought Alamofire might make this easier but after switching to Alamofire I find myself stuck at the same point. Open to a suggestion without Alamofire in order to learn how to do this.

Answer 1

This is not build into Alamofire per se because it is callback based but you can use the callbacks and nest each call to Alamofire into the other like that:

Alamofire.request("url").responseJSON { _ in
    Alamofire.request("anotherUrl").responseJSON { _ in 
        Alamofire.request("yetAnotherUrl").responseJSON { _ in 
            //all three have finished with either response or error
        }
    }
}

This leads to a pattern often seen with callbacks that is also sometimes referred to as Pyramid of doom.

As an alternative you can check out either Promises or I like to use a reactive library like RxSwift that will give your code more of a 'waterfall' look.

There is also a PromiseKit extension and a RxSwift extenstion for Alamofire.