Pattern for async callback between a view controller and a dataprovider singleton

DataProvider.swift

typealias ServiceResponse = (NSDictionary?, NSError?) -> Void

class DataProvider: NSObject {
    
    var client:AFHTTPRequestOperationManager?
    let LOGIN_URL = "/api/v1/login"
    
    class var sharedInstance:DataProvider {
        struct Singleton {
            static let instance = DataProvider()
        }
        return Singleton.instance
    }
    
    func setupClientWithBaseURLString(urlString:String) {
        client = AFHTTPRequestOperationManager(baseURL: NSURL.URLWithString(urlString))
        client!.operationQueue = NSOperationQueue.mainQueue()
        client!.responseSerializer = AFJSONResponseSerializer()
        client!.requestSerializer = AFJSONRequestSerializer()
    }
    
    func loginWithEmailPassword(email:String, password:String, onCompletion: ServiceResponse) -> Void {
        self.client!.POST(LOGIN_URL, parameters: ["email":email, "password":password] , success: {(operation:AFHTTPRequestOperation!, responseObject:AnyObject!) -> Void in
            
            self.setupClientWithBaseURLString("http://somebaseurl.com")
            
            let responseDict = responseObject as NSDictionary
                // Note: This is where you would serialize the nsdictionary in the responseObject into one of your own model classes (or core data classes)
                onCompletion(responseDict, nil)
            }, failure: {(operation: AFHTTPRequestOperation!, error:NSError!) -> Void  in
                onCompletion(nil, error)
            })
    }
}

MyViewController.swif

import UIKit

class MyViewController: UIViewController {

    override func viewDidLoad() {
        super.viewDidLoad()

        // Do any additional setup after loading the view.
    }

    override func viewWillAppear(animated: Bool)  {
        super.viewWillAppear(animated)
        DataProvider.sharedInstance.loginWithEmailPassword(email:"some@email.com", password:"somepassword") { (responseObject:NSDictionary?, error:NSError?) in
            
            if (error) {
                println("Error logging you in!")
            } else {
                println("Do something in the view controller in response to successful login!")
            }
        }
    }
   

   
}

if you define your client variable as non-Optional, then you don't need to add ! everytime

thanks....for this. I used to tell ppl learning Obj-C is like growing a third arm...Async Swift design seemed just as frustrating and painful...This really helps.