Completion Handlers in swift 4 - ios

Question

I have a problem I'm trying to wrap my head around relating to the use of completion handlers. I have 3 layers in my iOS program, the ViewController->Service->Networking. I need to load some data through API call from the view controller.
I have defined functions(completionHandlers) in the ViewController that should execute once the data request is complete and am comfortable when in implementing completion handlers when only two layers exists, but confused when in the following scenario:
DashboardViewController.swift
import UIKit
#IBDesignable class DashboardViewController: UIViewController {
#IBOutlet weak var stepCountController: ExpandedCardView!
var articles:[Article]?
let requestHandler = RequestHandler()
let dashboardService = DashboardService()
override func viewDidLoad() {
super.viewDidLoad()
dashboardService.getDashboardData(completionHandler: getDashboardDataCompletionHandler)
}
func getDashboardDataCompletionHandler(withData: DashboardDataRequest) {
print(withData)
}
}
DashboardService.swift
import Foundation
class DashboardService: GeneralService {
var requestHandler: DashboardRequestHandler
override init() {
requestHandler = DashboardRequestHandler()
super.init()
}
//this function should execute requestHandler.requestDashboardData(), and then execute convertDashboardData() with the result of previous get request
func getDashboardData(completionHandler: #escaping (DashboardDataRequest) -> Void) {
//on network call return
guard let url = URL(string: apiResourceList?.value(forKey: "GetDashboard") as! String) else { return }
requestHandler.requestDashboardData(url: url, completionHandler: convertDashboardData(completionHandler: completionHandler))
}
func convertDashboardData(completionHandler: (DashboardDataRequest) -> Void) {
//convert object to format acceptable by view
}
}
DashboardRequestHandler.swift
import Foundation
class DashboardRequestHandler: RequestHandler {
var dataTask: URLSessionDataTask?
func requestDashboardData(url: URL, completionHandler: #escaping (DashboardDataRequest) -> Void) {
dataTask?.cancel()
defaultSession.dataTask(with: url, completionHandler: {(data, response, error) in
if error != nil {
print(error!.localizedDescription)
}
guard let data = data else {
return
}
do {
let decodedJson = try JSONDecoder().decode(DashboardDataRequest.self, from: data)
completionHandler(decodedJson)
} catch let jsonError {
print(jsonError)
}
}).resume()
}
}
If you look at the comment in DashboardService.swift, my problem is quite obvious. I pass a completion handler from ViewController to Service and Service has its own completion handler that it passes to RequestHandler where the view controller completion handler (getDashboardDataCompletionHandler) should be executed after the Service completion handler (convertDashboardData())
Please help me in clarifying how to implement this. Am I making a design mistake by trying to chain completionHandlers like this or am I missing something obvious.
Thank you
--EDIT--
My Request Handler implementation is as follows:
import Foundation
class RequestHandler {
// let defaultSession = URLSession(configuration: .default)
var defaultSession: URLSession!
init() {
guard let path = Bundle.main.path(forResource: "Api", ofType: "plist") else {
print("Api.plist not found")
return
}
let apiResourceList = NSDictionary(contentsOfFile: path)
let config = URLSessionConfiguration.default
if let authToken = apiResourceList?.value(forKey: "AuthToken") {
config.httpAdditionalHeaders = ["Authorization": authToken]
}
defaultSession = URLSession(configuration: config)
}
}

Answer 1

In this case is more clear to use delegation, for example like this:
protocol DashboardServiceDelegate {
func didLoaded(_ viewModel: DashboardViewModel)
}
class DashboardViewController: UIViewController {
override func viewDidLoad() {
super.viewDidLoad()
dashboardService.delegate = self
dashboardService.getDashboardData()
}
}
extension DashboardViewController: DashboardServiceDelegate {
func didLoaded(_ viewModel: DashboardViewModel) {
///
}
}
protocol DashboardRequestHandlerDelegate() {
func didLoaded(request: DashboardDataRequest)
}
class DashboardService: GeneralService {
private lazy var requestHandler: DashboardRequestHandler = { reqHandler in
reqHandler.delegate = self
return reqHandler
}(DashboardRequestHandler())
func getDashboardData() {
guard let url = ...
requestHandler.requestDashboardData(url: url)
}
}
extension DashboardService: DashboardRequestHandlerDelegate {
func didLoaded(request: DashboardDataRequest) {
let viewModel = convert(request)
delegate.didLoaded(viewModel)
}
}