Swift 3.0 iOS10 Passing Async data through a segue sender for Firebase

Question

I have a log in page that collects a username and password. On submit, Its sends to the database to retrieve our servers access key. I do this through an asynchronous JSON POST using session.dataTask. When I retrieve the JSON Object I parse the key out of it. I want to pass it to the next page, retrieve a firebase token and then send both pieces of data back to the server for DB storage. I have created a "prepare for segue" function that collects the variable and passes it to a variable on the next page. I believe I am not setting up the sequence of events correctly or that the data isn't making it out of the Async container. Can someone have a look at these two files and see where I am getting it wrong?

Here is the first page I want to segue away from after making the REST web service call...

loginVC.swift:

import UIKit

class LoginVC: UIViewController {

    @IBOutlet weak var username: UITextField!
    @IBOutlet weak var password: UITextField!
    @IBOutlet weak var validationBox: UITextView!
    @IBAction func logInAction(_ sender: UIButton) {
        guard let user = username.text, !user.isEmpty else {
            validationBox.text = "Please enter valid credentials"
            return
        }
        guard let pass = password.text, !pass.isEmpty else {
            validationBox.text = "Please enter valid credentials"
            return
        }

        let params = ["sUser": username.text!, "sPass": password.text!]

        let url = URL(string: "restWebServiceURL")!
        let session = URLSession.shared
        var request = URLRequest(url: url)
        request.httpMethod = "POST"
        do {
            request.httpBody = try JSONSerialization.data(withJSONObject: params, options: .prettyPrinted)
        } catch let error {
            print(error.localizedDescription)
        }
        request.addValue("application/json", forHTTPHeaderField: "Content-Type")
        request.addValue("application/json", forHTTPHeaderField: "Accept")

        let task = session.dataTask(with: request as URLRequest, completionHandler: { data, response, error in

            guard error == nil else { return }
            guard let data = data else { return }

            do {
                if let parsedJSON = try JSONSerialization.jsonObject(with: data, options: .mutableContainers) as? [String: Any] {
                    let parsedData = parsedJSON["d"] as! [String:Any]
                    let key = parsedData["key"] as! String
                    DispatchQueue.main.async {
                        print(key)
                        self.performSegue(withIdentifier: "FirebaseVC", sender: key)
                    }

                }
            } catch let error {
                print(error.localizedDescription)
            }
        })
        task.resume()
    }
    func sayHello() {
        print("Hello!")
    }
    func sayGoodbye() {
        print("Goodbye!")
    }



    override func viewDidLoad() {
        super.viewDidLoad()
        validationBox.text = "Ready..."
        func prepare(for segue: UIStoryboardSegue, sender: Any?) {
            if let FirebaseInit = segue.destination as? FirebaseVC {
                if let sKey = sender as? String {
                    print("prepare - " + sKey)
                    FirebaseInit.sessionKey = sKey
                }
            }
        }
    }

    override func didReceiveMemoryWarning() {
        super.didReceiveMemoryWarning()
        // Dispose of any resources that can be recreated.
    }


}

Here is the page I want to go to to receive the data access key ...

FirebaseVC.swift:

import UIKit

class FirebaseVC: UIViewController {

    private var _sessionKey = String()

    var sessionKey : String {
        get { return _sessionKey }
        set { _sessionKey = newValue }
    }

    @IBOutlet weak var sessionKeyTestBox: UITextView!

    override func viewDidLoad() {
        super.viewDidLoad()
        print(_sessionKey)

    }

    override func didReceiveMemoryWarning() {
        super.didReceiveMemoryWarning()
        // Dispose of any resources that can be recreated.
    }



}

Feel free to suggest a better way to pass the data to the next page. Thanks...

Answer 1

It turns out I was correct in my assumption the the chain of events was off. Following the model suggested by @achrefGassoumi, I moved the datatask to a Singleton Service here:

import Foundation

struct CallWebService {

    static let sharedInstance = CallWebService()

    func logInToCaduceus(u: String, p: String, completion: @escaping (_ sKey: String) -> ()) {
        let params = ["sUser": u, "sPass": p]

        let url = URL(string: "https://telemed.caduceususa.com/ws/telemed.asmx/telemedLogin")!
        let session = URLSession.shared
        var request = URLRequest(url: url)
        request.httpMethod = "POST"
        do {
            request.httpBody = try JSONSerialization.data(withJSONObject: params, options: .prettyPrinted)
        } catch let error {
            print(error.localizedDescription)
        }
        request.addValue("application/json", forHTTPHeaderField: "Content-Type")
        request.addValue("application/json", forHTTPHeaderField: "Accept")

        let task = session.dataTask(with: request as URLRequest, completionHandler: { data, response, error in

            guard error == nil else { return }
            guard let data = data else { return }

            do {
                if let parsedJSON = try JSONSerialization.jsonObject(with: data, options: .mutableContainers) as? [String: Any] {
                    let parsedData = parsedJSON["d"] as! [String:Any]
                    let key = parsedData["key"] as! String
                    DispatchQueue.main.async {
                        completion(key)
                    }
                }
            } catch let error {
                print(error.localizedDescription)
            }
        })
        task.resume()

    }

}

Then my two controllers look like this:

LoginVC

import UIKit

class LoginVC: UIViewController {

    override func prepare(for segue: UIStoryboardSegue, sender: Any?) {
        if (segue.destination.isKind(of: FirebaseVC.self)) {
            let vc = segue.destination as! FirebaseVC
            if let sKey = sender as? String {
                vc.sessionKey = sKey
            }
        }
    }

    @IBOutlet weak var username: UITextField!
    @IBOutlet weak var password: UITextField!
    @IBOutlet weak var validationBox: UITextView!
    @IBAction func logInAction(_ sender: UIButton) {
        guard let user = username.text, !user.isEmpty else {
            validationBox.text = "Please enter valid credentials"
            return
        }
        guard let pass = password.text, !pass.isEmpty else {
            validationBox.text = "Please enter valid credentials"
            return
        }

        CallWebService.sharedInstance.logInToCaduceus(u: username.text!, p: password.text!, completion: {(sessionKey: String) -> Void in
                print(sessionKey)
                self.performSegue(withIdentifier: "FirebaseVC", sender: sessionKey)
            }
        )

    }

    override func viewDidLoad() {
        super.viewDidLoad()
        //validationBox.textAlignment = .center
        validationBox.text = "Ready..."
    }

    override func didReceiveMemoryWarning() {
        super.didReceiveMemoryWarning()
        // Dispose of any resources that can be recreated.
    }


}

AND THE receiving FirebaseVC

import UIKit

class FirebaseVC: UIViewController {

    private var _sessionKey = String()

    var sessionKey : String {
        get { return _sessionKey }
        set { _sessionKey = newValue }
    }

    @IBOutlet weak var sessionKeyTestBox: UITextView!

    override func viewDidLoad() {
        super.viewDidLoad()
        sessionKeyTestBox.text = _sessionKey
    }

    override func didReceiveMemoryWarning() {
        super.didReceiveMemoryWarning()
    }

}

Excuse my (non-swift) Javascript terminology but essentially I moved the data call into a service and then place a callback method in the service with the completion method to ensure the the performSegue doesn't fire until the data has been received and parsed out. So when i submit the log in form data to the server the segue doesn't fire until that async call has been completed.