Nil values passed after segue - ios

Question

I have an issue that has been driving me crazy, i had my values working inside my main VC which is named ViewController for both Age and Gender, i tested this with print statement and it works, this is below
#IBAction func uploadBtnAction(_ sender: UIButton) {
NetworkServices.instance.getGender(image: imageView1.image!) { (gender) in
self.Gender = gender
// print(self.Gender!) // This prints a value
}
NetworkServices.instance.getAge(image: imageView1.image!) { (age) in
self.Age = age
// print(self.Age!) //This prints a value
}
performSegue(withIdentifier: "updates", sender: self)
}
override func prepare(for segue: UIStoryboardSegue, sender: Any?) {
if segue.identifier == "updates" {
let destinationVC = segue.destination as! SecondViewController
destinationVC.userImage = image1
destinationVC.genderFromUser = Gender
destinationVC.ageFromUser = Age
}
}
but when i segue to another VC named SecondViewController which is below, i suddenly get an error "Thread 1: Fatal error: Unexpectedly found nil while unwrapping an Optional value", i have created variables to hold my values which will come from the first VC but this first VC does not pass the values after seque, please help, i have checked stackoverflow and tried all solutions to no avail, i though this should be the easiest part, funny enough, my image successfully passes over from the first VC to second during seque. Please advice.
import UIKit
import CLTypingLabel
class SecondViewController: UIViewController {
var userImage: UIImage?
var genderFromUser: String?
var ageFromUser: Int?
override func viewDidLoad() {
super.viewDidLoad()
imageView2.image = userImage
genderlabel.text = "Gender:"
ageLabel.text = "Age:"
print(ageFromUser!) // fatal error here
print(genderFromUser!) // fatal error here
print(userImage) // This is always successful
// ageUser.text =
// genderUser.text = userGender
// Do any additional setup after loading the view.
}
#IBOutlet weak var ageUser: UILabel!
#IBOutlet weak var genderUser: UILabel!
#IBOutlet weak var imageView2: UIImageView!
#IBOutlet weak var genderlabel: CLTypingLabel!
#IBOutlet weak var ageLabel: CLTypingLabel!
}
My goal is to set labels to the values passed over from the first VC at the second VC.

Answer 1

The problem is the order of operations. Your NetworkServices.instance.get... methods are asynchronous. I will use numbers to show you the order in which your code actually runs:
#IBAction func uploadBtnAction(_ sender: UIButton) {
NetworkServices.instance.getGender(image: imageView1.image!) { (gender) in
self.Gender = gender // 2 or 3
}
NetworkServices.instance.getAge(image: imageView1.image!) { (age) in
self.Age = age // 3 or 2
}
performSegue(withIdentifier: "updates", sender: self) // 1
}
So the segue is performed, and prepare is called, and the instance variables genderFromUser and ageFromUser are set, before you have set up your self.Gender and your self.Age. Therefore they are set to nil, because that's what they are at the time.
Later, of course, your asynchronous code comes along and does set self.Gender and self.Age, but it's too late; the train has left the station. SecondViewController already exists, and it has been configured with nil values.
Of course, since it does exist, you could now come along and set those instance properties again and get the SecondViewController to update its interface in response. Or you might work out some other solution. In general, this is a tricky problem — the problem of transitioning to another view controller at a time when its data is not ready yet and has to be fetched asynchronously.
[Also, you need to ask yourself, in fashioning a solution, what should happen if the data never arrives. What if the network chooses to go down at the exact moment the user taps the button? Welcome to the wild and wooly world of real life!]