Alamofire request doesn't return - ios

Question

I want to create an app that with a textField and a label. The textField will read the input and using Alamofire to check if the input exist. (The url will return JSON Bool.) A helper function called getReturnVal is to get the return value from the url.
However, the nameExist variable is always nil and it seems like the helper function never goes into the two cases. Please advise how should I fix this problem.
Thank you in advance.
My code is as below.
import UIKit
import Alamofire
class ViewController: UIViewController, UITextFieldDelegate {
#IBOutlet weak var nameField: UITextField!
#IBOutlet weak var nameLabel: UILabel!
var nameValid: Bool = false
func textFieldDidEndEditing(_ textField: UITextField) {
if textField == nameField {
guard let username = textField.text, !username.isEmpty else {
nameValid = false
return
}
let url = "https://bismarck.sdsu.edu/hometown/nicknameexists?name=" + username
print(url)
let nameExist = getReturnVal(url: url, type: Bool.self)
if nameExist! {
nameLabel.text = "Exist"
nameValid = false
} else {
nameLabel.text = "Valid"
nameValid = true
}
}
}
func textFieldShouldReturn(_ textField: UITextField) -> Bool {
textField.resignFirstResponder()
return true
}
func getReturnVal<T>(url: String, type: T.Type) -> T? {
var jsonObject: T?
print("enter------------")
Alamofire.request(url).validate().responseJSON { response in
switch response.result {
case .success:
if let JSON = response.result.value {
jsonObject = (JSON as! T)
print("SUCCESS--------")
print("JSON: \(String(describing: jsonObject))")
}
case .failure(let error):
print("--------------")
print(error)
jsonObject = false
}
}
return jsonObject
}
override func viewDidLoad() {
super.viewDidLoad()
nameField.delegate = self
// Do any additional setup after loading the view, typically from a nib.
}
override func didReceiveMemoryWarning() {
super.didReceiveMemoryWarning()
// Dispose of any resources that can be recreated.
}
}

Answer 1

The Alamofire function is asynchronous, because it’s a network request. This means the function returns before the data from the network request is returned, so it will always return nil because jsonObject is returned before it’s modified. What you need to do is pass in a callback function as a parameter, so that the callback function body gets called when the Alamofire function returns.
The callback signature could be something like this:
callback: (T) -> ()
Then inside the getReturnVal function you can call callback(jsonObject) right after the variable jsonObject is set (so twice). Then when you when you call getReturnVal you need to pass in a function that takes as a parameter that jsonObject.