Should perform segue on request - swift

Question

i'm starting with Swift3 and i'm having a recurrent problem due to the asynchronism. But until now, i always find a solution with callback.
I have a textField and a button, when i click on the button, i check on the API if there is a existing user named as in the textField.
Using shouldPerformSegue, i return the value if the users exist or no.
I have a separated class for handling calls on the Api
class Api {
static let urlApi = "https://XXXXXXXXXXXXX"
private let CUSTOMER_ID = "XXXXXXXX"
private let CUSTOMER_SECRET = "XXXXXXXX"
private var access_token : String? = nil
private var userInfo : User?
init() {
self.connect()
}
func connect() {
// Do the connect...
}
func get(user: String, callback: #escaping (_ status: Bool) -> Void) {
Alamofire.request(URL(string: "\(Api.urlApi)/v2/users/\(user)")!,
method: .get,
parameters: nil,
encoding: URLEncoding.default,
headers: ["Authorization": "Bearer \(self.access_token!)"])
.responseJSON(completionHandler: { response in
if response.result.isFailure {
print("ERROR: GET USER", response)
callback(false)
} else {
print("SUCCESS Getting user ", user)
callback(true)
}
})
}
}
And in my shouldPerformSegue
override func shouldPerformSegue(withIdentifier identifier: String, sender: Any?) -> Bool {
var userExist : Bool? = nil
let dispatchQueue = DispatchQueue(label: "getUser")
let semaphore = DispatchSemaphore(value: 1)
dispatchQueue.sync {
self.api?.get(user: self.userTextField.text!, callback: { status in
userExist = status
print("1 USEREXIST", userExist)
})
}
semaphore.wait()
print("2 USEREXIST", userExist)
return userExist ?? false // always false because userExist == nil
}
Sorry for the function mess, i don't really find the right way to do my DispachQueue and my Semaphore .. All googling answer look that i need those

Answer 1

The proper way to handle this scenario would be to make the request when the user taps on the button. If there is an error, you would present some error that says the username already exists. Then they would try again.
If the request is successful and that username has not been taken, then you would call performSegueWithIdentifier. The link below shows a good demonstration of the steps to take after this. Your current implementation isn't necessary.
https://stackoverflow.com/a/37823730/653839