URLSession and UI related tasks - ios

Question

I'm trying to authenticate login by retrieving a boolean from my web server using URLSession, and show an Alert Controller if the login fails.
func requestLogin() {
let url = URL(string: "http://mywebserver/login.php")
var request = URLRequest(url: url!)
request.httpMethod = "POST"
let postString = "username=\(txtUsername.text!)&password=\(txtPassword.text!)"
request.httpBody = postString.data(using: .utf8)
let task = URLSession.shared.dataTask(with: request, completionHandler: { data, response, error in
guard data != nil else {
self.promptMessage(message: "No data found")
return
}
do {
if let jsonData = try JSONSerialization.jsonObject(with: data!, options: .mutableContainers) as? NSDictionary {
let success = jsonData.value(forKey: "success") as! Bool
if (success) {
self.dismiss(animated: false, completion: { action in
//Move to next VC
})
return
} else {
self.dismiss(animated: false, completion: { action in
self.promptMessage(message: "The username or password that you have entered is incorrect. Please try again.")}
)
return
}
} else {
self.dismiss(animated: false, completion: { action in
self.promptMessage(message: "Error: Could not parse JSON!")
})
}
} catch {
self.dismiss(animated: false, completion: { action in
self.promptMessage(message: "Error: Request failed!")
})
}
})
showOverlayOnTask(message: "Logging in...")
task.resume()
}
func promptMessage(message: String) {
let alert = UIAlertController(title: "Login Failed", message: message, preferredStyle: .alert)
let okAction = UIAlertAction(title: "OK", style: .default, handler: nil)
alert.addAction(okAction)
self.present(alert, animated: true, completion: nil)
}
func showOverlayOnTask(message: String) {
let alert = UIAlertController(title: nil, message: message, preferredStyle: .alert)
let loadingIndicator = UIActivityIndicatorView(frame: CGRect(x: 10, y: 5, width: 50, height: 50))
loadingIndicator.hidesWhenStopped = true
loadingIndicator.activityIndicatorViewStyle = UIActivityIndicatorViewStyle.gray
loadingIndicator.startAnimating();
alert.view.addSubview(loadingIndicator)
self.present(alert, animated: true, completion: nil)
}
The weird problem I'm getting is that my Logging In alert controller sometimes does not dismiss. It gets stuck until I tap on the screen, which then will dismiss and show the next alert controller. It's very annoying and I don't know where I'm doing wrong.
How do I fix this?

Answer 1

Maybe the problem is that you're trying to dismiss the controller without executing on the main thread, normally the UI changes/updates should be executed on the main thread.
Try this and check if works:
DispatchQueue.main.async {
self.dismiss(animated: false, completion: { action in
self.promptMessage(message: "Error: Could not parse JSON!")
})
}