How To Fix Warning Attempt To Present UIAlertController On ViewController Whose View Is Not In The Window Hierarchy

I want to implement an effect that when the iOS app starts, it popup an alert dialog on the screen and displays some messages. But during the process, I meet an error message like below, and the alert dialog is not displayed as I want. This article will tell you how to fix this issue.

1. Error Message.

1. Below is the error message.

   TestProject[2673:43682] Warning: Attempt to present <UIAlertController: 0x7f8ae603c000> on <TestProject.ViewController: 0x7f8ae5d097d0> whose view is not in the window hierarchy!

2. How To Fix Above Error.

1. The reason for this error is because I create the instance of the swift UIAlertController class in the ViewController class’s viewDidLoad() method.
2. To fix this error, I need to override ViewController class’s viewDidAppear method and put the UIAlertController class initialization code in the viewDidAppear method like below.
3. ViewController.swift

   //
   //  ViewController.swift
   //  TestProject
   //
   //  Copyright © 2019 dev2qa.com. All rights reserved.
   //
   
   import UIKit
   
   class ViewController: UIViewController {
       
       var message:String = ""
   
       override func viewDidLoad() {
           super.viewDidLoad()
           
           message = "Hello Swift"
   
           print(message)
               
       }
       
       /* Override the viewDidAppear method and display iOS swift alert popup dialog in it. */
       override func viewDidAppear(_ animated: Bool) {
   
           /* Below method will display a swift alert dialog window. */
           presentAlertDialog()
       }
       
       /* Below function will create an iOS swift alert dialog and display the message in it. */
       func presentAlertDialog() -> Void{
   
           /* Create a UIAlertController object with provided title, message and alert dialog style. */
           let alertController:UIAlertController = UIAlertController(title: "Message", message: message, preferredStyle: UIAlertController.Style.alert)
   
        
           /* Create a UIAlertAction object with title to implement the alert dialog OK button. */    
           let alertAction:UIAlertAction = UIAlertAction(title: "OK", style: UIAlertAction.Style.default, handler:nil)
           /* Add above UIAlertAction button to the UIAlertController object. */        
           alertController.addAction(alertAction)
           
   
           /* Display the swift alert dialog window. */
           present(alertController, animated: true, completion: nil)
       }
   
   
   }

4. Below is the screen when executing the above app.
   

Reference

1. How To Prompt Alert Dialog When Click A Button In Swift