How to make UIAlertController modal in iOS 9.3?

The alert completion block is run as soon as alert has been displayed, not waiting for user answer.

You need to put actions that require user interaction in the UIAlertAction handlers.

Here is an example of an alert, with request for text entry for a name. We wait for entry validated to close the alertCalled when tapping a button.

    @objc func alertTextFieldDidChange(_ sender: UITextField) {

     alert?.actions[0].isEnabled = sender.text!.count > 0
    }

    @IBAction func testAlert(_ sender: Any) {
   
        //1. Create the alert controller.
        alert = UIAlertController(title: "Recherche de la Ville", message: "Nom de la ville ", preferredStyle: .alert)
      
        //2. Add the text field.
        alert?.addTextField(configurationHandler: { (textField) -> Void in
            textField.placeholder = ""
            textField.keyboardType = UIKeyboardType.emailAddress 
          // alertTextFieldDidChange activate searchButton
            textField.addTarget(self, action: #selector(self.alertTextFieldDidChange(_:)), for: UIControl.Event.editingChanged)
        })
      
        //3. Grab the value from the text field.
        let yesAction = UIAlertAction(title: "Search", style: .default, handler: { (action) -> Void in
            let textField = self.alert?.textFields![0]
            if textField?.text! == "" {
                print("empty")
            } else {
                print("not empty", textField?.text! ?? "")
            }
        })
        yesAction.isEnabled = false
        alert?.addAction(yesAction)
      
        // 4. Present the alert.
        self.present(alert!, animated: true, completion: nil)
      
    }

If you replace line 32 by

     self.present(alert!, animated: true, completion: { print("fired immediately") })

You will see that completion block is executed before any user interaction