警报 iOS swift

Question

When I test for bad data in a text field and try to show an alert for missing or bad data the alert does not display. But I can get an alert to display in the viewDidLoad function

Here is the code

 import UIKit
 import CoreData

class SellViewController: UIViewController, UIPickerViewDataSource, UIPickerViewDelegate  {
 @IBOutlet weak var customer: UITextField!
@IBOutlet weak var bales: UITextField!

override func viewDidLoad() {
    super.viewDidLoad()

    let alert = UIAlertController(title: "Hey", message: "@ SellViewController viewDidLoad ", preferredStyle: UIAlertControllerStyle.Alert)
    alert.addAction(UIAlertAction(title: "Working!!", style: UIAlertActionStyle.Default, handler: nil))
    self.presentViewController(alert, animated: true, completion: nil)

func checkDataInput(){

    print("checking data input customer.text  \(customer.text)")
    print("checking data input bales.text \(bales.text)")

    if (customer.text!.isEmpty) {
        customer.text = "REQUIRED"
        missingCustomer()
    }
if (bales.text!.isEmpty){
        availableAlert()
    }else{
        newQuantity = Int(bales.text!)!
     }

 func availableAlert() {
    print(" at availableAlert")

    let alert = UIAlertController(title: "Hey", message: "@ SellViewController func missingValues ", preferredStyle: UIAlertControllerStyle.Alert)
    alert.addAction(UIAlertAction(title: "Working!!", style: UIAlertActionStyle.Default, handler: nil))
    self.presentViewController(alert, animated: true, completion: nil)


func missingCustomer() {
    print(" at missingCustomer")
    let alert = UIAlertController(title: "Hey", message: "@ SellViewController func missingCustomer ", preferredStyle: UIAlertControllerStyle.Alert)
    alert.addAction(UIAlertAction(title: "Working!!", style: UIAlertActionStyle.Default, handler: nil))
    self.presentViewController(alert, animated: true, completion: nil)
    }

This displays alert at viewDidLoad but no alert when I have missing data. The print statements when I have missing data return as.

checking data input customer.text Optional("")
checking data input bales.text Optional("")
at missingCustomer
at availableAlert
fatal error: unexpectedly found nil while unwrapping an Optional value

The fatal error happens because the user is not able to correct their response when they get the alert.

What am I doing wrong? Shouldn't the alert display immediately when my missingData functions are triggered and the be able to correct their entries?

Answer 1

This sounds like a threading issue. Your data evaluation code is most likely executing on the main thread (you haven't posted it so cant say for sure) so when you try to do UI work, presenting an alert in this case, it will not execute. To fix this, try this code instead in your missingData function:

let alert = UIAlertController(title: "Hey", message: "@ SellViewController viewDidLoad ", preferredStyle: UIAlertControllerStyle.Alert)
alert.addAction(UIAlertAction(title: "Working!!", style: UIAlertActionStyle.Default, handler: nil))
dispatch_async(dispatch_get_main_queue()) {
    self.presentViewController(alert, animated: true, completion: nil)
}

Answer 2

let alert = UIAlertController(title: "Hey", message: "@ SellViewController viewDidLoad ", preferredStyle: UIAlertControllerStyle.Alert)
alert.addAction(UIAlertAction(title: "Working!!", style: UIAlertActionStyle.Default, handler: nil))
self.presentViewController(alert, animated: true, completion: nil)

you can change the code to ->

 let alert = UIAlertController(title: "Hey", message: "@ SellViewController viewDidLoad ", preferredStyle: UIAlertControllerStyle.Alert)
 let alertButton = UIAlertAction(title: "Working!!", style: UIAlertAction.Style.default)
alert.addAction(alertButton)
present(alert, animated: true)