How could I allow only actual Integer in the UITextField, i.e. 00 or 01230 is not acceptable, but 0 and 1230

Question

I have a code below that will have an UIAlertController with an UITextField that will only accept numbers.

class ViewController: UIViewController, UITextFieldDelegate {

    let priceText = UILabel()
    static let DefaultText = "Click to enter price"
    static let DefaultPlaceHolder = "In dollar"

    override func viewDidLoad() {
        super.viewDidLoad()
        priceText.text = ViewController.DefaultText
        view.addSubview(priceText)
        priceText.translatesAutoresizingMaskIntoConstraints = false
        priceText.backgroundColor = .yellow
        NSLayoutConstraint.activate([
            priceText.centerXAnchor.constraint(equalTo: view.centerXAnchor),
            priceText.centerYAnchor.constraint(equalTo: view.centerYAnchor),
        ])
        priceText.addGestureRecognizer(UITapGestureRecognizer(target: self, action: #selector(openEnterPriceDialog)))
        priceText.isUserInteractionEnabled = true
    }

    @objc private func openEnterPriceDialog() {
        let alert = UIAlertController(
            title: "Enter Price",
            message: "The value you want to sell",
            preferredStyle: UIAlertController.Style.alert)

        alert.addTextField {
            $0.placeholder = ViewController.DefaultPlaceHolder
            $0.keyboardType = .numberPad
            $0.delegate = self
        }
        alert.addAction(UIAlertAction(
            title: "Cancel",
            style: UIAlertAction.Style.default,
            handler: nil))
        alert.addAction(UIAlertAction(
            title: "Submit",
            style: UIAlertAction.Style.default,
            handler: nil }))
        self.present(alert, animated: true, completion: nil)
    }

    func textField(_ textField: UITextField, shouldChangeCharactersIn range: NSRange, replacementString string: String) -> Bool {
        let invalidCharacters = CharacterSet(charactersIn: "0123456789").inverted
        return string.rangeOfCharacter(from: invalidCharacters) == nil
    }
}

extension UITextField {
    open override func canPerformAction(_ action: Selector, withSender sender: Any?) -> Bool {
        return action == #selector(UIResponderStandardEditActions.cut) || action == #selector(UIResponderStandardEditActions.copy)
    }
}

However, it is still somehow allow key like 000 or even 01230, which is not a nice number. I would be okay with 0 or 1230, but not something start with 0.

How could I achieve that?

ps: most of stackoverflow answer like How can I declare that a text field can only contain an integer? only check to ensure digit from 0123456789 can be accepted, but still allow things like 00, or 01230.

Answer 1

Here is how you need to implement textField(_:shouldChangeCharactersIn:replacementString:) method,

func textField(_ textField: UITextField, shouldChangeCharactersIn range: NSRange, replacementString string: String) -> Bool {
    let invalidCharacters = CharacterSet(charactersIn: "0123456789").inverted
    if (string.rangeOfCharacter(from: invalidCharacters) == nil) {
        if let text = textField.text {
             var str = (text as NSString).replacingCharacters(in: range, with: string)
             if Set(str) == ["0"] {
                 textField.text = "0"
                 return false
             } else if str.first == "0" {
                 str.removeFirst()
                 textField.text = str
                 return false
             }
         }
         return true
    }
    return false
}

Answer 2

There is no need to override canPerformAction and/or to set your view controller as the text field delegate. You simply addTarget to your text field for UIControl.Event's .editingChanged and filter all non digits in the editingChanged selector method. You can also control the maximum value allowed and format the final price on the fly:

---

First create your alert controller as a property of your view controller and add all properties that your textField might need as well:

private let alert = UIAlertController(title: "Enter Price",
                              message: "The value you want to sell in dollars.",
                              preferredStyle: .alert)

private var price: Decimal = 0
private var maximum: Decimal = 999_999
private var lastValue: String?

---

Then customize your text field and add the target for editingChanged events inside viewDidLoad method:

override func viewDidLoad() {
    super.viewDidLoad()

    alert.addTextField { textField in
        textField.addTarget(self, action: #selector(self.editingChanged), for: .editingChanged)
        textField.keyboardType = .numberPad
        textField.textAlignment = .right
        textField.sendActions(for: .editingChanged)
    }
    alert.addAction(.init(title: "Submit", style: .default) { _ in
        print("Price:", Formatter.custom.string(for: self.price)!)
    })
    alert.addAction(.init(title: "Cancel", style: .default))
}

Next you can create your method to deal with the edit changed events as needed:

@objc func editingChanged(textField: UITextField) {
    guard let newValue = textField.text?.decimal, newValue <= maximum else {
        textField.text = lastValue
        return
    }
    price = newValue        
    let text = Formatter.custom.string(for: price)!
    print("Price:", text)
    textField.text = text
    lastValue = text
}

You will need to add a custom NumberFormatter to your view controller file and also a helper to filter the non digits from your string:

private extension Formatter {
    static let custom: NumberFormatter = {
        let formatter = NumberFormatter()
        formatter.locale = Locale(identifier: "en_US")
        formatter.numberStyle = .currency
        formatter.minimumFractionDigits = 0
        formatter.maximumFractionDigits = 0
        return formatter
    }()
}

---

private extension String {
    var decimal: Decimal { Decimal(string: filter { $0.isWholeNumber }) ?? 0 }
}

---

Now you can just present your alert when needed:

present(alert, animated: true)

Answer 3

Make a slightly shorter version using Decimal to avoid manually extracting the initial 0 from the String

    func textField(_ textField: UITextField, shouldChangeCharactersIn range: NSRange, replacementString string: String) -> Bool {
        let invalidCharacters = CharacterSet(charactersIn: "0123456789").inverted
        if (string.rangeOfCharacter(from: invalidCharacters) == nil) {
            if let text = textField.text {
                let str = (text as NSString).replacingCharacters(in: range, with: string)
                if let number = Decimal(string: str.filter { $0.isWholeNumber }) {
                    if (number <= Decimal(ViewController.maxValue)) { textField.text = "\(number)" }
                    return false
                }
            }
            return true
        }
        return false
    }