How can I declare that a text field can only contain an integer?

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Two things:

  1. Specify the keyboard type to only show the numeric keypad. So, set the keyboardType to .numberPad. This, however is not enough to stop the user from entering invalid characters in the text field. For example, the user is still able to paste text or switch keyboards when using an iPad.

  2. Specify the text field’s delegate and implement shouldChangeCharactersInRange that will not accept any characters other than the digits 0 though 9:

    class ViewController: UIViewController, UITextFieldDelegate {

    @IBOutlet weak var textField: UITextField!

    override func viewDidLoad() {
        super.viewDidLoad()

        // you can set the following two properties for the text field in Interface Builder, if you'd prefer

        textField.delegate = self
        textField.keyboardType = .numberPad
    }

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

    // or, alternatively:
    //
    // func textField(textField: UITextField, shouldChangeCharactersInRange range: NSRange, replacementString string: String) -> Bool {
    //     return string.range(of: "^\\d*$", options: .regularExpression) != nil
    // }

}

For Swift 2 rendition, see previous revision of this answer.

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