1. Your view controller should conform to UITextFieldDelegate, like below:

   class MyViewController: UIViewController, UITextFieldDelegate {}

2. Set the delegate of your textfield: myTextField.delegate = self

3. Implement the method in your view controller : textField(_:shouldChangeCharactersInRange:replacementString:)

All together:

class MyViewController: UIViewController,UITextFieldDelegate  //set delegate to class @IBOutlet var mytextField: UITextField             //  textfield variable override func viewDidLoad() {    super.viewDidLoad()    mytextField.delegate = self                  //set delegate}func textField(textField: UITextField, shouldChangeCharactersInRange range: NSRange,                       replacementString string: String) -> Bool{    let maxLength = 4    let currentString: NSString = textField.text    let newString: NSString =             currentString.stringByReplacingCharactersInRange(range, withString: string)    return newString.length <= maxLength}

For Swift 4

func textField(_ textField: UITextField, shouldChangeCharactersIn range: NSRange, replacementString string: String) -> Bool {    let maxLength = 1    let currentString: NSString = (textField.text ?? "") as NSString    let newString: NSString =        currentString.replacingCharacters(in: range, with: string) as NSString    return newString.length <= maxLength}

Allowing only a specified set of characters to be entered into a given text field

func textField(textField: UITextField, shouldChangeCharactersInRange range: NSRange, replacementString string: String) -> Bool {  var result = true    if mytextField == numberField {    if count(string) > 0 {      let disallowedCharacterSet = NSCharacterSet(charactersInString: "0123456789.-").invertedSet      let replacementStringIsLegal = string.rangeOfCharacterFromSet(disallowedCharacterSet) == nil      result = replacementStringIsLegal    }  }   return result}

How to program an iOS text field that takes only numeric input with a maximum length

Modern Swift

Note that much of the example code online regarding this problem is extremely out of date.

Paste the following into any Swift file in your project. (You can name the file anything, for example, "Handy.swift".)

This finally fixes one of the stupidest problems in iOS:

Your text fields now have a .maxLength.

It is completely OK to set that value in storyboard during development, or, set it in code while the app is running.

// simply have this in any Swift file, say, Handy.swiftimport UIKitprivate var __maxLengths = [UITextField: Int]()extension UITextField {    @IBInspectable var maxLength: Int {        get {            guard let l = __maxLengths[self] else {               return 150 // (global default-limit. or just, Int.max)            }            return l        }        set {            __maxLengths[self] = newValue            addTarget(self, action: #selector(fix), for: .editingChanged)        }    }    func fix(textField: UITextField) {        let t = textField.text        textField.text = t?.prefix(maxLength).string    }}

It's that simple.

Footnote - these days to safely truncate a String in swift, you simply .prefix(n)

An even simpler one-off version...

The above fixes all text fields in your project.

If you just want one particular text field to simply be limited to say "4", and that's that...

class PinCodeEntry: UITextField {        override func didMoveToSuperview() {                super.didMoveToSuperview()        addTarget(self, action: #selector(fixMe), for: .editingChanged)    }        @objc private func fixMe() { text = text?.prefix(4) }}

Phew! That's all there is to it.

(Just BTW, here's a similar very useful tip relating to UITextView,https://stackoverflow.com/a/42333832/294884 )

For the OCD programmer (like me)...

As @LeoDabus reminds, .prefix returns a substring. If you're incredibly caring, this

let t = textField.texttextField.text = t?.prefix(maxLength)

would be

if let t: String = textField.text {    textField.text = String(t.prefix(maxLength))}

Enjoy!

Swift 4, simply use:

public func textField(_ textField: UITextField, shouldChangeCharactersIn range: NSRange, replacementString string: String) -> Bool {    return range.location < 10}