How to add "Done" button to Numpad in iOS using Swift?

Swift 5 solution using @IBInpectable and extension. A dropdown will appear under the Attribute Inspector in the Interface Builder for every UITextField in the project.

extension UITextField{    
    @IBInspectable var doneAccessory: Bool{
        get{
            return self.doneAccessory
        }
        set (hasDone) {
            if hasDone{
                addDoneButtonOnKeyboard()
            }
        }
    }

    func addDoneButtonOnKeyboard()
    {
        let doneToolbar: UIToolbar = UIToolbar(frame: CGRect.init(x: 0, y: 0, width: UIScreen.main.bounds.width, height: 50))
        doneToolbar.barStyle = .default

        let flexSpace = UIBarButtonItem(barButtonSystemItem: .flexibleSpace, target: nil, action: nil)
        let done: UIBarButtonItem = UIBarButtonItem(title: "Done", style: .done, target: self, action: #selector(self.doneButtonAction))

        let items = [flexSpace, done]
        doneToolbar.items = items
        doneToolbar.sizeToFit()

        self.inputAccessoryView = doneToolbar
    }

    @objc func doneButtonAction()
    {
        self.resignFirstResponder()
    }
}

A Swift-3 Version (of Marko's solution - which worked for me)

(in my case, I have a UITextField identified as textfield)

func addDoneButtonOnKeyboard() {
    let doneToolbar: UIToolbar = UIToolbar(frame: CGRect(x: 0, y: 0, width: 320, height: 50))
    doneToolbar.barStyle       = UIBarStyle.default        
    let flexSpace              = UIBarButtonItem(barButtonSystemItem: UIBarButtonSystemItem.flexibleSpace, target: nil, action: nil)
    let done: UIBarButtonItem  = UIBarButtonItem(title: "Done", style: UIBarButtonItemStyle.done, target: self, action: #selector(ViewController.doneButtonAction))

    var items = [UIBarButtonItem]()
    items.append(flexSpace)
    items.append(done)

    doneToolbar.items = items
    doneToolbar.sizeToFit()

    self.textfield.inputAccessoryView = doneToolbar
}

func doneButtonAction() {
    self.textfield.resignFirstResponder()
}

As far as I know, you can't add the Done button on the keyboard part; you'd have add a inputAccessoryView to the UITextField or UITextView (if that's what you're using).

Check the documentation for more info.

Edit: Check this question for an example on how to do that.

Edit 2: Similar example in Swift.

Edit 3: Code from edit 2, as link may expire.

override func viewDidLoad()
{
    super.viewDidLoad()

    //--- add UIToolBar on keyboard and Done button on UIToolBar ---//
    self.addDoneButtonOnKeyboard()
}

//--- *** ---//

func addDoneButtonOnKeyboard()
{
    var doneToolbar: UIToolbar = UIToolbar(frame: CGRectMake(0, 0, 320, 50))
    doneToolbar.barStyle = UIBarStyle.BlackTranslucent

    var flexSpace = UIBarButtonItem(barButtonSystemItem: UIBarButtonSystemItem.FlexibleSpace, target: nil, action: nil)
    var done: UIBarButtonItem = UIBarButtonItem(title: "Done", style: UIBarButtonItemStyle.Done, target: self, action: Selector("doneButtonAction"))

    var items = NSMutableArray()
    items.addObject(flexSpace)
    items.addObject(done)

    doneToolbar.items = items
    doneToolbar.sizeToFit()

    self.textView.inputAccessoryView = doneToolbar
    self.textField.inputAccessoryView = doneToolbar

}

func doneButtonAction()
{
    self.textViewDescription.resignFirstResponder()
}

Swift 4.2

func addDoneButtonOnKeyboard(){
        let doneToolbar: UIToolbar = UIToolbar(frame: CGRect.init(x: 0, y: 0, width: UIScreen.main.bounds.width, height: 50))
        doneToolbar.barStyle = .default

        let flexSpace = UIBarButtonItem(barButtonSystemItem: .flexibleSpace, target: nil, action: nil)
        let done: UIBarButtonItem = UIBarButtonItem(title: "Done", style: .done, target: self, action: #selector(self.doneButtonAction))

        let items = [flexSpace, done]
        doneToolbar.items = items
        doneToolbar.sizeToFit()

        txtMobileNumber.inputAccessoryView = doneToolbar
    }

    @objc func doneButtonAction(){
        txtMobileNumber.resignFirstResponder()
    }