func textFieldShouldReturn(textField: UITextField!) -> Bool {
    textField.resignFirstResponder()  //if desired
    return true
}

textField.returnKeyType = UIReturnKeyType.Done

func textView(textView: UITextView, shouldChangeTextInRange range: NSRange, replacementText text: String) -> Bool {
    if (text == "n") {
        textView.resignFirstResponder()
    }
    return true
}

class ViewController: UIViewController,UITextFieldDelegate

override func viewDidLoad() {

    userNameTextField.delegate = self
    userNameTextField.tag = 0
    userNameTextField.returnKeyType = .next
    passwordTextField.delegate = self
    passwordTextField.tag = 1
    passwordTextField.returnKeyType = .go
}

func textFieldShouldReturn(_ textField: UITextField) -> Bool
{
    if let nextField = textField.superview?.viewWithTag(textField.tag + 1) as? UITextField {
        nextField.becomeFirstResponder()
    } else {
        textField.resignFirstResponder()
        return true;
    }
    return false
}