您可以使用UITextFieldDelegate的shouldChangeCharactersInRange方法将用户的输入限制为数字：

func textField(textField: UITextField,
    shouldChangeCharactersInRange range: NSRange,
    replacementString string: String) -> Bool {

    // Create an `NSCharacterSet` set which includes everything *but* the digits
    let inverseSet = NSCharacterSet(charactersInString:"0123456789").invertedSet

    // At every character in this "inverseSet" contained in the string,
    // split the string up into components which exclude the characters
    // in this inverse set
    let components = string.componentsSeparatedByCharactersInSet(inverseSet)

    // Rejoin these components
    let filtered = components.joinWithSeparator("")  // use join("", components) if you are using Swift 1.2

    // If the original string is equal to the filtered string, i.e. if no
    // inverse characters were present to be eliminated, the input is valid
    // and the statement returns true; else it returns false
    return string == filtered
}

为Swift 3更新：

 func textField(_ textField: UITextField, 
    shouldChangeCharactersIn range: NSRange, 
    replacementString string: String) -> Bool {

    // Create an `NSCharacterSet` set which includes everything *but* the digits
    let inverseSet = NSCharacterSet(charactersIn:"0123456789").inverted

    // At every character in this "inverseSet" contained in the string,
    // split the string up into components which exclude the characters
    // in this inverse set
    let components = string.components(separatedBy: inverseSet)

    // Rejoin these components
    let filtered = components.joined(separator: "")  // use join("", components) if you are using Swift 1.2

    // If the original string is equal to the filtered string, i.e. if no
    // inverse characters were present to be eliminated, the input is valid
    // and the statement returns true; else it returns false
    return string == filtered  
}