银行卡号输入格式化-swift

 func textField(textField: UITextField, shouldChangeCharactersInRange range: NSRange, replacementString string: String) -> Bool {
        
        var text = textField.text! as NSString
        //设置格式为数字
        let characterSet = NSCharacterSet(charactersInString: "0123456789")
        //去掉空格
        let str = string.stringByReplacingOccurrencesOfString(" ", withString: "")
        let nsStr = str as NSString
        if nsStr.rangeOfCharacterFromSet(characterSet.invertedSet).location != NSNotFound {
            return false
        }
        //将输入的数字添加给textfield
        text = text.stringByReplacingCharactersInRange(range, withString: string)
        //去掉空格
        text = text.stringByReplacingOccurrencesOfString(" ", withString: "")
        
        var newString = ""
        while text.length > 0 {
            let substring = text.substringToIndex(getMin(text.length, num2: 4))
            newString = newString.stringByAppendingString(substring)
            if substring.characters.count == 4 {
                newString = newString.stringByAppendingString(" ")
            }
            text = text.substringFromIndex(getMin(text.length, num2: 4))
        }
        newString = newString.stringByTrimmingCharactersInSet(characterSet.invertedSet)
        if newString.characters.count >= 20 { //设置银行卡位数为16
            return false
        }
        numberTextField.text = newString
        return false
        
    }
    
    
    func getMin(num1 : Int, num2: Int) -> Int {
        
        if num1 <= num2 {
            return num1
        }else{
            return num2
        }
        
    }