How to stop user entering a specific character in Textfield - Swift?

Question

I need to implement a validation in TextField that will stop the user to enter anything besides the allowed requirements.

The validations must contain:

Allow only numbers and decimal points .
The highest allowed number should be 9999999999
Stop users from writing two . in the TextField
Allow a maximum of two decimal numbers after the point (1, 1.9, and 1.99 are acceptable, 1.999 is not acceptable).
It should be aware that if the user tries to paste some text or any number that doesn't pass the requirements, it should not allow that.

I tried an implementation, but sometimes is not working. I don't understand how is that affected.

var dotLocation = Int()

func textField(_ textField: UITextField, shouldChangeCharactersIn range: NSRange, replacementString string: String) -> Bool {
    
    if string == "," {
        textField.text = textField.text! + "."
        return false
    }
    
    let nonNumberSet = CharacterSet(charactersIn: "0123456789.").inverted
    let allowedCharacters = CharacterSet(charactersIn:"0123456789.")//Here change this characters based on your requirement
    let characterSet = CharacterSet(charactersIn: string)
    
    if Int(range.length) == 0 && string.count == 0 {
        return true
    }
    
    if (string == ".") {
        if Int(range.location) == 0 {
            return false
        }
        if dotLocation == 0 {
            dotLocation = range.location
            return true
        } else {
            return false
        }
    }
    
    if range.location == dotLocation && string.count == 0 {
        dotLocation = 0
    }
    
    if dotLocation > 0 && range.location > dotLocation + 2 {
        return false
    }
    
    if range.location >= 10 {
        
        if dotLocation >= 10 || string.count == 0 {
            return true
        } else if range.location > dotLocation + 2 {
            return false
        }
        
        var newValue = (textField.text as NSString?)?.replacingCharacters(in: range, with: string)
        newValue = newValue?.components(separatedBy: nonNumberSet).joined(separator: "")
        textField.text = newValue
        
        return allowedCharacters.isSuperset(of: characterSet)
        
    } else {
        return allowedCharacters.isSuperset(of: characterSet)
    }
    
    return true
}

What would be the best implementation in such a case? Thanks for your contribution.

score 0 · Answer 1 · answered Sep 02 '22 at 05:34

There may be multiple ways of achieving the expected result. This is the first thing came to my mind.

    func textField(_ textField: UITextField, shouldChangeCharactersIn range: NSRange, replacementString string: String) -> Bool {
        
        //hamdle the maximum and minimum decimal counts
        let formatter = NumberFormatter()
        formatter.numberStyle = .none
        formatter.maximumFractionDigits = 2
        formatter.minimumFractionDigits = 0
        
        textField.textColor = UIColor(red: -238.0/255, green: -87.0/255, blue: -61.0/255, alpha: 1.0)
        
        let finalString = "\(textField.text ?? "")\(string)"
        let dpRemovedString = finalString.filter{$0 != "."}
        let dpCount = finalString.filter{$0 == "."}.count
        
        //check for the decimal poin count
        if dpCount > 1{
            return false
        }
        
        //check there is only numbers
        if CharacterSet.decimalDigits.isSuperset(of: CharacterSet(charactersIn: dpRemovedString)){
            if dpRemovedString == ""{
                return false
            }
            var formattedNumber = formatter.string(for: Double(finalString))
            //here highest alllowed number is 999
            if Float(formattedNumber!)! >= 999{
                return false
            }else{
                if string == "."{
                    formattedNumber = formattedNumber! + string
                }
                textField.text = formattedNumber
            }
            return false
        }else{
            return false
        }
    
    }

Check the answers in here to stop user from pasting things in the textfield.

score 0 · Answer 2 · answered Sep 06 '22 at 08:36

You can use Regex to shorten your validation logic.

Make your number checking here:

    fileprivate func isValidNumber(numStr: String) -> Bool {
        var returnValue = true
        let numberRegex = "^[0-9]*(.?)[0-9]{0,2}$"
        do {
            let regex = try NSRegularExpression(pattern: numberRegex)
            let nsString = numStr as NSString
            let results = regex.matches(in: numStr, range: NSRange(location: 0, length: nsString.length))
            if results.isEmpty {
                returnValue = false
            }
        } catch let error as NSError {
            print("invalid regex: \(error.localizedDescription)")
            returnValue = false
        }
        return  returnValue
    }

Then use it inside your func textField(_ textField: UITextField, shouldChangeCharactersIn range: NSRange, replacementString string: String) -> Bool:

    func textField(_ textField: UITextField, shouldChangeCharactersIn range: NSRange, replacementString string: String) -> Bool {
        let currentString = (textField.text ?? "") as NSString
        
        // get final text
        let newString = currentString.replacingCharacters(in: range, with: string)
        
        // allow backspace
        if string.isEmpty && range.length > 0 { return true }
        
        if self.isValidNumber(numStr: newString) {
            if let currentDecimalVal = Decimal.init(string: currentString as String) {
                // If current is already equal max value, skip change
                if currentDecimalVal.isEqual(to: self.maxNumber) { return false }
            }
            if let newDecimalVal = Decimal.init(string: newString) {
                // If new is still less than max, allow change
                return newDecimalVal.isLess(than: self.maxNumber)
            }
        }
        return false
    }

You can test your regex here: https://www.regextester.com/.

You may need to add \ before characters that need to be escaped, in Regex string.

How to stop user entering a specific character in Textfield - Swift?

2 Answers2