How do you dynamically format a number to have commas in a UITextField entry?

Question

I want to have commas dynamically added to my numeric UITextField entry while the user is typing.

For example: 123,456 and 12,345,678

Answer 1

For Swift 4.0 Version of Lyndsey Scott's answer:

   func textField(_ textField: UITextField, shouldChangeCharactersIn range: NSRange, replacementString string: String) -> Bool {
        if ((string == "0" || string == "") && (textField.text! as NSString).range(of: ".").location < range.location) {
            return true
        }

        // First check whether the replacement string's numeric...
        let cs = NSCharacterSet(charactersIn: "0123456789.").inverted
        let filtered = string.components(separatedBy: cs)
        let component = filtered.joined(separator: "")
        let isNumeric = string == component

        // Then if the replacement string's numeric, or if it's
        // a backspace, or if it's a decimal point and the text
        // field doesn't already contain a decimal point,
        // reformat the new complete number using
        if isNumeric {
            let formatter = NumberFormatter()
            formatter.numberStyle = .decimal
            formatter.maximumFractionDigits = 8
            // Combine the new text with the old; then remove any
            // commas from the textField before formatting
            let newString = (textField.text! as NSString).replacingCharacters(in: range, with: string)
            let numberWithOutCommas = newString.replacingOccurrences(of: ",", with: "")
            let number = formatter.number(from: numberWithOutCommas)
            if number != nil {
                var formattedString = formatter.string(from: number!)
                // If the last entry was a decimal or a zero after a decimal,
                // re-add it here because the formatter will naturally remove
                // it.
                if string == "." && range.location == textField.text?.count {
                    formattedString = formattedString?.appending(".")
                }
                textField.text = formattedString
            } else {
                textField.text = nil
            }
        }
        return false

    }