Python, безопасно, песочница [дубликат]

Порт Swift 4.2 с лучшим ответом здесь @ almas-adlibek

Набор переменных конфигурации:

private let kMaxTextLength = 8
private let kZeroDotted = "0."
private let kZero = "0"
private let kDoubleDot = ".."
private let kDot = "."
private let kPeriod = ","

Теперь Swift 4 преобразует часть кода.

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

    guard let oldText = textField.text, let swiftRange = Range(range, in: oldText) else {
        return true
    }

    let newText = oldText.replacingCharacters(in: swiftRange, with: string)

    var currentText =  textField.text?.trimmingCharacters(in: CharacterSet.whitespacesAndNewlines)

    // Change the "," (appears in other locale keyboards, such as russian) key ot "."
    currentText = currentText?.replacingOccurrences(of: kPeriod, with: kDot)

    // Check the statements of decimal value.
    if (newText == kDot) {
        textField.text = kZeroDotted;
        return false
    }

    if (newText.range(of: kDoubleDot) != nil) {
        textField.text = newText.replacingOccurrences(of: kDoubleDot, with: kDot);
        return false
    }

    // If second dot is typed, ignore it.
    let dots = newText.components(separatedBy: kDot)
    if(dots.count > 2) {
        textField.text = currentText;
        return false
    }

    // If first character is zero and second character is > 0, replace first with second. 05 => 5;
    if(newText.count == 2) {
        if(newText[0...0] == kZero && newText != kZeroDotted) {
            textField.text = newText[1...1]
            return false
        }
    }

    // Check the max characters typed.
    let oldLength = textField.text?.count ?? 0
    let replacementLength = string.count
    let rangeLength = range.length

    let newLength = oldLength - rangeLength + replacementLength;
    let returnKey = string.rangeOfCharacter(from: CharacterSet.newlines) != nil

    return newLength <= kMaxTextLength || returnKey;
}