Formatting while editing phone number

[k06a]

I am trying to achieve UITextField with formatted phone number while user input progress.
My algo:

1. Get only digits from phone field
2. Format digits as phone number
3. Set UITextFields text property new formatted phone number

All works fine, until user tries to edit phone number in the middle.
I can not properly save and restore selectedRange because of formatting chars " -()"

Is there any solution for this problem?

[k06a]

Solved via saving number of digits to the right from cursor position:

func countDigitsRighterThatCursor(textField: UITextField) -> Int {
    let cursorPos = textField.selectedTextRange.start
    var digits = -1
    var index = 0
    for ch in textField.text.unicodeScalars {
        if digits == -1 && cursorPos == textField.positionFromPosition(textField.beginningOfDocument, offset: index) {
            digits = 0
        }
        if digits >= 0 && NSCharacterSet.decimalDigitCharacterSet().longCharacterIsMember(ch.value) {
            digits++
        }
        index++;
    }

    return digits
}

func setCursor(textField: UITextField, whereHaveDigitsFromRight digits: Int) {
    let length = countElements(textField.text)
    var index = 0
    var digitsSkipped = 0
    for ch in reverse(textField.text.unicodeScalars) {
        if digitsSkipped == digits {
            let pos = textField.positionFromPosition(textField.endOfDocument, inDirection: .Left, offset: index)
            textField.selectedTextRange = textField.textRangeFromPosition(pos, toPosition: pos)
            return
        }
        if NSCharacterSet.decimalDigitCharacterSet().longCharacterIsMember(ch.value) {
            digitsSkipped++
        }
        index++
    }
}

@IBAction func phoneChanged(textField: UITextField) {
    if countElements(textField.text) == 0 {
        return
    }

    let rightCountDigits = countDigitsRighterThatCursor(textField)
    textField.text = formattedPhoneNumber(textField.text)
    setCursor(textField, whereHaveDigitsFromRight: rightCountDigits)
}