Simple Phone Number Formatting for String + UITextField

PhoneFormat.swift

// Usage:
// ------
//  /* add the action somewhere: */
//  phoneTextField.addTarget(self, action: #selector(updatePhoneFormat), for: .editingChanged)
//  /* implement the update action: */
//  func updatePhoneFormat() { phoneTextField.formatPhoneNumber() }

import UIKit

extension String {
    var phoneFormat: String {
        /**
         *  NOTE: This assumes US country code.
         */
        
        var text = trimNonNumbers()
        let length = text.lengthOfBytes(using: .utf8)
        switch length {
        case 11 where text.characters.first == "1":
            // example: `1 (222) 333 4444`
            text.insert(" ", at: text.index(text.startIndex, offsetBy: 7))
            text.insert(contentsOf: ") ".characters, at: text.index(text.startIndex, offsetBy: 4))
            text.insert(contentsOf: " (".characters, at: text.index(text.startIndex, offsetBy: 1))
        case 8...10 where text.characters.first != "1":
            // example: `(222) 333 4444`
            text.insert(contentsOf: " ".characters, at: text.index(text.startIndex, offsetBy: 6))
            text.insert(contentsOf: ") ".characters, at: text.index(text.startIndex, offsetBy: 3))
            text.insert(contentsOf: "(".characters, at: text.index(text.startIndex, offsetBy: 0))
        default:
            break
        }
        return text
    }
    
    func trimNonNumbers() -> String {
        return components(separatedBy: CharacterSet.decimalDigits.inverted).joined()
    }
}

extension UITextField {
    func formatPhoneNumber() {
        /**
         *  Note: This dance is to keep the cursor position in the same relative spot
         *  while editing characters in the middle of the phone number.
         */
        
        let pos = selectedTextRange?.start
        
        let range = textRange(from: beginningOfDocument, to: pos ?? endOfDocument)
        
        let diffBefore: Int = {
            guard let range = range else { return 0 }
            let leftOfCursorLength = text(in: range)?.lengthOfBytes(using: .utf8) ?? 0
            let leftOfCursorTrimmed = text(in: range)?.trimNonNumbers().lengthOfBytes(using: .utf8) ?? 0
            return leftOfCursorTrimmed - leftOfCursorLength
        }()
        
        text = text?.phoneFormat
        
        let diffAfter: Int = {
            guard let range = range else { return 0 }
            let leftOfCursorLength = text(in: range)?.lengthOfBytes(using: .utf8) ?? 0
            let leftOfCursorTrimmed = text(in: range)?.trimNonNumbers().lengthOfBytes(using: .utf8) ?? 0
            let temp = leftOfCursorTrimmed - leftOfCursorLength
            guard
                let pos = pos,
                let newPos = position(from: pos, offset: -temp),
                let newRange = textRange(from: beginningOfDocument, to: newPos)
            else {
                return temp
            }
            let newLeftOfCursorLength = text(in: newRange)?.lengthOfBytes(using: .utf8) ?? 0
            let newLeftOfCursorTrimmed = text(in: newRange)?.trimNonNumbers().lengthOfBytes(using: .utf8) ?? 0
            return newLeftOfCursorTrimmed - newLeftOfCursorLength
        }()
        
        if let oldPos = pos, let newPos = position(from: oldPos, offset: diffBefore - diffAfter) {
            selectedTextRange = textRange(from: newPos, to: newPos)
        }
    }
}