currencyID.swift

import UIKit

class CurrencyViewController: UIViewController {
    @IBOutlet weak var curent: UITextField!
    @IBOutlet weak var resultValue: UILabel!
    
    override func viewDidLoad() {
        super.viewDidLoad()
        view.backgroundColor = .base
        curent.addTarget(self, action: #selector(myTextFieldDidChange), for: .editingChanged)
    }
    
    override var preferredStatusBarStyle: UIStatusBarStyle {
        return .lightContent
    }
    
    @objc func myTextFieldDidChange(_ textField: UITextField) {
        if let amountString = curent.text?.currencyInputFormatting() {
            curent.text = amountString
        }
    }
    
    @IBAction func didAddValue(_ sender: Any) {
        resultValue.text = curent.text
        curent.text = nil
    }
}

extension String {
    
    // formatting text for currency textField
    func currencyInputFormatting() -> String {
        
        var number: NSNumber!
        let formatter = NumberFormatter()
        formatter.numberStyle = .currencyAccounting
        formatter.currencySymbol = "$"
        formatter.maximumFractionDigits = 2
        formatter.minimumFractionDigits = 2
        
        var amountWithPrefix = self
        
        // remove from String: "$", ".", ","
        // for count string characters in swift2 use "self.characters.count" or swift4+ "self.count"
        let regex = try! NSRegularExpression(pattern: "[^0-9]", options: .caseInsensitive)
        amountWithPrefix = regex.stringByReplacingMatches(in: amountWithPrefix, options: NSRegularExpression.MatchingOptions(rawValue: 0), range: NSMakeRange(0, self.count), withTemplate: "")
        
        let double = (amountWithPrefix as NSString).doubleValue
        number = NSNumber(value: (double / 100))
        
        // if first number is 0 or all numbers were deleted
        guard number != 0 as NSNumber else {
            return ""
        }
        
        return formatter.string(from: number)!
    }
}