How to input currency format on a text field (from right to left) using Swift?

For Swift 3. Input currency format on a text field (from right to left)

override func viewDidLoad() {
    super.viewDidLoad()

    textField.addTarget(self, action: #selector(myTextFieldDidChange), for: .editingChanged)
}

@objc func myTextFieldDidChange(_ textField: UITextField) {

    if let amountString = textField.text?.currencyInputFormatting() {
        textField.text = amountString
    }
}

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: "$", ".", ","
        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)!
    }
}

I started with Leo Dabus' answer (which didn't work out of the box for me) and in the process of trying to simplify and make it work ended up with this, which I think is pretty lean & clean if I do say so myself 😎

class CurrencyTextField: UITextField {

    /// The numbers that have been entered in the text field
    private var enteredNumbers = ""

    private var didBackspace = false

    var locale: Locale = .current

    override init(frame: CGRect) {
        super.init(frame: frame)
        commonInit()
    }

    required init?(coder: NSCoder) {
        super.init(coder: coder)
        commonInit()
    }

    private func commonInit() {
        addTarget(self, action: #selector(editingChanged), for: .editingChanged)
    }

    override func deleteBackward() {
        enteredNumbers = String(enteredNumbers.dropLast())
        text = enteredNumbers.asCurrency(locale: locale)
        // Call super so that the .editingChanged event gets fired, but we need to handle it differently, so we set the `didBackspace` flag first
        didBackspace = true
        super.deleteBackward()
    }

    @objc func editingChanged() {
        defer {
            didBackspace = false
            text = enteredNumbers.asCurrency(locale: locale)
        }

        guard didBackspace == false else { return }

        if let lastEnteredCharacter = text?.last, lastEnteredCharacter.isNumber {
            enteredNumbers.append(lastEnteredCharacter)
        }
    }
}

private extension Formatter {
    static let currency: NumberFormatter = {
        let formatter = NumberFormatter()
        formatter.numberStyle = .currency
        return formatter
    }()
}

private extension String {
    func asCurrency(locale: Locale) -> String? {
        Formatter.currency.locale = locale
        if self.isEmpty {
            return Formatter.currency.string(from: NSNumber(value: 0))
        } else {
            return Formatter.currency.string(from: NSNumber(value: (Double(self) ?? 0) / 100))
        }
    }
}

You can create a currency text field subclassing UITextField. Add a target for UIControlEvents .editingChanged. Add a selector method to filter the digits from your textfield string. After filtering all non digits from your string you can format again your number using NumberFormatter as follow:

Xcode 11.5 • Swift 5.2 or later

import UIKit

class CurrencyField: UITextField {
    var decimal: Decimal { string.decimal / pow(10, Formatter.currency.maximumFractionDigits) }
    var maximum: Decimal = 999_999_999.99
    private var lastValue: String?
    var locale: Locale = .current {
        didSet {
            Formatter.currency.locale = locale
            sendActions(for: .editingChanged)
        }
    }
    override func willMove(toSuperview newSuperview: UIView?) {
        // you can make it a fixed locale currency if needed
        // self.locale = Locale(identifier: "pt_BR") // or "en_US", "fr_FR", etc
        Formatter.currency.locale = locale
        addTarget(self, action: #selector(editingChanged), for: .editingChanged)
        keyboardType = .numberPad
        textAlignment = .right
        sendActions(for: .editingChanged)
    }
    override func deleteBackward() {
        text = string.digits.dropLast().string
        // manually send the editingChanged event
        sendActions(for: .editingChanged)
    }
    @objc func editingChanged() {
        guard decimal <= maximum else {
            text = lastValue
            return
        }
        text = decimal.currency
        lastValue = text
    }
}

---

extension CurrencyField {
    var doubleValue: Double { (decimal as NSDecimalNumber).doubleValue }
}

---

extension UITextField {
     var string: String { text ?? "" }
}

---

extension NumberFormatter {
    convenience init(numberStyle: Style) {
        self.init()
        self.numberStyle = numberStyle
    }
}

---

private extension Formatter {
    static let currency: NumberFormatter = .init(numberStyle: .currency)
}

---

extension StringProtocol where Self: RangeReplaceableCollection {
    var digits: Self { filter (\.isWholeNumber) }
}

---

extension String {
    var decimal: Decimal { Decimal(string: digits) ?? 0 }
}

---

extension Decimal {
    var currency: String { Formatter.currency.string(for: self) ?? "" }
}

---

extension LosslessStringConvertible {
    var string: String { .init(self) }
}

---

View Controller

class ViewController: UIViewController {

    @IBOutlet weak var currencyField: CurrencyField!
    override func viewDidLoad() {
        super.viewDidLoad()
        currencyField.addTarget(self, action: #selector(currencyFieldChanged), for: .editingChanged)
        currencyField.locale = Locale(identifier: "pt_BR") // or "en_US", "fr_FR", etc
    }
    @objc func currencyFieldChanged() {
        print("currencyField:",currencyField.text!)
        print("decimal:", currencyField.decimal)
        print("doubleValue:",(currencyField.decimal as NSDecimalNumber).doubleValue, terminator: "\n\n")
    }
}

Sample project

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SwiftUI version of this post here