This gist provides code for handling appropriate live-formatting of currency input for the current locale with respect to the placement of the currency's symbol.

CurrencyService.swift

internal final class CurrencyService {
    
    // MARK: - Currency Symbol Position
    
    internal static var currencySymbolPosition: CurrencySymbolPosition {
        let currencyFormat = CFNumberFormatterGetFormat(CFNumberFormatterCreate(nil, Locale.current as CFLocale, .currencyStyle)) as NSString
        let positiveNumberFormat = currencyFormat.components(separatedBy: ";")[0] as NSString
        let currencySymbolLocation = positiveNumberFormat.range(of: "¤").location
        let position: CurrencySymbolPosition = currencySymbolLocation == 0 ? .before : .after
        return position
    }
    
    // MARK: - CurrencySymbolPosition
    
    internal enum CurrencySymbolPosition {
        case before
        case after
    }
    
}

HorizontallyExpandableTextField.swift

internal final class HorizontallyExpandableTextField: UITextField {
    
    // MARK: - Properties
    
    private var previousText: String?
    
    internal override var intrinsicContentSize: CGSize {
        var size = super.intrinsicContentSize
        if isEditing, let lastTextBeforeEditing = previousText {
            let originalSize = (lastTextBeforeEditing as NSString).size(attributes: typingAttributes)
            size.width = size.width - originalSize.width
        }
        return size
    }
    
    // MARK: - Initialization
    
    internal override init(frame: CGRect) {
        super.init(frame: frame)
        setupTextChangeNotification()
    }
    
    internal required init?(coder aDecoder: NSCoder) {
        super.init(coder: aDecoder)
        setupTextChangeNotification()
    }
    
    deinit { NotificationCenter.default.removeObserver(self) }
    
    // MARK: - Notification Handling
    
    internal func setupTextChangeNotification() {
        NotificationCenter.default.addObserver(forName: .UITextFieldTextDidChange, object: self, queue: OperationQueue.main) { (notification) in
            self.invalidateIntrinsicContentSize()
        }
        NotificationCenter.default.addObserver(forName: .UITextFieldTextDidBeginEditing, object: self, queue: OperationQueue.main) { (notification) in
            self.previousText = self.text
        }
    }
    
}

ViewController.swift

internal func textField(_ textField: UITextField, shouldChangeCharactersIn range: NSRange, replacementString string: String) -> Bool {
        if string.characters.count == 0 && range.length >= 1 { return true }
        guard let text = textField.text else { return false }
        guard let currencySymbol = formatter.currencySymbol else { return false }
        guard let decimalSeparator = formatter.currencyDecimalSeparator else { return false }
        guard let groupingSeparator = formatter.currencyGroupingSeparator else { return false }
        guard let newText = (text as NSString?)?.replacingCharacters(in: range, with: string) else { return false }
        let symbolsStripped = newText.replacingOccurrences(of: currencySymbol, with: "").replacingOccurrences(of: decimalSeparator, with: "").replacingOccurrences(of: groupingSeparator, with: "")
        guard let integer = Int(symbolsStripped) else { return false }
        let divisor = Int(pow(Double(10), Double(formatter.maximumFractionDigits)))
        let double = Double(integer) / Double(divisor)
        guard let formatted = formatter.string(from: double as NSNumber) else { return false }
        textField.text = formatted.replacingOccurrences(of: currencySymbol, with: "").trimmingCharacters(in: .whitespaces)
        return false
    }

This code is useful for handling formatted currency input for the current locale. It takes into account the location of the currency's symbol so that the currency can be live-formatted upon input without having to worry about disabling the ability to delete the currency symbol in the text input view.

For example, if you place an UILabel on either side of an UITextField, you can determine the position of the currency's symbol and set the appropriate label to contain the symbol. You can then set the text of the unused label to nil. This will display the currency symbol and the currency's value appropriately.

Coupled with an instance of HorizontallyExpandableTextField, the trailing label can be configured to be pinned to the trailing edge of the text field. This will allow the label to keep an explicit distance from the trailing edge of the text field for currencies that display the symbol after the value.