UITextField with groupingSeparator when inputting number, tested with localization support.

DecimalTextFieldCoordinator.swift

import Foundation
import UIKit

// Implements UITextFieldDelegate to support input features like showing thousand separator
// Useful for financial App currency number input, tested for multiple localizations, copy and paste numbers into the field.
// Usage: 
//
// 1. Setup
// let textField = UITextField()
// let coordinator = DecimalTextFieldCoordinator(balanceTextField)
// textField.delegate = coordinator
//
// 2. Get the value from the text field:
// balanceTextFieldCoordinator.value
class DecimalTextFieldCoordinator: NSObject, UITextFieldDelegate {
    static func formatter(digits: Int) -> NumberFormatter {
        let formatter = NumberFormatter()
        formatter.locale = Locale.current
        formatter.numberStyle = .decimal
        formatter.usesSignificantDigits = false
        formatter.minimumFractionDigits = 0
        formatter.maximumFractionDigits = digits
        return formatter
    }

    static func allowSaveStoreOnTyping(currentText: String?) -> Bool {
        if currentText?.contains(NumberFormatter().decimalSeparator) == true,
           let last = currentText?.last,
            NSMutableCharacterSet(charactersIn: NumberFormatter().decimalSeparator + "0").isSuperset(of: CharacterSet(charactersIn: String(last))) {
            // Don't change stored value when user is typing decimal value
            return false
        } else {
            return true
        }
    }

    static func allowInput(currentText: String?, string: String) -> Bool {
        let mutableSet = NSMutableCharacterSet(charactersIn: NumberFormatter().decimalSeparator)
        mutableSet.formUnion(with: NSCharacterSet.decimalDigits)
        mutableSet.addCharacters(in: NumberFormatter().groupingSeparator)
        let isNumberSet = mutableSet.isSuperset(of: CharacterSet(charactersIn: string))
        return isNumberSet
    }

    private let formatter: NumberFormatter
    let textField: UITextField

    init(_ textField: UITextField) {
        self.textField = textField

        self.formatter = Self.formatter(digits: .percentoMaxDigits)

        super.init()
        self.textField.addTarget(self, action: #selector(textFieldDidChange(_:)), for: .editingChanged)
    }

    @objc func textFieldDidChange(_ textField: UITextField) {
        guard DecimalTextFieldCoordinator.allowSaveStoreOnTyping(currentText: textField.text) else {
            return
        }
        textField.text = formatter.string(for: value)
    }

    func textField(_ textField: UITextField,
                   shouldChangeCharactersIn range: NSRange,
                   replacementString string: String) -> Bool {
        return DecimalTextFieldCoordinator.allowInput(currentText: textField.text, string: string)
    }

    func textFieldDidBeginEditing(_ textField: UITextField) {
        // clear the text if 0
        if value == 0 {
            textField.text = ""
        }
    }

    private func valueFor(text: String?) -> Decimal {
        let originalText = text?.replacingOccurrences(of: NumberFormatter().groupingSeparator, with: "")

        if let currentValue = originalText {
            let number = formatter.number(from: currentValue) ?? 0.0
            let result = number.decimalValue
            return result
        } else {
            return 0
        }
    }

    var value: Decimal {
        valueFor(text: textField.text)
    }

    func textFieldDidEndEditing(_ textField: UITextField,
                                reason: UITextField.DidEndEditingReason) {
        textField.text = formatter.string(for: value)
    }
}