Is it possible to dynamically update the currency formatting of SwiftUI TextField?

I posted about the related issue that specifically relates to the formatting of Euros (EUR) here.

I think this works. Try on simulator. Preview gives unpredictable results.

import SwiftUI

struct Amount {
    var value: Decimal
    var currency: CurrencyCode
}

struct ContentView: View {
    @State var amount = Amount(value: Decimal(), currency: .GBP)

    var body: some View {
        CurrencyAmount(title: "Some label", amount: $amount)
    }
}

struct ContentView_Previews: PreviewProvider {
    static var previews: some View {
        ContentView(amount: Amount(value: Decimal(), currency: .GBP))
    }
}

struct CurrencyAmount: View {
    let title: String
    let prompt: String = ""
    @Binding var amount: Amount
    @State var currency: CurrencyCode = .GBP

    var body: some View {
        HStack {
            Spacer(minLength: 80)
            TextField(title, value: $amount.value, format: .currency(code: amount.currency.rawValue), prompt: Text(prompt))
            CurrencyPicker(selected: $currency)
            Spacer(minLength: 80)
        }
        .onChange(of: currency) { newValue in
            amount.currency =  newValue
        }
    }
}

struct CurrencyPicker: View {
    @Binding var selected: CurrencyCode
    let label = "Currency"
    var body: some View {
        Picker(label, selection: $selected) {
            ForEach(CurrencyCode.allCases) {
                Text($0.rawValue)
                    .tag($0)
            }
        }
    }
}

enum CurrencyCode: String, CaseIterable, Identifiable {
    var id: String { self.rawValue }
    case AUD, CAD, EUR, GBP, NZD, USD
}

I understand that you want to change the currency symbol, but also want to change the TextField to show the new currency in the proper format.

I had the same problem. While working on it I realized that currency symbol and number formatting are actually different things, and not necessarily connected. If an American speaks about ten thousand dollars, he will write "$10,000.00". If a German speaks about a thousand Euros, he will write "10.000,00 €". And if he speaks about a thousand dollars, in his own country, he will probably write "10.000,00 $". A Frenchman will write a thousand Euros as "10 000,00 €". A Dutchman will write "€ 10.000,00". Etc.

So, to properly format a currency you need to know both the valuta and the region where the expression is used.

This information is in SwiftUI captured in NumberFormatter.locale, which contains both a language code and a country code. Examples in String form: "en-US", "en-GB", etc. The following code takes care of that.

A trick is used to update the TextField whenever another locale is chosen in the Picker. See the function setLocale(:). This is because the TextField does not monitor the .locale property of the NumberFormatter class object, and I know of no way to make it do so. Therefore amount, which is monitored, is changed and then changed back. An ugly hack, but I think safe and it works (at least until the compiler optimizes it away... and there are ways around that I suppose.) Anyway, I would like to learn of a more Swifty way.

I hope this is what you are looking for!

import SwiftUI

struct ContentView: View {

    private var numberFormatter: NumberFormatter

    init(numberFormatter: NumberFormatter = NumberFormatter()) {
        self.numberFormatter = numberFormatter
        self.numberFormatter.usesGroupingSeparator = true
        self.numberFormatter.numberStyle = .currency
        self.numberFormatter.locale = Locale(identifier: "nl-NL")
    }

    @State private var amount = 1234567.89

    let locales = ["nl-NL", "de-DE", "fr-FR", "en-US", "en-GB", "th-TH", "az-AZ"]

    @State private var locale = "nl-NL"
    @State private var newLocale = "nl-NL"

    func setLocale(to locale: String) {
        numberFormatter.locale = Locale(identifier: locale)
        amount += 1
        amount -= 1
    }

    var body: some View {

        NavigationView {

            Form {
                Text("Current locale: \(locale)")
                TextField("Amount", value: $amount, formatter: numberFormatter)
                Picker("Choose your new locale:", selection: $newLocale) {
                    ForEach(locales, id: \.self) { locale in
                        Text(locale)
                    }
                }
                .onChange(of: newLocale) { newValue in
                    setLocale(to: newLocale)
                    locale = newLocale
                }
            }
        }
    }
}

Disclaimer: Only tested on the XCode simulator.