SwiftUI NumberField - TextField with custom display/edit number formatters

NumberField.swift

import SwiftUI

public struct NumberField: View {

    public init(_ title: String,
                value: Binding<NSNumber?>,
                decorator: @escaping (String) -> String = { $0 }) {
        self.title = title
        self.value = value
        let formatter = NumberFormatter()
        formatter.maximumFractionDigits = -1
        self.displayFormatter = formatter
        self.editingFormatter = formatter
        self.decorator = decorator
    }

    public init(_ title: String,
                value: Binding<NSNumber?>,
                formatter: NumberFormatter,
                decorator: @escaping (String) -> String = { $0 }) {
        self.title = title
        self.value = value
        self.displayFormatter = formatter
        self.editingFormatter = formatter
        self.decorator = decorator
    }

    public init(_ title: String,
                value: Binding<NSNumber?>,
                displayFormatter: NumberFormatter,
                editingFormatter: NumberFormatter,
                decorator: @escaping (String) -> String = { $0 }) {
        self.title = title
        self.value = value
        self.displayFormatter = displayFormatter
        self.editingFormatter = editingFormatter
        self.decorator = decorator
    }

    public var body: some View {
        TextField(title, text: Binding(get: {
            self.isEditing ? self.editingValue : self.formattedValue
        }, set: { newValue in
            self.editingValue = newValue
            self.updateValue(with: newValue)
        }), onEditingChanged: { isEditing in
            self.isEditing = isEditing
            self.editingValue = self.formattedValue
        })
    }

    let title: String
    let value: Binding<NSNumber?>
    let displayFormatter: NumberFormatter
    let editingFormatter: NumberFormatter
    let decorator: (String) -> String

    @State private var editingValue: String = ""
    @State private var isEditing: Bool = false

    private var formattedValue: String {
        guard let value = self.value.wrappedValue else { return "" }
        let formatter = isEditing ? editingFormatter : displayFormatter
        guard let formattedValue = formatter.string(for: value) else { return "" }
        return isEditing ? formattedValue : decorator(formattedValue)
    }

    private func updateValue(with string: String) {
        let newValue = editingFormatter.number(from: string)
        let newString = newValue.map { editingFormatter.string(for: $0) } as? String
        value.wrappedValue = newString.map { editingFormatter.number(from: $0) } as? NSNumber
    }

}

NumberField_Example.swift

import SwiftUI

struct ExampleView: View {

    @State private var amount: Decimal?

    var body: some View {
        Form {
            NumberField(
                "Enter amount",
                value: Binding(get: {
                    self.amount.map { NSDecimalNumber(decimal: $0) }
                }, set: { number in
                    self.amount = number?.decimalValue
                }),
                displayFormatter: .currency,
                editingFormatter: .currencyEditing,
                decorator: { "[ \($0) ]" }
            )
            Text("Amount: \(String(describing: amount))")
        }
    }

}

extension NumberFormatter {

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

    static let currencyEditing: NumberFormatter = {
        let formatter = NumberFormatter()
        formatter.numberStyle = .decimal
        formatter.groupingSeparator = ""
        formatter.minimumFractionDigits = NumberFormatter.currency.minimumFractionDigits
        formatter.maximumFractionDigits = NumberFormatter.currency.maximumFractionDigits
        return formatter
    }()

}

This NumberField is super handy. I have one question:

        formatter.maximumFractionDigits = -1

why is it set to -1?

Honestly, I don't remember, I wrote this code four years ago. It could be a workaround for an early-SwiftUI bug. Anyway, this code is a bit outdated, we should not use TextField.init(_:text:onEditingChanged:) as it was deprecated by Apple.

Makes sense.
Thanks for the reply (and for the snippet!).