example of custom binding

ContentView.swift

//
//  ContentView.swift
//  tmp073
//
//  Created by Ivo Vacek on 03/03/2020.
//  Copyright © 2020 Ivo Vacek. All rights reserved.
//

import SwiftUI
struct UnitPicker: View {
    let label: String
    let data: [String]
    @Binding var selection: Int
    var body: some View {
        Picker(label, selection: $selection) {
            ForEach(0 ..< data.count) { (idx) in
                Text(self.data[idx])
            }
        }.pickerStyle(WheelPickerStyle())
    }
}
struct MyTextFieldStyle: TextFieldStyle {
    @Binding var focused: Bool
    func _body(configuration: TextField<Self._Label>) -> some View {
        configuration
            .padding(10)
            .background(
                RoundedRectangle(cornerRadius: 10, style: .continuous)
                    .stroke(focused ? Color.secondary : Color.clear, lineWidth: 3)
        ).padding()
    }
}
struct ContentView: View {
    let units = [UnitTemperature.celsius, UnitTemperature.fahrenheit, UnitTemperature.kelvin]
    @State var value = Measurement(value: 0.0, unit: UnitTemperature.celsius)
    @State var toUnit = 0
    @State var empty = true
    var result: String {
        format(value: value.converted(to: units[toUnit]).value)
    }
    var body: some View {
        let unitBinding = Binding<Int>(get: {
            self.units.firstIndex(of: self.value.unit) ?? 0
        }) { (idx) in
            let v = self.value.value
            self.value = Measurement(value: v, unit: self.units[idx])
        }
        let binding = Binding<String>(get: {
            if self.empty { return "" } else {
                return self.format(value: self.value.value)
            }
        }) { (str) in
            self.empty = str.isEmpty
            // before string to double
            // replace decimal separator to "." !!!
            let dot = Locale.current.decimalSeparator ?? "."
            let s = str.replacingOccurrences(of: dot, with: ".")
            self.value.value = Double(s) ?? 0.0
        }
        return Form {
            Section(header: Text("Temperature").font(.title)) {
                HStack {
                    TextField("0", text: binding)
                        .font(.title)
                        .textFieldStyle(MyTextFieldStyle(focused: .constant(true)))
                        .keyboardType(.numbersAndPunctuation)
                    Spacer(minLength: 20)
                    UnitPicker(label: "", data: units.map {$0.symbol }, selection: unitBinding)
                        .frame(width: 50, height: 100).clipped()
                }
                HStack {
                    TextField("0", text: .constant(result))
                        .font(.title)
                        .disabled(true)
                        .textFieldStyle(MyTextFieldStyle(focused: .constant(false)))
                    Spacer(minLength: 20)
                    UnitPicker(label: "", data: units.map {$0.symbol }, selection: $toUnit)
                        .frame(width: 50, height: 100).clipped()
                }
            }
        }.padding()
    }
    func format(value: Double) -> String {
        // for presentation
        // replace "." with decimal separator
        let dot = Locale.current.decimalSeparator ?? "."
        var s = String(format: "%f", value)
            .replacingOccurrences(of: ".", with: dot)
            .reversed()
            .drop(while: {$0 == "0"})
            .reversed()
            .map(String.init)
            .joined()
        if let last = s.last, String(last) == dot {
            s.removeLast()
        }
        return s
    }
}
struct ContentView_Previews: PreviewProvider {
    static var previews: some View {
        ContentView()
    }
}