Trying (and failing) to use formatters SwiftUI TextFields

FormattedTextFieldView.swift

import Foundation
import SwiftUI


struct FormattedTextFieldView : View {
    @State private var double = 0.5
    @State private var nsNumber: NSNumber = 0.5


    static let numberFormatter: NumberFormatter = {
        let formatter = NumberFormatter()
        formatter.numberStyle = .decimal
        formatter.minimum = 0
        formatter.maximum = 10
        formatter.isPartialStringValidationEnabled = true
        formatter.isLenient = true
        return formatter
    }()


    var body: some View {
        Form {
            Section(header: Text("Bound to Double")) {
                HStack {
                    Text("Double")
                    Spacer()
                    TextField("Double Value", value: $double.asNSNumber(), formatter: type(of: self).numberFormatter)
                        .multilineTextAlignment(.trailing)
                }

                Slider(value: $double, from: 0, through: 10)
            }

            Section(header: Text("Bound to NSNumber")) {
                HStack {
                    Text("NSNumber")
                    Spacer()
                    TextField("NSNumber Value", value: $nsNumber, formatter: type(of: self).numberFormatter)
                        .multilineTextAlignment(.trailing)
                }

                Slider(value: $nsNumber.asDouble(), from: 0, through: 10)
            }
        }
    }
}


extension Binding where Value == Double {
    func asNSNumber() -> Binding<NSNumber> {
        return Binding<NSNumber>(getValue: {
            NSNumber(value: self.value)
        }, setValue: {
            self.value = $0.doubleValue
        })
    }
}


extension Binding where Value == NSNumber {
    func asDouble() -> Binding<Double> {
        return Binding<Double>(getValue: {
            self.value.doubleValue
        }, setValue: {
            self.value = NSNumber(value: $0) }
        )
    }
}