Hallo, I am trying to write a slightly generic view for user to enter weights in different units (there is a textfield and picker for the units), but I am not able to pass in the data in correct format. Can you please advise on how to resolve this?

import SwiftUI

//Mock data object, real one has more properties
class PersonalData: ObservableObject
{
    @Published var weight: Measurement<UnitMass>?

    init(_ weight: Double)
    {
        self.weight = Measurement(value: weight, unit: .kilograms)
    }

}

//Mock data object, real one has more properties
class ToolData: ObservableObject
{
    @Published var payload: Measurement<UnitMass>?
}

// My "generic" view to handle user input of weight
struct  EntryFieldView: View
{
    private let kg = UnitMass.kilograms
    private let pounds = UnitMass.pounds
    private var nf: NumberFormatter
                                    {
                                        let result = NumberFormatter()
                                        result.maximumFractionDigits = 1
                                        result.numberStyle = .none
                                        return result
                                    }

    private var mf: MeasurementFormatter
                                    {
                                        let result = MeasurementFormatter()
                                        result.unitOptions = .providedUnit
                                        result.unitStyle = .medium
                                        return result
                                    }

    @Binding var mass: Measurement<UnitMass>?   //this seems to be the issue
    @State var currentUnit : UnitMass = (Locale.current.usesMetricSystem ? .kilograms : .pounds)

    private var valueBinding: Binding<Double>
    {
        Binding(
            get:
                {
                    mass!.value
                },
            set:
                { newVal in
                    mass!.value = newVal
                })
    }

    var entry: some View
    {
        VStack{

            if self.mass == nil
            {
                Button("+"){
                    mass = Measurement(value: 0, unit: currentUnit)
                }
            }
            else
            {
                TextField("Weight", value: valueBinding, formatter: nf)
            }
        }
    }

    var unitPicker: some View
    {
        VStack{
            Picker("\(mf.string(from: currentUnit))", selection: $currentUnit){
                Text(mf.string(from: kg)).tag(kg)
                Text(mf.string(from: pounds)).tag(pounds)
            }.onChange(of: currentUnit, perform: { value in
                mass?.convert(to: currentUnit)
            }).pickerStyle(SegmentedPickerStyle())
            .disabled(mass == nil)

        }
    }

    var body: some View
    {
        HStack
        {
            entry
            unitPicker
        }
    }
}

struct MasterView: View
{
    @EnvironmentObject var person: PersonalData
    @EnvironmentObject var tool : ToolData

    var body: some View
    {
        NavigationView
        {
            VStack
            {
                NavigationLink(
                    destination: EntryFieldView(mass: Binding(person.weight), currentUnit: .kilograms), //+++++ HERE I GET ERROR
                    label: {

                        Text("Person weight: \(person.weight?.description ?? "none")")
                    })
                NavigationLink(
                    destination: EntryFieldView(mass: Binding(tool.payload), currentUnit: .kilograms), //+++++ HERE I GET ERROR
                    label: {
                        Text("Tool payload: \(tool.payload?.description ?? "none")")
                    })

            }.navigationTitle("Master")
        }
    }
}

struct MasterView_Previews: PreviewProvider {
    static var previews: some View {

        MasterView()
            .environmentObject(PersonalData(80))
            .environmentObject(ToolData())

    }
}

The error I get is: "cannot convert value of type 'Measurement<UnitMass>?' to expected argument type 'Binding<Measurement<UnitMass>>'"

thank you for any help, I am sure there has to be an elegant way how to do this but I could not find one.

You can't just pass a value directly into the Binding initialiser to create one. To access the two-way binding value from an @EnvironmentObject property (in your case), you need to use the $ syntax like this:

EntryFieldView(mass: $person.weight, currentUnit: .kilograms)

Check out this page regarding the error message, and this page explaining two-way bindings.

@njansari thank you for the answer, however it does not work either.

The error message is now

cannot convert value of type 'Binding<Published<Measurement<UnitMass>?>.Publisher>' to expected argument type 'Binding<Measurement<UnitMass>?>'

You must have changed something else in your code because when I run your original code with the changes I suggested, everything compiles and I do not see you new error message.

Having looked at your new error message, though, it seems you may have wrapped one of your measurement properties in a Published struct. Is this the case, or is there something else?

Sorry, you are probably right. I was experimenting a lot... I have resolved the issue now, thank you!