Simple SwiftUI view for testing https://stackoverflow.com/questions/58369407/change-the-options-in-a-picker-dynamically-using-distinct-arrays

Converter.swift

import SwiftUI

extension String: Identifiable {
    
    public var id: String { self }
}

struct ContentView: View {
    @State private var baseNumber = ""
    @State private var dimensionSelection: String = ""
    @State private var baseUnitSelection: String = ""
    @State private var convertedUnitSelection: String = ""

    let temperatureUnits: [String] = ["Celsius", "Fahrenheit", "Kelvin"]
    let lengthUnits: [String] = ["meters", "kilometers", "feet", "yards", "miles"]
    let timeUnits: [String] = ["seconds", "minutes", "hours", "days"]
    let volumeUnits: [String] = ["milliliters", "liters", "cups", "pints", "gallons"]
    let dimensionChoices: [String] = ["Temperature", "Length", "Time", "Volume"]
    let dimensions: [(String, [String])]

    init () {
        dimensions = zip(dimensionChoices, [temperatureUnits, lengthUnits, timeUnits, volumeUnits])
            .reduce([], { acc, val in acc + [(val.0, val.1)] })
    }

    var convertedValue: Double {

        var result: Double = 0
        let base = Double(baseNumber) ?? 0
        if baseUnitSelection == "Celsius" {
            if convertedUnitSelection == "Celsius" {
                result = base
            } else if convertedUnitSelection == "Fahrenheit" {
                result = base * 9/5 + 32
            } else if convertedUnitSelection == "Kelvin" {
                result = base + 273.15
            }
        }

        return result
    }

    
    var dimensionUnitsView: some View {
        (dimensions.first(where: {(key, _) in key == dimensionSelection })?.1).map { options in
            Group {
                Section(header: Text("Select the base unit")) {
                    Picker("Base Unit", selection: self.$baseUnitSelection) {
                        ForEach(options) { selection in
                            Text(selection)
                        }
                    }.pickerStyle(SegmentedPickerStyle())
                }
                
                Section(header: Text("Select the unit to convert to")) {
                    Picker("Converted Unit", selection: self.$convertedUnitSelection) {
                        ForEach(options) { selection in
                            Text(selection)
                        }
                    }.pickerStyle(SegmentedPickerStyle())
                }
            }
        }
    }

    var body: some View {
        NavigationView {
            Form {
                Section {
                    TextField("Enter a number", text: $baseNumber)
                        .keyboardType(.decimalPad)
                }

                Section(header: Text("Select the type of conversion")) {
                    Picker("Dimension", selection: $dimensionSelection) {
                        ForEach(dimensions.map({ $0.0 })) { dimension in
                            Text(dimension)
                        }
                    }.pickerStyle(SegmentedPickerStyle())
                }
                
                dimensionUnitsView
                
                Section(header: Text("The converted value is")) {
                    Text("\(convertedValue)")
                }
                
            }.navigationBarTitle("Unit Converter")
        }
    }
}

struct ContentView_Previews: PreviewProvider {
    static var previews: some View {
        ContentView()
    }
}