My solution for #100DaysOfSwiftUI day 19

ContentView.swift

import SwiftUI

enum UnitTypes: CaseIterable, Hashable, Identifiable {
  case temperature
  case length
  case time
  case volume
  
  var name: String { "\(self)".capitalized }
  var id: UnitTypes { self }
  
  var subTypes: [Dimension] {
    switch self {
      case .temperature:
        return [UnitTemperature.celsius, UnitTemperature.fahrenheit, UnitTemperature.kelvin]
      case .length:
        return [UnitLength.meters, UnitLength.kilometers, UnitLength.feet, UnitLength.yards, UnitLength.miles]
      case .time:
        return [UnitDuration.seconds, UnitDuration.minutes, UnitDuration.hours, UnitLength(symbol: "day", converter: UnitConverterLinear(coefficient: 86_400))]
      case .volume:
        return [UnitVolume.milliliters, UnitVolume.liters, UnitVolume.cups, UnitVolume.pints, UnitVolume.gallons]
    }
  }
}

struct ContentView: View {
  @State private var input: String = ""
  @State private var unitType: UnitTypes = UnitTypes.length {
    willSet {
      inputUnit = newValue.subTypes[0]
      outputUnit = newValue.subTypes[1]
    }
  }
  @State var inputUnit: Dimension = UnitLength.feet
  @State var outputUnit: Dimension = UnitLength.meters
  
  private var inputValue: Double { Double(input) ?? 0 }
  
  var output: String {
    let measurement = Measurement(value: inputValue, unit: inputUnit)
    return "\(measurement.converted(to: outputUnit))"
  }
  
  var body: some View {
    NavigationView {
      Group {
        Form {
          Section {
            
            Picker(selection: $unitType, label: Text("Unit category")) {
              ForEach(UnitTypes.allCases) { unit in
                Text(unit.name)
              }
            }.pickerStyle(SegmentedPickerStyle())
            
            HStack {
              
              TextField("Input", text: $input)
                .keyboardType(.decimalPad)
              
              Divider()
              
              Picker(selection: $inputUnit, label: Text("Type:")) {
                ForEach(unitType.subTypes, id: \.self) { type in
                  Text(type.symbol)
                }
              }
              
            }
          }
          
          Section(header: Text("Converted to")) {
            Picker(selection: $outputUnit, label: Text("Output type")) {
              ForEach(unitType.subTypes, id: \.self) { type in
                Text(type.symbol)
              }
            }.pickerStyle(SegmentedPickerStyle())
            
            Text("Result: ") + Text(output)
          }
          
        }
        
      }.navigationBarTitle("Unit Converter")
      
    }
  }
  
}

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

I am still working on bugs in the pickers and the unitType willSet property. My intention is that changing the unitType @State property would set sensible defaults for the input and output type @State properties. Unfortunately, the willSet doesn't seem to have any impact.

Additionally, the outputType picker retains some extraneous options after changing the unitType picker.