SwiftUI : Picker does not update correctly when changing datasource

I'm not sure why SwiftUI behaves like this, seems like a bug to me (Correct me if I'm wrong). All I can suggest is to add separate pickers for temperature and length and hide those based on the current selected type. For code re-usability I've added the picker to another file.

MyCustomPicker

struct MyCustomPicker: View {
    var pickerData: [String]
    @Binding var binding: Int
    var body: some View {
        Picker("Convert", selection: $binding) {
            ForEach(0 ..< pickerData.count, id: \.self)
            { i in
                Text(self.pickerData[i])
            }
        }
        .pickerStyle(SegmentedPickerStyle())
    }
}

ContentView

struct ContentView: View {

    @State var selectedType = 0
    @State var inputTempUnit = 0
    @State var outputTempUnit = 1
    @State var inputLenUnit = 0
    @State var outputLenUnit = 1

    let arrTypes = ["Temperature", "Length"]
    let tempData = ["Celsius", "Fahrenheit", "Kelvin"]
    let lenData  = ["meters", "kilometers", "feet", "yards", "miles"]


    var body: some View {
        NavigationView {
            Form {
                Section(header: Text("Choose type")) {
                    MyCustomPicker(pickerData: arrTypes, binding: $selectedType)
                }

                Section(header: Text("From")) {
                    if selectedType == 0 {
                        MyCustomPicker(pickerData: tempData, binding: $inputTempUnit)
                    } else {
                        MyCustomPicker(pickerData: lenData, binding: $inputLenUnit)
                    }
                }

                Section(header: Text("To")) {
                    if selectedType == 0 {
                        MyCustomPicker(pickerData: tempData, binding: $outputTempUnit)
                    } else {
                        MyCustomPicker(pickerData: lenData, binding: $outputLenUnit)
                    }
                }
            }
        }
    }
}

Note: You have to use different state variables to keep track the temperature and length selection.

Nick Polychronakis solved it in this fork: https://github.com/nickpolychronakis/100DaysOfSwiftUI/tree/master/UnitCoverter

The solution is to add .id(:identifier:) to your picker so it is unique.

Observable var:

@State var unit = 0

Main picker:

Picker("Length", selection: $unit) {
                    ForEach(0 ..< inputUnitTypes.count) {
                        Text("\(self.inputUnitTypes[$0].description)")
                    }
                }
                .pickerStyle(SegmentedPickerStyle())

One of secondary pickers which content is determined by the unit variable.

Picker("Length", selection: $inputUnit) {
                        ForEach(0 ..< selected.count) {
                            Text("\(self.selected[$0].description)")
                        }
                    }
                    .id(unit)