The picker doesn't update the number of segments it has when a value is changed during execution - xcode

Question

I have recently started learning swiftUI and I'm facing some issues here. This is the code:
struct ContentView: View {
#State private var measurementType = 0
#State private var inputValue = ""
#State private var inputUnit = 0
#State private var outputUnit = 1
var outputValue = ""
let measurementTypes = ["Temp", "Length", "Time", "Volume"]
var typeDictionary = [
["Celsius", "Fahrenheit", "Kelvin"],
["Meters", "Kilometers", "Feet", "Yards", "Miles"],
["Seconds", "Minutes", "Hours", "Days"],
["Milliliters", "Liters", "Cups", "Pints", "Gallons"]
]
var body: some View {
NavigationView {
Form {
Section(header: Text("Choose the type of measurement")) {
Picker("The type of measurement", selection: $measurementType) {
ForEach(0 ..< measurementTypes.count) {
Text("\(measurementTypes[$0])")
}
}
.id(measurementType)
.pickerStyle(SegmentedPickerStyle())
}
Section {
TextField("Enter the value", text: $inputValue)
.keyboardType(.decimalPad)
}
Section(header: Text("Choose the input unit")) {
Picker("The input unit", selection: self.$inputUnit) {
ForEach(0 ..< typeDictionary[measurementType].count) {
Text("\(typeDictionary[measurementType][$0])")
}
}
.pickerStyle(SegmentedPickerStyle())
}
Section(header: Text("Choose the output unit")) {
}
Section(header: Text("Converted Value")) {
Text("")
}
}
}
}
}
struct ContentView_Previews: PreviewProvider {
static var previews: some View {
ContentView()
}
}
Whenever I change the measurementType while the code is running, the values of the picker in the input units section update, but the number of segments don't. If I start with the value of measurementType as 1, then the app simply crashes when I choose Temp or Time from the picke.

Answer 1

When using ForEach, you need to provide an id so SwiftUI can uniquely identify each element in the array and know when to redraw the view. You can use id: \.self to use the item's value as its id.
Here's an updated working version:
import SwiftUI
struct ContentView: View {
#State private var measurementType = 0
#State private var inputValue = ""
#State private var inputUnit = 0
#State private var outputUnit = 1
var outputValue = ""
let measurementTypes = ["Temp", "Length", "Time", "Volume"]
var typeDictionary = [
["Celsius", "Fahrenheit", "Kelvin"],
["Meters", "Kilometers", "Feet", "Yards", "Miles"],
["Seconds", "Minutes", "Hours", "Days"],
["Milliliters", "Liters", "Cups", "Pints", "Gallons"]
]
var body: some View {
NavigationView {
Form {
Section(header: Text("Choose the type of measurement")) {
Picker("The type of measurement", selection: $measurementType) {
ForEach(0 ..< measurementTypes.count, id: \.self) {
Text("\(measurementTypes[$0])")
}
}
.id(measurementType)
.pickerStyle(SegmentedPickerStyle())
}
Section {
TextField("Enter the value", text: $inputValue)
.keyboardType(.decimalPad)
}
Section(header: Text("Choose the input unit")) {
Picker("The input unit", selection: self.$inputUnit) {
ForEach(0 ..< typeDictionary[measurementType].count, id: \.self) {
Text("\(typeDictionary[measurementType][$0])")
}
}
.pickerStyle(SegmentedPickerStyle())
}
Section(header: Text("Choose the output unit")) {
}
Section(header: Text("Converted Value")) {
Text("")
}
}
}
}
}
struct ContentView_Previews: PreviewProvider {
static var previews: some View {
ContentView()
}
}