I have no idea why my code is not working. There are no errors or warnings. Result always shows 0. When i return input.value instead of output.value it works fine but it refuses to return output.value for some reason.

import SwiftUI

struct ContentView: View {
    @State private var lengthInput = ""
    @State private var inputUnit = "m"
    @State private var outputUnit = "m"

    @FocusState private var lenghtIsFocused: Bool

    let units = ["m", "km", "ft", "yd", "mi"]

    var result: Double {
        guard lengthInput.isEmpty == false else {
            return 0
        }
        let input = Measurement(value: Double(lengthInput)!, unit: UnitLength(symbol: inputUnit))
        let output = input.converted(to: UnitLength(symbol: outputUnit))
        return output.value
    }

    var body: some View {
        Form {
            Section {
                TextField("Length", text: $lengthInput)
                    .keyboardType(.decimalPad)
                    .focused($lenghtIsFocused)
            }

            Section {
                Picker("Input Unit", selection: $inputUnit) {
                    ForEach(units, id: \.self) {
                        Text($0)
                    }
                }
                .pickerStyle(.segmented)
            } header: {
                Text("Input Unit")
            }

            Section {
                Picker("Output Unit", selection: $outputUnit) {
                    ForEach(units, id: \.self) {
                        Text($0)
                    }
                }
                .pickerStyle(.segmented)
            } header: {
                Text("Output Unit")
            }

            Section("Result") {
                Text(result, format: .number)
            }
        }
        .toolbar {
            ToolbarItemGroup(placement: .keyboard) {
                Spacer()
                Button("Done") {
                    lenghtIsFocused = false
                }
            }
        }
    }
}

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

The problem was me not reading the documentation properly after all... I added two switch statements to solve the problem. This is how my computed property looks like in the working code.

var result: Double {
        guard lengthInput.isEmpty == false else {
            return 0
        }
        var inputting: UnitLength
        var outputting: UnitLength

        switch inputUnit {
        case "m":
            inputting = .meters
        case "km":
            inputting = .kilometers
        case "ft":
            inputting = .feet
        case "yd":
            inputting = .yards
        default:
            inputting = .miles
        }

        switch outputUnit {
        case "m":
            outputting = .meters
        case "km":
            outputting = .kilometers
        case "ft":
            outputting = .feet
        case "yd":
            outputting = .yards
        default:
            outputting = .miles
        }

        let input = Measurement(value: Double(lengthInput)!, unit: inputting)
        let output = input.converted(to: outputting)
        return output.value
    }

Your code appears to be mostly correct. However, there are a couple of minor issues and improvements you can make. Here's the corrected code:

import SwiftUI

struct ContentView: View { @State private var lengthInput = "" @State private var inputUnit = "m" @State private var outputUnit = "m"

@FocusState private var lengthIsFocused: Bool

let units = ["m", "km", "ft", "yd", "mi"]

var result: Double {
    guard let inputValue = Double(lengthInput) else {
        return 0
    }
    let input = Measurement(value: inputValue, unit: UnitLength(symbol: inputUnit))
    let output = input.converted(to: UnitLength(symbol: outputUnit))
    return output.value
}

var body: some View {
    Form {
        Section {
            TextField("Length", text: $lengthInput)
                .keyboardType(.decimalPad)
                .focused($lengthIsFocused)
        }

        Section(header: Text("Input Unit")) {
            Picker("Input Unit", selection: $inputUnit) {
                ForEach(units, id: \.self) {
                    Text($0)
                }
            }
            .pickerStyle(.segmented)
        }

        Section(header: Text("Output Unit")) {
            Picker("Output Unit", selection: $outputUnit) {
                ForEach(units, id: \.self) {
                    Text($0)
                }
            }
            .pickerStyle(.segmented)
        }

        Section(header: Text("Result")) {
            Text("\(result, specifier: "%.2f")")
        }
    }
    .toolbar {
        ToolbarItemGroup(placement: .keyboard) {
            Spacer()
            Button("Done") {
                lengthIsFocused = false
            }
        }
    }
}

}

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

Here are the changes made:

Renamed lenghtIsFocused to lengthIsFocused for consistency and corrected its usage in the code.

Added optional binding for converting the lengthInput to a Double and guarding against non-numeric input.

Modified the Text view in the "Result" section to format the result with two decimal places using specifier: "%.2f".

These changes should make your code work as expected.