Error handling (try, catch, throw) in iOS Swift - Mini App: Build & Ship

Build: Error Handling Demo

This screen lets the user enter a number and tries to convert it to an integer. If the input is invalid, it shows an error message using Swift's try, catch, and throw error handling.

Target UI

-------------------------
| Error Handling Demo   |
-------------------------
| Enter a number:       |
| [______________]     |
| [Convert]            |
|                      |
| Result:              |
|                      |
-------------------------

A TextField for user input of a number

A Convert button below the TextField

When Convert is tapped, try to convert input to Int using a throwing function

If conversion succeeds, show 'Valid number: X' in Result label

If conversion fails, catch the error and show 'Error: Invalid number' in Result label

Use Swift's try, catch, and throw keywords properly

Starter Code

iOS Swift

import SwiftUI

struct ErrorHandlingDemoView: View {
    @State private var userInput = ""
    @State private var resultMessage = ""

    var body: some View {
        VStack(spacing: 20) {
            Text("Error Handling Demo")
                .font(.title)

            TextField("Enter a number", text: $userInput)
                .textFieldStyle(RoundedBorderTextFieldStyle())
                .keyboardType(.numberPad)
                .padding(.horizontal)

            Button("Convert") {
                // TODO: Call the function that tries to convert input
                // and update resultMessage accordingly
            }
            .buttonStyle(.borderedProminent)

            Text("Result: \(resultMessage)")
                .padding()

            Spacer()
        }
        .padding()
    }

    // TODO: Add a throwing function to convert String to Int
}

Task 1

Task 2

Task 3

Task 4

Solution

iOS Swift

import SwiftUI

enum ConversionError: Error {
    case invalidNumber
}

struct ErrorHandlingDemoView: View {
    @State private var userInput = ""
    @State private var resultMessage = ""

    var body: some View {
        VStack(spacing: 20) {
            Text("Error Handling Demo")
                .font(.title)

            TextField("Enter a number", text: $userInput)
                .textFieldStyle(RoundedBorderTextFieldStyle())
                .keyboardType(.numberPad)
                .padding(.horizontal)

            Button("Convert") {
                do {
                    let number = try convertToInt(userInput)
                    resultMessage = "Valid number: \(number)"
                } catch {
                    resultMessage = "Error: Invalid number"
                }
            }
            .buttonStyle(.borderedProminent)

            Text("Result: \(resultMessage)")
                .padding()

            Spacer()
        }
        .padding()
    }

    func convertToInt(_ input: String) throws -> Int {
        if let number = Int(input) {
            return number
        } else {
            throw ConversionError.invalidNumber
        }
    }
}

This app screen uses a TextField to get user input as a string. The convertToInt function tries to convert the string to an integer. If it fails, it throws a custom error ConversionError.invalidNumber.

When the user taps the Convert button, the app uses a do-try-catch block to call convertToInt. If conversion succeeds, it updates the result message with the valid number. If it fails, it catches the error and shows an error message.

This demonstrates how to handle errors gracefully in Swift using try, catch, and throw.

Final Result

Completed Screen

-------------------------
| Error Handling Demo   |
-------------------------
| Enter a number:       |
| [12345__________]    |
| [Convert]            |
|                      |
| Result:              |
| Valid number: 12345  |
-------------------------

User types a number in the TextField

User taps Convert button

If input is a valid number, Result label shows 'Valid number: X'

If input is invalid (e.g., letters), Result label shows 'Error: Invalid number'

Stretch Goal

Add a Clear button that resets the input and result message

💡 Hint

Add another Button below Convert that sets userInput and resultMessage to empty strings