Error Handling and Concurrency

This lesson covers two essential topics: how Swift handles errors safely using throw/catch, and how Swift's structured concurrency model enables safe asynchronous and parallel code with async/await and actors.

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Part 1: Error Handling

Defining Errors

Errors in Swift conform to the Error protocol. Enums are the most common way to define error types:

enum NetworkError: Error {
    case badURL
    case requestFailed(statusCode: Int)
    case decodingFailed
    case noData
}

enum ValidationError: Error {
    case tooShort(minimum: Int)
    case tooLong(maximum: Int)
    case invalidFormat(String)
}

Throwing Functions

Functions that can fail are marked with throws:

func fetchUser(id: Int) throws -> User {
    guard id > 0 else {
        throw ValidationError.invalidFormat("ID must be positive")
    }
    guard let data = loadData(for: id) else {
        throw NetworkError.noData
    }
    return try decode(data)
}

Catching Errors

Use do-catch to handle thrown errors:

do {
    let user = try fetchUser(id: 42)
    print("Found user: \(user.name)")
} catch NetworkError.noData {
    print("No data available")
} catch NetworkError.requestFailed(let statusCode) {
    print("Request failed with status \(statusCode)")
} catch {
    print("Unexpected error: \(error)")
}

try? and try!