Error Handling

There are four ways to handle errors in Swift:

1. Handle Errors with Do-Catch

func readFileIntoStringDoCatch(fileName: String, fileExtension: String) {
    if let fileURL = Bundle.main.url(forResource: fileName, withExtension: fileExtension) {
        do {
            let content = try String(contentsOf: fileURL, encoding: .utf8)
            print(content)
        } catch {
            print("error handled immediately!")
        }
    }
}

readFileIntoStringDoCatch(fileName: "swift", fileExtension: "png")

2. Convert Error to Optional with try?

func readFileIntoStringOptional(fileName: String, fileExtension: String) {
    if let fileURL = Bundle.main.url(forResource: fileName, withExtension: fileExtension) {
        if let content = try? String(contentsOf: fileURL, encoding: .utf8) {
            print(content)
        } else {
            print("could not read contents of file into string")
        }
    }
}

readFileIntoStringOptional(fileName: "swift", fileExtension: "png")

3. Ignore Error with try!

func readFileIntoStringOrCrash(fileName: String, fileExtension: String) {
    if let fileURL = Bundle.main.url(forResource: fileName, withExtension: fileExtension) {
        let content = try! String(contentsOf: fileURL, encoding: .utf8)
        print(content)
    }
}

readFileIntoStringOrCrash(fileName: "swift", fileExtension: "txt")

4. Propagate Error

func readFileIntoStringOrPropagate(fileName: String, fileExtension: String) throws {
    if let fileURL = Bundle.main.url(forResource: fileName, withExtension: fileExtension) {
        let content = try String(contentsOf: fileURL, encoding: .utf8)
        print(content)
    }
}

do {
    try readFileIntoStringOrPropagate(fileName: "swift", fileExtension: "png")
} catch {
    print("the error was propagated to me, and I handled it!")
}

Custom Error Handling

Create custom errors via enums that are of type Error

enum PurchaseError: Error {
    case invalidAddress
    case cardRejected
    case invalidPromo
    case invalidCardInformation
}

Use associated values to add helpful debugging information.

enum PurchaseError: Error {
    case invalidAddress
    case cardRejected
    case invalidPromo(String)
    case invalidCardInformation(String)
}

do {
    try attemptPurchase(withPromoCode: "FREE")
} catch let error as PurchaseError {
    switch error {
    case let .invalidPromo(code): /* extract the associated value */
        print("Invalid Promo Code: \(code)")
    default:
        print(error)
        break
    }
} catch {
    print(error)
}

LocalizedError and CustomNSError

Add extra debugging information to a custom error by also implementing the LocalizedError and CustomNSError protocols.

extension PurchaseError: LocalizedError {   
    var failureReason: String? {
        switch self {
        case .invalidAddress:
            return NSLocalizedString("Address invalid or contained empty fields", comment: "")
        case .cardRejected:
            return NSLocalizedString("Card Rejected", comment: "")
        case let .invalidPromo(code):
            return NSLocalizedString("Invalid Promo Code: \(code)", comment: "")
        case .invalidCardInformation(String):
            return NSLocalizedString("Invalid Card information: \(info)", comment: "")
        }
    }
}


extension PurchaseError: CustomNSError {
    // domain and error code...

    /// The user-info dictionary.
    var errorUserInfo: [String: Any] {
        switch self {
        case let .invalidPromo(code):
            return [
                "promoCode": code,
                "dateExpired":getExpirationDate(code) ?? "n/a"
            ]
        case let .invalidCardInformation(info):
            return [
                "cardInformation": info
            ]
        default:
            return [:]
        }
    }
}