Swift custom LocalizedError lose failureReason in caller CatchBlock

Question

trying first time to play around with error handling and error concepts. So I found this approach to create Error enums. Conforming to LocalizedError protocol I expected to have the failureReason as further explanation. Following is the enum for doing this.

enum NetworkError : LocalizedError {
  case invalidInput(String)
  case unexpected
  case invalidURL(_ url: URL)
  case apiError(statusCode: Int)
  
  var faliureReason: String? {
    switch self {
    case .invalidInput:
      return "Request body is nil."
    case .unexpected:
      return "..."
    case .invalidURL(_):
      return "..."
    case .apiError(statusCode: let statusCode):
      return "..."
    }
  }
}

in my caller function I've an catch block for this but the error.failureReason property remains nil.

func updateItem() async {
    guard let car = car else { return }
    
    let resource = PostCarResource(car: car)
    let request = APIRequest(resource: resource)
    
    var errorMessage = nil
    
    do {
      try await request.execute()
    } catch let error as LocalizedError {
      self.errorMessage = error.localizedDescription
      print(".")
      print("self.errorMessage")
      print(self.errorMessage)
      print(".")
      print("error.failureReason")
      print(error.failureReason ?? "no Reason")
      print(".")
      print("error")
      print(error)
      print(".")
      print("dump")
      dump(error)
    } catch {
      dump(error)
    }
  }

I'd expect that after the error has been set correctly to .invalidInput (what happens correctly) I could use error.failureReason but this is not the case. So why I'm not getting the String "Request body is nil."

The output of my console looks like:

.
self.errorMessage
Optional("The operation couldn’t be completed. (CarUSell.NetworkError error 0.)")
.
error.failureReason
no Reason
.
error
invalidInput("Request body is nil.")
.
dump
▿ -.NetworkError.invalidInput
  - invalidInput: "Request body is nil."

Answer 1

Looks like a simple typo. Your code defines

var faliureReason

That is not the same as failureReason. That's all, really.

---

However, although this is not quite what you asked, note that you are using LocalizedError incorrectly. You do not need to catch a LocalizedError in a special way; it is an Error. LocalizedError is merely a way of constructing the error. So instead of

} catch let error as LocalizedError {

You should should just be saying

} catch {

The error will arrive as an Error called error, so now just read that. If you want to check whether it has a failure reason, cast to an NSError and check the localizedFailureReason:

} catch {
    print((error as NSError).localizedFailureReason)
}

Here is a complete playground example:

enum NetworkError : LocalizedError {
    case invalidInput(String)
    case unexpected
    case invalidURL(_ url: URL)
    case apiError(statusCode: Int)

    var failureReason: String? {
        switch self {
        case .invalidInput:
            return "Request body is nil."
        case .unexpected:
            return "..."
        case .invalidURL(_):
            return "..."
        case .apiError(statusCode: _):
            return "..."
        }
    }
}

func test() {
    do {
        throw NetworkError.invalidInput("what")
    } catch {
        print((error as NSError).localizedFailureReason) // "Request body is nil"
    }
}

test()

Observe too that you have defined invalidInput(String) but you are not using the String in your failureReason. So I didn't use it either (but I had to supply a dummy string, when throwing, in order to get the example to compile). You probably meant something like this (I've marked the key changes with comments):

enum MyNetworkError : LocalizedError {
    case invalidInput(String)
    case unexpected
    case invalidURL(_ url: URL)
    case apiError(statusCode: Int)

    var failureReason: String? {
        switch self {
        case .invalidInput(let message): // <--
            return message // <--
        case .unexpected:
            return "..."
        case .invalidURL(_):
            return "..."
        case .apiError(statusCode: _):
            return "..."
        }
    }
}

func test() {
    do {
        throw MyNetworkError.invalidInput("Request body is nil") // <--
    } catch {
        print((error as NSError).localizedFailureReason) // "Request body is nil"
    }
}

test()