Change Error localizedDescription

Question

I've an error class that is:

public enum ModelError: Error {
  case invalidArray(model: String)

  var localizedDescription: String {
    switch self {
    case .invalidArray(model: let model):
      return "\(model) has an invalid array"
    default:
      return "modelError"
    }
  }
}

and when passed as an Error in a callback function, I want to access its custom localizedDescription. For instance:

func report(_ error: Error) {
  print("Error report: \(error.localizedDescription)")
}

But calling report(ModelError.invalidArray(model: "test")) prints:

"The operation couldn’t be completed. (ModelError error 0.)"

Such things seems feasible with NSError since I can override the localizedDescription property there. But I don't want to use NSError since it's not really a swift thing and a lot of libraries work with Error.

Shouldn't `ModelError(model: "test")` be `ModelError.invalidArray(model: "text")` ? — rmaddy, Nov 10 '16 at 23:53
I can make your code work if `ModelError` conforms to `CustomStringConvertible` and `localizeDescription` is renamed to `description`. And then print `error` instead of `error.localizedDescription`. — rmaddy, Nov 11 '16 at 00:07
@rmaddy Isn't that kind of outside the bounds of CustomStringConvertible? — GetSwifty, Nov 11 '16 at 00:08
@PEEJWEEJ I don't know, is it? I thought that protocol can be used by anything that wants to provide a `description` method. I'm still learning Swift myself. — rmaddy, Nov 11 '16 at 00:10
It can be, but the intention is that it describes the Type, not gives other info. https://developer.apple.com/reference/swift/customstringconvertible the description should be something like ModelError.invalidArray(test) — GetSwifty, Nov 11 '16 at 00:11

GetSwifty · Answer 1 · 2016-12-05T21:04:14.053

According to the Documentation, localizedDescription is implemented in a protocol extension, not in the protocol declaration, which means there's nothing to adhere to or override. There is a type-wide interface for enums that adhere to Error.

My way I get around this is to use a wrapper protocol:

protocol LocalizedDescriptionError: Error {
    var localizedDescription: String { get }
}

public enum ModelError: LocalizedDescriptionError {
    case invalidArray(model: String)

    var localizedDescription: String {
        switch self {
        case .invalidArray(model: let model):
            return "\(model) has an invalid array"
        default:
            return "modelError"
        }
    }
}

let error: LocalizedDescriptionError = ModelError.invalidArray(model: "Model")
let text = error.localizedDescription // Model Has an invalid array

The above works only because you are declaring the `error` constant to be of the (specialized) `LocalizedDescriptionError` type. If you allow it to be of the (base) `Error` type, it will dispatch to the default implementation within the `Error` protocol, since - as you mentioned - `localizedDescription` is not declared within the `Error` protocol itself and thus cannot be overridden. Must use the new `LocalizedError` protocol, instead. — Gary, Jan 18 '17 at 16:18

Change Error localizedDescription

1 Answers1