How to provide a localized description with an Error type in Swift?

Question

I am defining a custom error type with Swift 3 syntax and I want to provide a user-friendly description of the error which is returned by the localizedDescription property of the Error object. How can I do it?

public enum MyError: Error {   case customError    var localizedDescription: String {     switch self {     case .customError:       return NSLocalizedString("A user-friendly description of the error.", comment: "My error")     }   } }  let error: Error = MyError.customError error.localizedDescription // "The operation couldn’t be completed. (MyError error 0.)" 

Is there a way for the localizedDescription to return my custom error description ("A user-friendly description of the error.")? Note that the error object here is of type Error and not MyError. I can, of course, cast the object to MyError

(error as? MyError)?.localizedDescription 

but is there a way to make it work without casting to my error type?

Answer 1

As described in the Xcode 8 beta 6 release notes,

Swift-defined error types can provide localized error descriptions by adopting the new LocalizedError protocol.

In your case:

public enum MyError: Error {     case customError }  extension MyError: LocalizedError {     public var errorDescription: String? {         switch self {         case .customError:             return NSLocalizedString("A user-friendly description of the error.", comment: "My error")         }     } }  let error: Error = MyError.customError print(error.localizedDescription) // A user-friendly description of the error. 

You can provide even more information if the error is converted to NSError (which is always possible):

extension MyError : LocalizedError {     public var errorDescription: String? {         switch self {         case .customError:             return NSLocalizedString("I failed.", comment: "")         }     }     public var failureReason: String? {         switch self {         case .customError:             return NSLocalizedString("I don't know why.", comment: "")         }     }     public var recoverySuggestion: String? {         switch self {         case .customError:             return NSLocalizedString("Switch it off and on again.", comment: "")         }     } }  let error = MyError.customError as NSError print(error.localizedDescription)        // I failed. print(error.localizedFailureReason)      // Optional("I don\'t know why.") print(error.localizedRecoverySuggestion) // Optional("Switch it off and on again.") 

By adopting the CustomNSError protocol the error can provide a userInfo dictionary (and also a domain and code). Example:

extension MyError: CustomNSError {      public static var errorDomain: String {         return "myDomain"     }      public var errorCode: Int {         switch self {         case .customError:             return 999         }     }      public var errorUserInfo: [String : Any] {         switch self {         case .customError:             return [ "line": 13]         }     } }  let error = MyError.customError as NSError  if let line = error.userInfo["line"] as? Int {     print("Error in line", line) // Error in line 13 }  print(error.code) // 999 print(error.domain) // myDomain