Swift Error Handling and metadata of the error

Question

I want to build a fully Swift error handling and propagation system that I can use throughout an entire application. The implementation is fairly straightforward, if you have something like this:

enum AnnouncerError: Error {
  /// A network error occured
  case networkError
  /// The program was unable to unwrap data from nil to a non-nil value
  case unwrapError
  /// The parser was unable to validate the XML
  case validationError
  /// The parser was unable to parse the XML
  case parseError
}

I can get the type of the error in a delegate function like this using a simple switch statement:

func feedFinishedParsing(withFeedArray feedArray: [FeedItem]?, error: AnnouncerError?) {
  // unwrap the error somewhere here
  switch error {
  case .networkError:
    print("Network error occured")
  case .parseError:
    print("Parse error occured")
  case .unwrapError:
    print("Unwrap error occured")
  default:
    print("Unknown error occured")
  }
}

However, I have the additional data from a particular case in the error enum and that's when the problems arise:

enum AnnouncerError: Error {
  /// A network error occured
  case networkError(description: String) //note this line
  /// The program was unable to unwrap data from nil to a non-nil value
  case unwrapError
  /// The parser was unable to validate the XML
  case validationError
  /// The parser was unable to parse the XML
  case parseError
}

How should I get the description string from the Error type if I call the delegate method with a .networkError?

As an extension, if there is no straightforward way to directly get the description from a .networkError, should I continue using the current architecture where the delegate method has an optional (nullable) Error type where I can check for the type of the error, or should I use a completely different architecture such as a try-catch system and if so, how should I implement it?

Possibly related: [Swift 3 errors with additional data](http://stackoverflow.com/questions/41202869/swift-3-errors-with-additional-data) and [How to provide a localized description with an Error type in Swift?](http://stackoverflow.com/questions/39176196/how-to-provide-a-localized-description-with-an-error-type-in-swift). — Martin R, Dec 24 '16 at 13:36

dirtydanee · Accepted Answer · 2016-12-24T14:12:50.927

If you would like to access the description property of networkError case , you do not need to do too much.

func feedFinishedParsing(withFeedArray feedArray: [FeedItem]?, error: AnnouncerError?) {
    // unwrap the error somewhere here
    switch error {
    // Just declare description in the case, and use it if u need it.
    case .networkError(let description):
        print("Network error occured")
        print(description)
    case .parseError:
        print("Parse error occured")
    case .unwrapError:
        print("Unwrap error occured")
    default:
        print("Unknown error occured")
    }
}

Of course, you can build a robust machine for error handling, but if you just wanna access this property, the above solution will do the trick.

Regardless, i would recommend to read further into Swift pattern matching, what explains in detail the advanced usage of the switch language element is Swift.

Swift Error Handling and metadata of the error

1 Answers1