Swift2.0：如何通过try try获取返回值和错误

Question

I have a function in Objective-C. This function always returns a value, event if an error occurs:

-(NSString *)test:(NSError **)error;

I override it in Swift1 as following, so that I can still get the value and the error at the same time.

override func test(error: NSErrorPointer) -> String {
    var error: NSError?
    let result = super.test(&error)
    ...
}

But now in swift2, I can only override the function like

override func test() throws -> String {

} 

In this case, how to get the value and error at the same time?

I am doing this because I need to override a function in AFNetworking. Both return value and error are needed.

- (id)responseObjectForResponse:(NSURLResponse *)response
                           data:(NSData *)data
                          error:(NSError *__autoreleasing *)error;    

Answer 1

I know this isn't answering the question, although I could offer guidance into the way error handling works in Swift, but this might be a lot easier for you.

AFNetworking has a dedicated Swift library called AlamoFire. If your project is in Swift, it might be beneficial for you to use the swift version of the AFNetworking library. It can be found here:

https://github.com/Alamofire/Alamofire

Answer 2

I think you don't really need the BOOL value returned. It should be true if everything is OK and false otherwise.

With Swift 2 you should use do , try and catch . You can catch the error and cast it as an NSError instance.

Example :

do {
  try something()
  // continue if something() returned true otherwise -> catch
} catch let error as NSError {
  print(error.localizedDescription)
}

With AFNetworking, you could do something like this to change the NSError localized description with an error message from the JSON :

override func responseObjectForResponse(response: NSURLResponse!, data: NSData!) throws -> AnyObject {
  do {
    try self.validateResponse(response as! NSHTTPURLResponse, data: data)
  } catch let error as NSError {
    let json = JSON(data: data) // i'm using SwiftyJSON : https://github.com/SwiftyJSON/SwiftyJSON
    if let errorMessage = json["error"].string {
      var userInfo = error.userInfo
      userInfo[NSLocalizedDescriptionKey] = errorMessage
      throw NSError(domain: error.domain, code: error.code, userInfo: userInfo)
    }
  }

  return try super.responseObjectForResponse(response, data: data)
}

Maybe there is a better way to do this but i'm starting with Swift 2 :)