Can't get throws to work with function with completion handler

Question

I'm trying to add a throws to my existing function with a completion handler but I keep getting a warning saying no calls throwing functions occur within try expression. In the section where I throw the errors, I get an error saying

invalid conversion from throwing function of type '() throwing -> Void' to non-throwing function type.

enum LoginError: ErrorType {
    case Invalid_Credentials
    case Unable_To_Access_Login
    case User_Not_Found
}

@IBAction func loginPressed(sender: AnyObject) {

    do{
        try self.login3(dict, completion: { (result) -> Void in

            if (result == true)
            {
                self.performSegueWithIdentifier("loginSegue", sender: nil)
            }
        })
    }
    catch LoginError.User_Not_Found
    {
        //deal with it
    }
    catch LoginError.Unable_To_Access_Login
    {
        //deal with it
    }
    catch LoginError.Invalid_Credentials
    {
        //deal with it
    }
    catch
    {
        print("i dunno")
    }

}

func login3(params:[String: String], completion: (result:Bool) throws -> Void)
{
    //Request set up
    let task = session.dataTaskWithRequest(request, completionHandler: {data, response, error -> Void in
        do {
            let json = try NSJSONSerialization.JSONObjectWithData(data, options: .MutableLeaves) as? NSDictionary
            if let parseJSON = json
            {
                let userID = parseJSON["user_id"] as? Int
                let loginError = parseJSON["user_not_found"] as? String
                let validationError = parseJSON["invalid_credentials"] as? String
                let exception = parseJSON["unable_to_access_login"] as? String

                var responseArray = [(parseJSON["user_id"] as? Int)]
                if userID != nil
                {
                    dispatch_async(dispatch_get_main_queue()) {
                        completion(result:true)
                    }

                }
                else if loginError != ""
                {
                    dispatch_async(dispatch_get_main_queue()){
                        completion(result: false)
                        self.loginErrorLabel.text = loginError
                        throw LoginError.User_Not_Found
                    }
                }
                else if validationError != ""
                {
                    dispatch_async(dispatch_get_main_queue()){
                        completion(result:false)
                        self.validationErrorLabel.text = validationError
                        throw LoginError.Invalid_Credentials
                    }

                }
                else if exception != nil
                {
                    dispatch_async(dispatch_get_main_queue()){
                        completion(result:false)
                        self.exceptionErrorLabel.text = "Unable to login"
                        throw LoginError.Unable_To_Access_Login
                    }
                }
            }
            else
            {
            }
        }
        catch let parseError {
            // Log the error thrown by `JSONObjectWithData`
        })

        task.resume()

}

Answer 1

What you can do is encapsulating the error into a throwable closure like in the following code to achieve what you want:

func login3(params:[String: String], completion: (inner: () throws -> Bool) -> ()) {

   let task = session.dataTaskWithRequest(request, completionHandler: { data, response, error -> Void in

            let json = try NSJSONSerialization.JSONObjectWithData(data, options: .MutableLeaves) as? NSDictionary

            if let parseJSON = json {
               let userID = parseJSON["user_id"] as? Int
               let loginError = parseJSON["user_not_found"] as? String
               let validationError = parseJSON["invalid_credentials"] as? String
               let exception = parseJSON["unable_to_access_login"] as? String

               var responseArray = [(parseJSON["user_id"] as? Int)]
               if userID != nil {
                 dispatch_async(dispatch_get_main_queue()) {
                     completion(inner: { return true })
                 }

            }
            else if loginError != ""
            {
                dispatch_async(dispatch_get_main_queue()) {
                    self.loginErrorLabel.text = loginError
                    completion(inner: { throw LoginError.User_Not_Found })
                }
            }
            else if validationError != ""
            {
                dispatch_async(dispatch_get_main_queue()) {
                    self.validationErrorLabel.text = validationError
                    completion(inner: {throw LoginError.Invalid_Credentials})
                }
            }
            else if exception != nil
            {
                dispatch_async(dispatch_get_main_queue()){
                    self.exceptionErrorLabel.text = "Unable to login"
                    completion(inner: {throw LoginError.Unable_To_Access_Login})
                }
            }
        }
        else
        {
        }
    }

   task.resume()
}

And the you can call it like in the following way:

self.login3(dict) { (inner: () throws -> Bool) -> Void in
   do {
     let result = try inner()
     self.performSegueWithIdentifier("loginSegue", sender: nil)
   } catch let error {
      print(error)
   }
}

The trick is that the login3 function takes an additional closure called 'inner' of the type () throws -> Bool. This closure will either provide the result of the computation, or it will throw. The closure itself is being constructed during the computation by one of two means:

• In case of an error: inner: {throw error}
• In case of success: inner: {return result}

I strongly recommend you an excellent article about using try/catch in async calls Using try / catch in Swift with asynchronous closures

I hope this help you.