How to best modify error messages from -validate method on Alamofire

Question

I have this small snippet that makes a request to a login endpoint and it works fine.

public func loginWithEmail(email: String, password: String, completionHandler: Result<String, NSError> -> Void) {
    Alamofire.request(AuthenticationRouter.Login(email: email, password: password))
        .validate()
        .responseString { response in
            completionHandler(response.result)
    }
}

My question is: when the user inputs invalid credentials the server returns 403 which due to validate will generate a Result<NSError>. What is the best way to modify the error description / failure reason in this error?

Should I create my own error? Is there a way to modify it for all the requests?

Alamofire validate func snippet for reference:

public func validate<S: SequenceType where S.Generator.Element == Int>(statusCode acceptableStatusCode: S) -> Self {
    return validate { _, response in
        if acceptableStatusCode.contains(response.statusCode) {
            return .Success
        } else {
            let failureReason = "Response status code was unacceptable: \(response.statusCode)"
            return .Failure(Error.errorWithCode(.StatusCodeValidationFailed, failureReason: failureReason))
        }
    }
}

Answer 1

The easiest way would be to act on the error once it reaches your app code. You probably don't want to create a custom validate method that overrides the default Alamofire behavior because your case is VERY specific.

In general, you would set your acceptable status codes and use the validate(statusCodes:) method. Then in your response logic in your app code, switch on the error and create custom app representations of each case. In this case, a 403 means that the incorrect credentials were used which only your application knows. Hence, it should live in your app code, not in an Alamofire validation extension. My two cents...