Alamofire: status code: 404,but response.result = SUCCESS

You aren't validating the response status code, so it is possible that a 404 will return you a .Success case. All that the responseData serializer does is validate that there wasn't an error (which there won't be since you aren't validating the response status code) or that the data exists.

public static func dataResponseSerializer() -> ResponseSerializer<NSData, NSError> {
    return ResponseSerializer { _, _, data, error in
        guard error == nil else { return .Failure(error!) }

        guard let validData = data where validData.length > 0 else {
            let failureReason = "Data could not be serialized. Input data was nil or zero length."
            let error = Error.errorWithCode(.DataSerializationFailed, failureReason: failureReason)
            return .Failure(error)
        }

        return .Success(validData)
    }
}

In your case, you are receiving a 404 which isn't throwing an error, and the server is returning data. Therefore, your responseData serializer succeeds. If you expect a 404 to always throw an error, you should additionally use the .validate(statusCode: [200]) method before the responseData serializer. More info in the Validation Section of the README.

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