How could I throw an Error when response data does not contain an object to decode with Combine?

Question

I have a publisher wrapper struct where I can handle response status code. If the status code is not range in 200..300 it return with an object, otherwise it throws an Error. It works well.

public func anyPublisher<T:Decodable>(type: T.Type) -> AnyPublisher<T, Error> {
    return URLSession.shared.dataTaskPublisher(for: urlRequest)
        .tryMap { output in
            guard let httpResponse = output.response as? HTTPURLResponse, 200..<300 ~= httpResponse.statusCode else {
                throw APIError.unknown
            }
            return output.data
    }
    .decode(type: T.self, decoder: JSONDecoder())
    .receive(on: DispatchQueue.main)
    .eraseToAnyPublisher()
}

Using:

let sendNewUserPublisher = NetworkPublisher(urlRequest: request).anyPublisher(type: User.self)

cancellationToken = sendNewUserPublisher.sink(receiveCompletion: { completion in
    if case let .failure(error) = completion {
        NSLog("error: \(error.localizedDescription)")
    }
}, receiveValue: { post in
    self.post = post
})

As above, I would like to handle the error even if the response data does not contain an object to be decoded.

public func anyPublisher() -> AnyPublisher<URLSession.DataTaskPublisher.Output, URLSession.DataTaskPublisher.Failure> {
    return URLSession.shared.dataTaskPublisher(for: urlRequest)
        // I'd like to handle status code here, and throw an error, if needed
        .receive(on: DispatchQueue.main)
        .eraseToAnyPublisher()
}

Thank you in advance for any help you can provide.

Answer 1

I would suggest creating a Publisher that handles the HTTP response status code validation and using that for both of your other publishers - the one that handles an empty request body and the one that decodes the body.

If you need the HTTPURLResponse object even after validating its status code:

extension URLSession.DataTaskPublisher {
    /// Publisher that throws an error in case the data task finished with an invalid status code, otherwise it simply returns the body and response of the HTTP request
    func httpResponseValidator() -> AnyPublisher<Output, CustomError> {
        tryMap { data, response in
            guard let httpResponse = response as? HTTPURLResponse else { throw CustomError.nonHTTPResponse }
            let statusCode = httpResponse.statusCode
            guard (200..<300).contains(statusCode) else { throw CustomError.incorrectStatusCode(statusCode) }
            return (data, httpResponse)
        }
        .mapError { CustomError.network($0) }
        .eraseToAnyPublisher()
    }
}

Or if you don't care about any other properties of the response, only that its status code was valid:

func httpResponseValidator() -> AnyPublisher<Data, CustomError> {
    tryMap { data, response in
        guard let httpResponse = response as? HTTPURLResponse else { throw CustomError.nonHTTPResponse }
        let statusCode = httpResponse.statusCode
        guard (200..<300).contains(statusCode) else { throw CustomError.incorrectStatusCode(statusCode) }
        return data
    }
    .mapError { CustomError.network($0) }
    .eraseToAnyPublisher()
}

Then you can use this to rewrite both versions of your anyPublisher function:

extension URLSession.DataTaskPublisher {
    func anyPublisher<T:Decodable>(type: T.Type) -> AnyPublisher<T, Error> {
        httpResponseValidator()
            .decode(type: T.self, decoder: JSONDecoder())
            .receive(on: DispatchQueue.main)
            .eraseToAnyPublisher()
    }

    func anyPublisher() -> AnyPublisher<Output, CustomError> {
        httpResponseValidator()
            .receive(on: DispatchQueue.main)
            .eraseToAnyPublisher()
    }
}