handling HTTP status code with URLSession and Combine - swift

Question

I'm trying to handle the responses that arrive from a DataTaskPublisher reading its response status code.
When status code is greater than 299, I'd like to return a ServiceError type as Failure. In every examples that I've seen I've used .mapError and .catch... in this specific case, from a .flatMap, I really don't know how to handle the publisher response to return the Error instead of the TResponse...
return URLSession.DataTaskPublisher(request: urlRequest, session: .shared)
.mapError{error in return ServiceError.request}
.flatMap{ data, response -> AnyPublisher<TResponse, ServiceError> in
if let httpResponse = response as? HTTPURLResponse,
(200...299).contains(httpResponse.statusCode){
return Just(data)
.decode(type: TResponse.self, decoder: JSONDecoder())
.mapError{error in return ServiceError.decode}
.eraseToAnyPublisher()
}else{
//???? HOW TO HANDLE THE ERROR?
}
}
.receive(on: RunLoop.main)
.eraseToAnyPublisher()

Answer 1

enum ServiceErrors: Error {
case internalError(_ statusCode: Int)
case serverError(_ statusCode: Int)
}
return URLSession.shared.dataTaskPublisher(for: urlRequest)
.tryMap { data, response in
guard let httpResponse = response as? HTTPURLResponse,
200..<300 ~= httpResponse.statusCode else {
switch (response as! HTTPURLResponse).statusCode {
case (400...499):
throw ServiceErrors.internalError((response as! HTTPURLResponse).statusCode)
default:
throw ServiceErrors.serverError((response as! HTTPURLResponse).statusCode)
}
}
return data
}
.mapError { $0 as! ServiceErrors }
.decode(type: T.self, decoder: JSONDecoder())
.receive(on: RunLoop.main)
.eraseToAnyPublisher()
NOTE: I relied on this link to make my error handler.

Answer 2

If I correctly understood your goal, you need something like
}else{
return Fail(error: ServiceError.badServiceReply)
.eraseToAnyPublisher()
}
Simple example:
URLSession.shared
.dataTaskPublisher(for: URL(string: "https://www.google.com")!)
.receive(on: DispatchQueue.main)
.flatMap { _ in
Fail(error: URLError(URLError.unsupportedURL)).eraseToAnyPublisher()
} //for the sake of the demo
.replaceError(with: "An error occurred") //this sets Failure to Never
.assign(to: \.stringValue, on: self)
.store(in: &cancellableBag)
would always assign string "An error occurred" due to remap to Fail publisher

Answer 3

You can use tryMap here:
URLSession.shared.dataTaskPublisher(for: urlRequest)
.mapError { error in return ServiceError.request }
.tryMap { (data, response) throws -> TResponse in
guard let httpResponse = response as? HTTPURLResponse,
200...299 ~= httpResponse.statusCode else {
throw ServiceError.invalidStatusCode((response as? HTTPURLResponse)?.statusCode ?? 0)
}
return try JSONDecoder().decode(TResponse.self, from: data)
}
.receive(on: RunLoop.main)
.eraseToAnyPublisher()
This will also simplify your code as you don't need do create an extra publisher.
I also made a few other improvements to the original code:
used URLSession.shared.dataTaskPublisher(for: urlRequest) instead of URLSession.DataTaskPublisher(request: urlRequest, session: .shared)
used the ~= operator to check the status code
used a guard instead of a let as it's more idiomatic