Error handling in Swift: Error from one enum to another - swift

Question

I have this code in a ServiceClient. It handles service-level calls, like signIn(user, password, completion), listObjects(completion), addObject(objectID, content, completion), getObject(id, completion) etc. It contains (but doesn't subclass) an APIClient, which performs only basic HTTPS services like perform(request, completion).
I don't really want the controller that sits above this to deal with 404s as success, which means trapping the error in ServiceClient. So the idea is APIClient deals with networking errors whereas ServiceClient deals with unexpected HTTP results.
So I end up with this in ServiceClient, where errors like invalidURL are converted from an APIClient enum to a ServiceClient enum:
apiClient.perform(request) {result in
switch result {
case .success(let data):
guard data.statusCode == 200 else {
completion(.failure(.badResponse))
return
}
completion(.success(data))
case .failure(let error):
switch error {
case .invalidURL:
completion(.failure(.invalidURL))
case .requestFailed:
completion(.failure(.requestFailed))
case .decodingFailure:
completion(.failure(.decodingFailure))
}
}
}
I think in this case I'll just make APIClient handle invalid HTTP status codes, but what's the more general solution to this? At some point I'll want different error codes for different service clients, at which point this becomes a problem again.

Answer 1

I suggest using Int type enumeration for both ServiceClient and APIClient.
As I understood this is your custom enumerations.
So, assuming you have ServiceClientError and APIClientError you can implement them using this way:
enum ServiceClientError: Int {
case invalidURL, requestFailed, decodingFailure
}
enum APIClientError: Int {
case invalidURL, requestFailed, decodingFailure
}
You can create your custom conversion method:
extension ServiceClientError {
static func create(from apiClientError: APIClientError) -> ServiceClientError {
return ServiceClientError(rawValue: apiClientError.rawValue)
}
}
Wanted function:
apiClient.perform(request) {result in
switch result {
case .success(let data):
guard data.statusCode == 200 else {
completion(.failure(.badResponse))
return
}
completion(.success(data))
case .failure(let error):
guard let serviceClientError = ServiceClientError.create(from: error) else {
/// Handle incorrect behavior
fatalError("Wrong enumeration mapping")
return
}
completion(.failure(serviceClientError))
}
}