Replace nil with Empty or Error in Combine - swift

Question

I have a Combine publisher like this:
enum RemoteError: Error {
case networkError(Error)
case parseError(Error)
case emptyResponse
}
func getPublisher(url: URL) -> AnyPublisher<Entiy, RemoteError> {
return URLSession.shared
.dataTaskPublisher(for: url)
.map(\.data)
.decode(type: RemoteResponse.self, decoder: decoder)
.mapError { error -> RemoteError in
switch error {
case is URLError:
return .networkError(error)
default:
return .parseError(error)
}
}
.map { response -> Entiy in
response.enitities.last
}
.eraseToAnyPublisher()
}
struct RemoteResponse: Codable {
let enitities: [Entity]
let numberOfEntries: Int
}
struct Entity {
}
By the above setting, the compiler complains because response.enitities.last can be nil. The question is can I replace nil with Empty publisher and if not can I replace it with error emptyResponse in Combine chain? The first option is preferable.

Answer 1

You have a couple of options here.
If you don't want the publisher to publish anything in case entities is empty, you can use coampactMap instead of map:
.compactMap { response in
response.entities.last
}
If you would rather publish an error in such a case you can use tryMap which allows you to throw an Error. You would need mapError to come after it:
.tryMap { response in
guard let entity = response.entities.last else {
throw RemoteError.emptyResponse
}
return entity
}
.mapError { error -> RemoteError in
switch error {
case is URLError:
return .networkError(error)
case is DecodingError:
return .parseError(error)
default:
return .emptyResponse
}
}

Answer 2

You need a flat map in order to map to another publisher:
.flatMap {
$0.enitities.last.publisher
}
Optional has a convenient publisher property that gives you a publisher that publishes only that value if the value is not nil, and an empty publisher if it is nil. This is only available in iOS 14+. If you are targeting a lower version, you need to do something like:
.flatMap { (response) -> AnyPublisher<Entity, Never> in
if let last = response.entities.last {
return Just(last).eraseToAnyPublisher()
} else {
return Empty(completeImmediately: true).eraseToAnyPublisher()
}
}

Answer 3

Following the answer from #Sweeper, Here edit with below iOS 14.0
.setFailureType(to: NSError.self)
.flatMap { (response) -> AnyPublisher<Entity, Never> in
if let last = response.entities.last {
return Just(last).eraseToAnyPublisher()
} else {
return Empty(completeImmediately: true).eraseToAnyPublisher()
}
NSError -> will be error type which you are returning
Happy coding 🚀