Swift: Extending Combine operators - swift

Question

I'm trying to wrap a tryMap operator along the lines of this article.
extension Publisher where Output == Data {
func decode<T: Decodable>(as type: T.Type = T.self, using decoder: JSONDecoder = .init()) -> Publishers.Decode<Self, T, JSONDecoder> {
decode(type: type, decoder: decoder)
}
}
extension Publisher where Output == URLSession.DataTaskPublisher.Output {
func processData(_: #escaping (Self.Output) throws -> Data) -> Publishers.TryMap<Self, Data> {
tryMap { element -> Data in
guard let httpResponse = element.response as? HTTPURLResponse,
httpResponse.statusCode == 200
else {
throw URLError(.badServerResponse)
}
return element.data
}
}
}
While using it I'm getting a compiler error which I'm struggling with:
return urlSession
.dataTaskPublisher(for: request)
.processData // <- Value of type '(#escaping (URLSession.DataTaskPublisher.Output) throws -> Data) -> Publishers.TryMap<URLSession.DataTaskPublisher, Data>' (aka '(#escaping ((data: Data, response: URLResponse)) throws -> Data) -> Publishers.TryMap<URLSession.DataTaskPublisher, Data>') has no member 'decode'
.decode(as: InstantResponse.self)
.eraseToAnyPublisher()
What would be the correct way of doing it?
Thanks!

Answer 1

First of all, you aren't calling processData - you are missing the parentheses, which would actually execute the function. Second, your processData declaration is incorrect, it should not take a closure as its input argument, since you aren't using that closure anyways.
extension Publisher where Output == URLSession.DataTaskPublisher.Output {
func processData() -> Publishers.TryMap<Self, Data> {
tryMap { element -> Data in
guard let httpResponse = element.response as? HTTPURLResponse,
httpResponse.statusCode == 200
else {
throw URLError(.badServerResponse)
}
return element.data
}
}
}
return urlSession
.dataTaskPublisher(for: request)
.processData() // parentheses necessary here
.decode(as: InstantResponse.self)
.eraseToAnyPublisher()