APIKitとCombineを使う #Swift

　初投稿です。APIKit の Session に Combine の Publisher を生やそうと思います（Session に Publisher が生えていると、楽しいので）。

extension Session {
    struct Send<R>: Publisher where R: Request {
        typealias Output = R.Response
        typealias Failure = SessionTaskError

        let request: R

        func receive<S>(subscriber: S) where S: Subscriber, Failure == S.Failure, Output == S.Input {
            let task = Session.send(request) { result in
                switch result {
                case let .success(response):
                    _ = subscriber.receive(response)
                    subscriber.receive(completion: .finished)

                case let .failure(error):
                    subscriber.receive(completion: .failure(error))
                }
            }

            let subscription = Cancel(task: task)
            subscriber.receive(subscription: subscription)
        }
    }

    struct Cancel: Subscription {
        let combineIdentifier = CombineIdentifier()
        let task: SessionTask?

        func request(_: Subscribers.Demand) {}

        func cancel() {
            task?.cancel()
        }
    }
}

リクエストをつくります。

class GitHubAPI {
    struct SearchRepositories: Request {
        struct Response: Codable {
            struct Item: Codable {
                let fullName: String
            }

            let items: [Item]
        }

        let baseURL = URL(string: "https://api.github.com")!

        let method = HTTPMethod.get

        let path = "/search/repositories"

        var queryParameters: [String: Any]? {
            ["q": query]
        }

        let query: String

        func response(from object: Any, urlResponse _: HTTPURLResponse) throws -> Response {
            guard let data = object as? Data else {
                throw ResponseError.unexpectedObject(object)
            }

            let decoder = JSONDecoder()
            decoder.keyDecodingStrategy = .convertFromSnakeCase
            return try decoder.decode(Response.self, from: data)
        }
    }
}

// Codable のために APIKit のパースを止める

class PassthroughParser: DataParser {
    let contentType: String? = nil

    func parse(data: Data) throws -> Any {
        return data
    }
}

extension Request {
    var dataParser: DataParser {
        PassthroughParser()
    }
}

リクエストを送ります。

Session.Send(request: GitHubAPI.SearchRepositories(query: "SwiftUI"))
    .map { $0.items }
    .flatMap { Publishers.Sequence(sequence: $0) }
    .sink { print($0.fullName) }

すべてが Publisher になる。