Mock Swift APIClient with Combine

MockAPIClient+Combine.swift

@available(iOS 13.0, macOS 10.15, tvOS 13.0, watchOS 6.0, macCatalyst 13.0, *)

// MARK: - Request

protocol Request {
    associatedtype Response: Decodable
    var urlRequest: URLRequest { get }
}

// MARK: - APISession

public protocol APISession {
    func dataTaskPublisher<R: Request>(for request: R) -> AnyPublisher<(data: Data, response: URLResponse), URLError>
}

public protocol APISession {
    func dataTaskPublisher<R: Request>(for request: R) -> AnyPublisher<(data: Data, response: URLResponse), URLError>
}

// MARK: - APIClient

protocol APIClient {
    associatedtype Session: APISession
    var session: Session { get }
    func request<R: Request>(of request: R) -> AnyPublisher<R.Response, Error>
}

public extension APIClient {
    func request<R: Request>(of request: R) -> AnyPublisher<R.Response, Error> {
        session
            .dataTaskPublisher(for: request)
            .tryMap { data, response in
                     // something
        }
        .eraseToAnyPublisher()
    }
}

// MARK: - Mock

class MockAPIClient: APIClient {
    let session = MockAPISession()
}

class MockAPISession: APISession {
    func dataTaskPublisher<R: Request>(for request: R) -> AnyPublisher<(data: Data, response: URLResponse), URLError> {
        MockDataTaskPublisher(request: request)
            .eraseToAnyPublisher()
    }
}

struct MockDataTaskPublisher<R: Request>: Publisher {
    typealias Output = (data: Data, response: URLResponse)
    typealias Failure = URLError

    let request: R

    func receive<S>(subscriber: S) where S : Subscriber, Self.Failure == S.Failure, Self.Output == S.Input {
        // something
    }
}