Interactors.Authentication.swift

public class Authentication {

    public typealias Credentials = (username: String, password: String)

    public let credentials: SafeObserver<Credentials>
    public let token: SafeSignal<Token>

    public init(client: SafeClient) {

        let credentials = SafePublishSubject<Credentials>()

        self.token = credentials
            .flatMapLatest { username, password in
                let request = API.User.login(username: username, password: password)
                return client.response(for: request)
            }

        self.credentials = credentials.toObserver()
    }
}