Example of how to use SignalProducer to fetch multiple pages

GitHubAPI.swift

private func fetchAllUsers() -> SignalProducer<[User], Error> {
    let decoder = JSONDecoder()
    decoder.keyDecodingStrategy = .convertFromSnakeCase

    func fetchUsers(since lastSeenUserID: Int?) -> SignalProducer<[User], Error> {
        var components = URLComponents(string: "https://api.github.com/users")!

        if let userID = lastSeenUserID {
            components.queryItems = [
                URLQueryItem(name: "since", value: userID.description)
            ]
        }
        var request = URLRequest(url: components.url!)
        request.setValue("application/vnd.github.v3+json", forHTTPHeaderField: "Accept")

        return URLSession.shared.reactive
            .data(with: request)
            .flatMap(.concat) { (data, _) -> SignalProducer<[User], Error> in
                SignalProducer { () in
                    Result<[User], Error> {
                        try decoder.decode([User].self, from: data)
                    }
                }
            }
    }

    // Adapted from the `repeat` operator: https://github.com/ReactiveCocoa/ReactiveSwift/blob/3f4351d04115fd8797802d9b2d17b812cd761602/Sources/SignalProducer.swift#L2270-L2304
    return SignalProducer { observer, lifetime in
        func iterate(since lastSeenUserID: Int?) {
            fetchUsers(since: lastSeenUserID)
                .startWithSignal { (signal, signalDisposable) in
                    lifetime += signalDisposable

                    signal.observe { event in
                        switch event {
                        case .value(let users):
                            if let last = users.last, last.id < 300 {
                                // Emit the values we just got
                                observer.send(value: users)

                                // And get the next ones
                                iterate(since: last.id)
                            } else {
                                observer.sendCompleted()
                            }
                        case .failed(let error):
                            observer.send(error: error)
                        case .interrupted:
                            observer.sendInterrupted()
                        case .completed:
                            // Ignore completed events of the child streams
                            break
                        }
                    }
                }
        }

        // Kick off the request for the first page
        iterate(since: nil)
    }
}