TwitterOAuthRequestTokenPublisher.swift

func oAuthRequestTokenPublisher() -> AnyPublisher<TemporaryCredentials, OAuthError> {
    // 1
    let request = (baseURLString: "https://api.twitter.com/oauth/request_token",
                   httpMethod: "POST",
                   consumerKey: ClientCredentials.APIKey,
                   consumerSecret: ClientCredentials.APIKeySecret,
                   callbackURLString: "\(ClientCredentials.CallbackURLScheme)://")

    // 2
    guard let baseURL = URL(string: request.baseURLString) else {
        return Fail(error: OAuthError.urlError(URLError(.badURL)))
            .eraseToAnyPublisher()
    }

    // 3
    var parameters = [
        URLQueryItem(name: "oauth_callback", value: request.callbackURLString),
        URLQueryItem(name: "oauth_consumer_key", value: request.consumerKey),
        URLQueryItem(name: "oauth_nonce", value: UUID().uuidString),
        URLQueryItem(name: "oauth_signature_method", value: "HMAC-SHA1"),
        URLQueryItem(name: "oauth_timestamp", value: String(Int(Date().timeIntervalSince1970))),
        URLQueryItem(name: "oauth_version", value: "1.0")
    ]

    // 4
    let signature = oAuthSignature(httpMethod: request.httpMethod,
                                   baseURLString: request.baseURLString,
                                   parameters: parameters,
                                   consumerSecret: request.consumerSecret)

    // 5
    parameters.append(URLQueryItem(name: "oauth_signature", value: signature))

    // 6
    var urlRequest = URLRequest(url: baseURL)
    urlRequest.httpMethod = request.httpMethod
    urlRequest.setValue(oAuthAuthorizationHeader(parameters: parameters),
                        forHTTPHeaderField: "Authorization")

    return
        //7
        URLSession.shared.dataTaskPublisher(for: urlRequest)
        // 8
        .tryMap { data, response -> TemporaryCredentials in
            // 9
            guard let response = response as? HTTPURLResponse
            else { throw OAuthError.unknown }

            // 10
            guard response.statusCode == 200
            else { throw OAuthError.httpURLResponse(response.statusCode) }

            // 11
            guard let parameterString = String(data: data, encoding: .utf8)
            else { throw OAuthError.cannotDecodeRawData }

            // 12
            if let parameters = parameterString.urlQueryItems {
                // 13
                guard let oAuthToken = parameters["oauth_token"],
                      let oAuthTokenSecret = parameters["oauth_token_secret"],
                      let oAuthCallbackConfirmed = parameters["oauth_callback_confirmed"]
                else {
                    throw OAuthError.unexpectedResponse
                }

                // 14
                if oAuthCallbackConfirmed != "true" {
                    throw OAuthError.failedToConfirmCallback
                }

                // 15
                return TemporaryCredentials(requestToken: oAuthToken,
                                            requestTokenSecret: oAuthTokenSecret)
            } else {
                throw OAuthError.cannotParseResponse
            }
        }
        // 16
        .mapError { error -> OAuthError in
            switch (error) {
            case let oAuthError as OAuthError:
                return oAuthError
            default:
                return OAuthError.unknown
            }
        }
        // 17
        .eraseToAnyPublisher()
}