LongPollingTransport.swift

//
//  LongPollingTransport.swift
//  SignalRClient
//
//  Created by David Robertson on 13/07/2020.
//

import Foundation

public class LongPollingTransport: Transport {
    public var delegate: TransportDelegate?

    private let logger: Logger
    private let closeQueue = DispatchQueue(label: "LongPollingTransportCloseQueue")

    private var active = false
    private var opened = false
    private var closeCalled = false
    private var httpClient: HttpClientProtocol?
    private var url: URL?
    private var closeError: Error?

    public let inherentKeepAlive = true

    init(logger: Logger) {
        self.logger = logger
    }

    public func start(url: URL, options: HttpConnectionOptions) {
        logger.log(logLevel: .info, message: "Starting LongPolling transport")
        httpClient = options.httpClientFactory(options)
        self.url = url
        opened = false
        closeError = nil
        closeCalled = false
        active = true
        triggerPoll()
    }

    public func send(data: Data, sendDidComplete: @escaping (Error?) -> Void) {
        guard active, let httpClient = httpClient, let url = url else {
            sendDidComplete(SignalRError.invalidState)
            return
        }
        httpClient.post(url: url, body: data) { (responseOptional, errorOptional) in
            if let error = errorOptional {
                sendDidComplete(error)
            } else if let response = responseOptional {
                if response.statusCode == 200 {
                    sendDidComplete(nil)
                } else {
                    sendDidComplete(SignalRError.webError(statusCode: response.statusCode))
                }
            }
        }
    }

    public func close() {
        closeQueue.sync {
            if !closeCalled {
                closeCalled = true
                active = false
                self.logger.log(logLevel: .debug, message: "Sending LongPolling session DELETE request...")
                self.httpClient?.delete(
                    url: self.url!,
                    completionHandler: { (_, errorOptional) in
                        if let error = errorOptional {
                            self.logger.log(
                                logLevel: .error, message: "Error while DELETE-ing long polling session: \(error)")
                            self.delegate?.transportDidClose(error)
                        } else {
                            self.logger.log(logLevel: .info, message: "LongPolling transport stopped.")
                            self.delegate?.transportDidClose(self.closeError)
                        }
                    })
            } else {
                self.logger.log(logLevel: .debug, message: "closeCalled flag is already set")
            }
        }
    }

    private func triggerPoll() {
        if self.active {
            let pollUrl = self.getPollUrl()
            self.logger.log(logLevel: .debug, message: "Polling \(pollUrl)")
            self.httpClient?.get(url: pollUrl, completionHandler: self.handlePollResponse(response:error:))
        } else {
            self.logger.log(logLevel: .debug, message: "Long Polling transport polling complete.")
            self.close()
        }
    }

    private func handlePollResponse(response: HttpResponse?, error: Error?) {
        if let error = error {
            if (error as? URLError)?.errorCode == NSURLErrorTimedOut {
                self.logger.log(logLevel: .debug, message: "Poll timed out (client side), reissuing.")
            } else {
                self.logger.log(logLevel: .error, message: "Error during polling: \(error)")
                self.closeError = error
                self.active = false
            }

        } else if let response = response {
            switch response.statusCode {
            case 204:
                self.logger.log(logLevel: .info, message: "LongPolling transport terminated by server.")
                self.closeError = nil
                self.active = false

            case 200:
                if !self.opened {
                    // First response must be discarded.
                    self.opened = true
                    self.delegate?.transportDidOpen()
                } else if let data = response.contents, data.count > 0 {
                    self.logger.log(logLevel: .debug, message: "Message received: \(data)")
                    self.delegate?.transportDidReceiveData(data)
                } else {
                    self.logger.log(logLevel: .debug, message: "Poll timed out (server side), reissuing.")
                }

            case 404:
                // If we have a poll request in progress when .close() is called, the session will be destroyed and the server
                // will respond with 404. So if we get a 404 when the active flag is false, this is normal. Otherwise,
                // we should treat this as an unexpected response.
                if self.active {
                    fallthrough
                }
            default:
                self.logger.log(logLevel: .error, message: "Unexpected response code \(response.statusCode)")
                self.closeError = SignalRError.webError(statusCode: response.statusCode)
                self.active = false
            }
        }

        self.triggerPoll()
    }

    private func getPollUrl() -> URL {
        var components = URLComponents.init(url: self.url!, resolvingAgainstBaseURL: true)!
        if components.queryItems == nil {
            components.queryItems = []
        }
        let millisecondUnixTime = Int64(Date().timeIntervalSince1970 * 1000)
        components.queryItems?.append(URLQueryItem(name: "_", value: String(millisecondUnixTime)))
        let pollUrl = components.url
        return pollUrl!
    }

}