WebSocket in Swift: Building Real-Time Applications for iOS and macOS

Introduction: Why WebSocket is Important for a Swift Developer

In modern mobile and desktop development, users are accustomed to instant updates: the news feed updates without reloading, the interlocutor's status in a chat changes in real-time, and stock quotes arrive with millisecond delays. Traditional HTTP with its request-response model is inefficient for such tasks — it creates excessive load on the server and the device's battery.

The WebSocket protocol comes to the rescue. It establishes a permanent bidirectional connection between the client (an iOS/macOS app in Swift) and the server. In this article, we will look at two main ways to work with WebSocket in Swift: the native URLSessionWebSocketTask (iOS 13+ / macOS 10.15+) and the popular third-party library Starscream. You will learn how to create a real-time chat, configure automatic reconnection, and handle errors.

WebSocket Basics in Swift: URLSessionWebSocketTask

Starting with iOS 13 and macOS Catalina, Apple provides built-in WebSocket support through the URLSessionWebSocketTask class. This is a simple and secure way that does not require installing additional dependencies.

Creating and Configuring a Connection

First, let's create an instance of URLSession and set the configuration. Note: to work with WebSocket, you need to use the wss:// protocol (secure connection) or ws:// (unsecured, for development only).

import Foundation

class WebSocketManager: NSObject {    private var webSocketTask: URLSessionWebSocketTask?    private let session: URLSession        override init() {        let config = URLSessionConfiguration.default        config.timeoutIntervalForRequest = 30        config.waitsForConnectivity = true        self.session = URLSession(configuration: config, delegate: nil, delegateQueue: OperationQueue())        super.init()    }        func connect() {        guard let url = URL(string: "wss://echo.websocket.org") else { return }        webSocketTask = session.webSocketTask(with: url)        webSocketTask?.resume()        print("WebSocket connection established")        receiveMessage()    }}

Sending and Receiving Messages

WebSocket supports two types of messages: String and binary (Data). Let's look at sending:

func sendMessage(_ text: String) {    let message = URLSessionWebSocketTask.Message.string(text)    webSocketTask?.send(message) { error in        if let error = error {            print("Send error: \\(error.localizedDescription)")        } else {            print("Message sent: \\(text)")        }    }}

To receive messages, you need to implement a recursive call to the receive method:

private func receiveMessage() {    webSocketTask?.receive { [weak self] result in        switch result {        case .success(let message):            switch message {            case .string(let text):                print("Received text message: \\(text)")            case .data(let data):                print("Received binary data: \\(data.count) bytes")            @unknown default:                break            }            // Continue listening for the next messages            self?.receiveMessage()        case .failure(let error):            print("Receive error: \\(error.localizedDescription)")            // Here you can implement reconnection        }    }}

Closing the Connection and Error Handling

Properly terminating a WebSocket connection is an important part of a production application:

func disconnect() {    let closeCode = URLSessionWebSocketTask.CloseCode.normalClosure    webSocketTask?.cancel(with: closeCode, reason: nil)    p