CODESAMPLE

Message Broker - Swift

Message Broker Swift

The Message Broker pattern decouples application components by allowing them to communicate through intermediary message channels. Components don’t need to know about each other directly; they simply publish messages to the broker, and other components subscribe to the messages they’re interested in. This promotes scalability, maintainability, and flexibility.

This Swift implementation uses a simple NotificationCenter as the message broker. Publisher structs define message types. Components publish messages using the post function of NotificationCenter, and subscribe by observing notifications for specific message types. This approach leverages Swift’s built-in event handling mechanism, making it concise and idiomatic. The use of structs for messages aligns with Swift’s value-type philosophy, and the NotificationCenter handles the decoupling efficiently.

// Define message types as structs
struct UserCreated {
    let userId: Int
    let username: String
}

struct ProductPurchased {
    let productId: Int
    let userId: Int
}

// Message Broker (using NotificationCenter)
import Foundation

class MessageBroker {
    static let shared = MessageBroker()

    private init() {}

    func publish<T: Hashable>(message: T, userInfo: [AnyHashable : Any]? = nil) {
        NotificationCenter.default.post(name: NSNotification.Name(rawValue: String(describing: message)), object: nil, userInfo: userInfo)
    }

    func subscribe<T: Hashable>(observer: Any, selector: Selector, messageType: T.Type) {
        NotificationCenter.default.addObserver(observer, selector: selector, name: NSNotification.Name(rawValue: String(describing: messageType)), object: nil)
    }
}

// Example Component 1: User Service
class UserService {
    func createUser(userId: Int, username: String) {
        print("Creating user \(username) with ID \(userId)")
        let message = UserCreated(userId: userId, username: username)
        MessageBroker.shared.publish(message: message)
    }
}

// Example Component 2: Email Service
class EmailService {
    @objc func handleUserCreated(notification: Notification) {
        guard let userInfo = notification.userInfo, let userId = userInfo["userId"] as? Int, let username = userInfo["username"] as? String else {
            return
        }
        print("Sending welcome email to \(username) (ID: \(userId))")
    }

    func setupSubscription() {
        MessageBroker.shared.subscribe(observer: self, selector: #selector(handleUserCreated), messageType: UserCreated.self)
    }
}

// Example Usage
let userService = UserService()
let emailService = EmailService()
emailService.setupSubscription()

userService.createUser(userId: 123, username: "Alice")