observer-pattern-2.swift

/*
Observer Pattern
1. The subscriber is the "observer" object and receives updates.
2. The publisher is the "observable" object and sends updates.
3. The value is the underlying object that's changed.

Reference: https://howtodoinjava.com/design-patterns/behavioral/observer-design-pattern/
*/

import Foundation

struct Value {
    let text: String
}

protocol Subscriber: AnyObject {
    func update(value: Value)
}

protocol Publisher {
    func attach(subscriber: Subscriber)
    func detach(subscriber: Subscriber)
    func notifyUpdate(value: Value)
}

final class MessagePublisher: Publisher {
    
    private var subscribers: [Subscriber] = []
    
    func attach(subscriber: Subscriber) {
        if !subscribers.contains(where: { $0 === subscriber }) {
            subscribers.append(subscriber)
        }
    }
    
    func detach(subscriber: Subscriber) {
        if let index = subscribers.firstIndex(where: { $0 === subscriber }) {
            subscribers.remove(at: index)
        }
    }
    
    func notifyUpdate(value: Value) {
        for subscriber in subscribers {
            subscriber.update(value: value)
        }
    }
}

// MARK: - Subscribers

final class MessageSubscriberOne: Subscriber {
    func update(value: Value) {
        print("MessageSubscriberOne: \(value.text)")
    }
}

final class MessageSubscriberTwo: Subscriber {
    func update(value: Value) {
        print("MessageSubscriberTwo: \(value.text)")
    }
}

final class MessageSubscriberThree: Subscriber {
    func update(value: Value) {
        print("MessageSubscriberThree: \(value.text)")
    }
}

// MARK: -

let messageOne = MessageSubscriberOne()
let messageTwo = MessageSubscriberTwo()
let messageThree = MessageSubscriberThree()

let messagePublisher = MessagePublisher()
messagePublisher.attach(subscriber: messageOne)
messagePublisher.attach(subscriber: messageTwo)

messagePublisher.notifyUpdate(value: Value(text: "Hello"))

messagePublisher.detach(subscriber: messageTwo)
messagePublisher.attach(subscriber: messageThree)

messagePublisher.notifyUpdate(value: Value(text: "I'm Bradley"))

/*
MessageSubscriberOne: Hello
MessageSubscriberTwo: Hello
MessageSubscriberOne: I'm Bradley
MessageSubscriberThree: I'm Bradley
*/