CODESAMPLE

Observer - Kotlin

Observer Kotlin

The Observer pattern defines a one-to-many dependency between objects, so that when one object changes state, all its dependents are notified and updated automatically. This promotes loose coupling and improved modularity. Here, Subject maintains a list of Observers and notifies them when its data changes. The ConcreteSubject holds the data and notifies observers. Observer defines an update method, and ConcreteObserver implements this to react to the subject’s changes. Kotlin’s support for extension functions and first-class functions allows for a concise and flexible implementation. The use of interfaces promotes abstraction.

// Observer Pattern in Kotlin

// Observer Interface
interface Observer {
    fun update(data: String)
}

// Subject Interface
interface Subject {
    fun registerObserver(observer: Observer)
    fun unregisterObserver(observer: Observer)
    fun notifyObservers(data: String)
}

// Concrete Subject
class ConcreteSubject : Subject {
    private val observers = mutableListOf<Observer>()
    private var data: String = "Initial Data"

    override fun registerObserver(observer: Observer) {
        observers.add(observer)
    }

    override fun unregisterObserver(observer: Observer) {
        observers.remove(observer)
    }

    override fun notifyObservers(data: String) {
        this.data = data
        observers.forEach { it.update(data) }
    }

    fun getData(): String = data
}

// Concrete Observer
class ConcreteObserver(private val id: Int) : Observer {
    override fun update(data: String) {
        println("Observer $id: Data updated to: $data")
    }
}

// Example Usage
fun main() {
    val subject = ConcreteSubject()

    val observer1 = ConcreteObserver(1)
    val observer2 = ConcreteObserver(2)

    subject.registerObserver(observer1)
    subject.registerObserver(observer2)

    subject.notifyObservers("New Data 1")
    subject.notifyObservers("New Data 2")

    subject.unregisterObserver(observer1)
    subject.notifyObservers("Final Data")
}