Simple example of how to implement the Observable pattern in Kotlin with kotlin.properties.Delegates and Delegates.observable

ObservableExample.kt

import kotlin.properties.Delegates

interface Publisher {
    fun onNews(news: String)
}

class RadioChannel : Publisher {
    override fun onNews(news: String) = println("Heard on radio: $news")
}

class Newspaper : Publisher {
    override fun onNews(news: String) = println("Read in newspaper: $news")
}

class NewsAgency {
    private val listeners = mutableListOf<Publisher>()
    var news: String by Delegates.observable(initialValue = "") { _, old, new ->
        if (new != old) listeners.map { listener -> listener.onNews(new) }
    }

    fun subscribe(publisher: Publisher) = listeners.add(publisher)
    fun unsubscribe(publisher: Publisher) = listeners.remove(publisher)
}

fun main(args: Array<String>) {
    // Set up objects
    val radioChannel = RadioChannel()
    val newspaper = Newspaper()
    val newsAgency = NewsAgency()

    // Add listeners
    newsAgency.subscribe(radioChannel)
    newsAgency.subscribe(newspaper)

    // Spread news, is redundant
    newsAgency.news = "Celtics won!"
    newsAgency.news = "Celtics won!"
    newsAgency.news = "Celtics won!"

    // Newspaper went out of business
    newsAgency.unsubscribe(newspaper)
    newsAgency.news = "Traffic jam on I93!"
}