MainObservable.kt

import java.util.*
import kotlin.properties.Delegates

interface ValueChangedListener {
    fun onValueChanged(newValue: String)
}

class PrintingTextChangedListener: ValueChangedListener {
    override fun onValueChanged(newValue: String) {
        println("Value of the text is: $newValue")
    }
}

class ObservableText(listener: ValueChangedListener) {
    var text: String by Delegates.observable(
            initialValue = "",
            onChange = {
                property, oldValue, newValue ->
                listener.onValueChanged(newValue)
            }
    )
}

fun main(args: Array<String>) {
    val initialObservableObject = ObservableText(PrintingTextChangedListener())

    initialObservableObject.text = "Hello"
    initialObservableObject.text = "World"
}