ObservableProperty.kt

import kotlin.reflect.KProperty
import kotlin.reflect.KProperty0
import kotlin.reflect.jvm.isAccessible


class SomeClass {

    var someProperty by observable(1)
}


fun main() {
    val someObject = SomeClass()

    someObject::someProperty.observe { new, old ->
        println("Property has changed from $old to $new.")
    }

    someObject.someProperty = 5
    // Property has changed from 1 to 5.
}


class ObservableProperty<T>(private var value: T) {

    private val listeners = mutableListOf<(T, T) -> Unit>()


    fun addListener(listener: (new: T, old: T) -> Unit) {
        listeners += listener
    }


    operator fun getValue(thisRef: Any?, property: KProperty<*>) =
        value


    operator fun setValue(thisRef: Any?, property: KProperty<*>, value: T) {
        val oldValue = this.value

        this.value = value

        for (listener in listeners)
            listener(value, oldValue)
    }
}


fun <T> observable(initialValue: T) =
    ObservableProperty(initialValue)


fun <T> KProperty0<T>.observe(listener: (new: T, old: T) -> Unit) {
    isAccessible = true
    (getDelegate() as ObservableProperty<T>).addListener(listener)
}