Mastering High Performance with Kotlin

Observable delegates

The observer pattern is a commonly-used pattern in any programming language. The Kotlin standard library contains a special delegate that implements this pattern:

var subject: String by Delegates.observable("init value") {    property, old, new ->    println("$old -> $new")}

In this code snippet, we can see a subject variable that has an observer. The observer is a lambda that's passed as the second parameter to the observable function. The observable function looks like this:

public inline fun <T> observable(initialValue: T, crossinline onChange: (property: KProperty<*>, oldValue: T, newValue: T) -> Unit):    ReadWriteProperty<Any?, T> = object : ObservableProperty<T>(initialValue) {        override fun afterChange(property: KProperty<*>, ...