Kotlin Programming Cookbook

How to do it…

The observable delegates take in a default value and a construct where we have old and new values. Let's take a look at the next example:

fun main(args: Array<String>) {    var a:String by Delegates.observable("",{_,oldValue,newValue ->        println("old value: $oldValue, new value: $newValue ")    })    a="a"    a="b">}//Output:old value: , new value: a          old value: a, new value: b

In the preceding example, we have provided the initial value as an empty string. The construct will be executed every time we try to update the value of the a property. We have changed the value of a two times and hence we are seeing two print statements.