Can't count the value with lambda - kotlin

Question

Please help to understand why the value of clicks doesn't change. I started read "Kotlin in action" and now have more questions than answers (it's joke book is great, but I'm not)
I tried to set clicks before function, it was advice from book "kotlin in action", but the value of var clicks is always 0.
I have the following interface :
interface Button {
fun onClick(function: () -> Unit)
}
And Kotlin file
fun main() {
var clicks = 0
fun tryToClickOnButton(button: Button): Int{
button.onClick{println("UPP $clicks")}
button.onClick{clicks++}
button.onClick{println("UPP $clicks")}
return clicks
}
class SpecialButton: Button{
override fun onClick(function: () -> Unit) {
println("AAAAAA $clicks")
}
}
val button = object: Button {
override fun onClick(function: () -> Unit) {
println("BBBBB $clicks")
}
}
println(tryToClickOnButton(button))
println(tryToClickOnButton(object : Button{
override fun onClick(function: () -> Unit) {
println("CCCCCCC $clicks")
}
}))
println(tryToClickOnButton(SpecialButton()))
}

Answer 1

The problem is that you have created a callback that takes a function, but you never call that function.
Let's remove some code, to make the example simpler:
fun main() {
var clicks = 0
fun tryToClickOnButton(button: Button): Int {
button.onClick { println("UPP $clicks") }
button.onClick { clicks++ }
button.onClick { println("UPP $clicks") }
return clicks
}
val button = object : Button {
override fun onClick(function: () -> Unit) {
println("BBBBB $clicks")
}
}
println(tryToClickOnButton(button))
}
You can see that in your Button's onClick callback, you accept an argument called function of type "function", but you never use it. Because of that, you'll never print "UPP ${clicks}" and you'll never increment clicks value.
To solve the issue, you simply need to call function, like this:
fun main() {
var clicks = 0
fun tryToClickOnButton(button: Button): Int {
button.onClick { println("UPP $clicks") }
button.onClick { clicks++ }
button.onClick { println("UPP $clicks") }
return clicks
}
val button = object : Button {
override fun onClick(function: () -> Unit) {
println("BBBBB $clicks")
function() // this was missing
}
}
println(tryToClickOnButton(button))
}
That code prints:
BBBBB 0
UPP 0
BBBBB 0
BBBBB 1
UPP 1
1