What does ".()" mean in Kotlin?

Question

I've seen examples where a function has an argument given by ClassName.() This doesn't seem to be an extension function, which is ClassName.Function()

An example is Kotterknife:

private val View.viewFinder: View.(Int) -> View?     get() = { findViewById(it) } 

Which I don't quite know the function of,

and MaterialDrawerKt

fun Activity.drawer(setup: DrawerBuilderKt.() -> Unit = {}): Drawer {     val builder = DrawerBuilderKt(this)     builder.setup()     return builder.build() } 

Where the code allows you to directly call

drawer {     ... } 

rather than give it arguments surrounded by the parentheses.

Is there any documentation on this anywhere?

Answer 1

A function that takes in nothing and returns nothing in Kotlin looks like:

var function : () -> Unit 

The difference is that the function in your code takes in nothing, returns nothing, but is invoked on an object.

For example,

class Builder (val multiplier: Int) {      fun invokeStuff(action: (Builder.() -> Unit)) {         this.action()     }      fun multiply(value: Int) : Int {         return value * multiplier     } } 

The important bit here is the way we've declared the type of 'action'

action: (Builder.() -> Unit) 

This is a function that returns nothing, takes in nothing but is invoked on an object of type "Builder".

Refer more here.