Functional variables

Let's quickly refresh how to work with function variables. We can define a variable with a functional type like this:

var onClickHandler: (ViewState) -> Unit = {}

We can then reassign that stored function as with any other variable:

fun main() {
    onClickHandler = { viewState ->
        println("viewState -> ${viewState.title} 
                ${viewState.subtitle}")
    }
}

And finally, since the variable represents a function, we can invoke the function in several different ways by referencing the variable name, as follows:

fun main() {
    onClickHandler = { viewState ->
        println("viewState -> ${viewState.title} 
                ${viewState.subtitle}")
    }
    
    val viewState = ViewState("Hello", "Kotlin")
    onClickHandler(viewState)
    onClickHandler.invoke(viewState)
}

By allowing us to store and update variables with a functional type, Kotlin enables us to start thinking in terms of functions and writing code that more heavily relies on functional operations.