The great power of .with

And now we've come to the last of the Kotlin scope functions in our journey: .with(). The context object is passed in as an argument to it, but inside the corresponding lambda you can access it as a receiver using the keyword this. Additionally, .with() as it turns out, is not an extension function, and it has two neat uses.

The first is to use the given object to perform a set of calculations or tasks and return the result. For example, let's say I had a list of users and wanted to sort the list and then print certain properties from each user. I could do something like this.

data class Avenger(
    val alias: String,
    val realName: String,
)

fun main() {
    val avengers = mutableListOf(
        Avenger("Captain America", "Steve Rogers"),
        Avenger("Black Widow", "Natasha Romanoff"),
        Avenger("Iron Man", "Tony Stark"),
        Avenger("Scarlet Witch", "Wanda Maximoff"),
    )

    with(avengers) {
        this.sortBy { it.alias }
        this.forEach { 
            println("${it.alias}: ${it.realName}") 
        }
    }

    // Black Widow: Natasha Romanoff
    // Captain America: Steve Rogers
    // Iron Man: Tony Stark
    // Scarlet Witch: Wanda Maximoff
}

A second neat option for using .with() that is outlined in the official documentation is create a helper object and use its properties and methods to calculate a value. For my example, I'll pretend that there was a tournament between all the Avengers and the first person on the list was the winner. I could write the following.

val tourneyResults = with(avengers) {
       "${first().alias} won the tournament over ${last().alias}"
   }

    println(tourneyResults)
    // Black Widow won the tournament over Scarlet Witch

I know what you're thinking, those are some pretty controversial tourney results 😆. But regardless, it's an awesome use case.

Lastly, this article would not be complete without the following joke.

val greatPower = "great responsibility"
with(greatPower) {
    println("With great power comes $this")
}
// With great power comes great responsibility