when to use an inline function in Kotlin?

Question

I know that an inline function will maybe improve performance & cause the generated code to grow, but I\'m not sure when it is correct to use one.

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Answer 1

One simple case where you might want one is when you create a util function that takes in a suspend block. Consider this.

fun timer(block: () -> Unit) {
    // stuff
    block()
    //stuff
}

fun logic() { }

suspend fun asyncLogic() { }

fun main() {
    timer { logic() }

    // This is an error
    timer { asyncLogic() }
}

In this case, our timer won't accept suspend functions. To solve it, you might be tempted to make it suspend as well

suspend fun timer(block: suspend () -> Unit) {
    // stuff
    block()
    // stuff
}

But then it can only be used from coroutines/suspend functions itself. Then you'll end up making an async version and a non-async version of these utils. The problem goes away if you make it inline.

inline fun timer(block: () -> Unit) {
    // stuff
    block()
    // stuff
}

fun main() {
    // timer can be used from anywhere now
    timer { logic() }

    launch {
        timer { asyncLogic() }
    }
}

Here is a kotlin playground with the error state. Make the timer inline to solve it.