Is the event lambda necessary? Will it affect performance?

Question

In a Composable, Usually we use function param to handle the event, such as the following onClickHandler: () -> Unit

In the official documentation, using a lambda to call this onClickHandler function.

@Composable
fun FilledButtonExample(onClickHandler: () -> Unit) {
    Button(onClick = { onClickHandler() }) {
        Text("Filled")
    }
}

Is it better if we assign the onClickHandler function directly to the onClick variable? such as Button(onClick = onClickHandler )

@Composable
fun FilledButtonExample(onClickHandler: () -> Unit) {
    Button(onClick = onClickHandler ) {
        Text("Filled")
    }
}

Answer 1

Removing the redundant lamdba will be better, but the impact on performance will be neglegible.

Let's run a test to see if the compiled output of the two variants is actually different, as the Kotlin Compiler does many optimizations to remove redundancy.

I built two minimal code samples that are anough to test the case:

Lambda.kt

fun main() {
    val onClickHandler: () -> Unit = { println("Hello World") }
    val onClick: () -> Unit  = { onClickHandler() }
    onClick()
}

NoLambda.kt

fun main() {
    val onClickHandler: () -> Unit = { println("Hello World") }
    val onClick: () -> Unit = onClickHandler
    onClick()
}

We can compile a Kotlin file with the following command:

kotlinc Lamdba.kt -d Lambda

When we compare the outputs, we see the following:

Lamdba Output

NoLamdba Output

The Kotlin Compiler generates a separate class file for each function. So by omitting the redundant lamdba, we actually reduce the number of generated class files.
The practical impact on performance will be neglegible though.