How to use function receiver type with SAM interfaces in Kotlin-CodePudding

I'm coming from Java and am new to Kotlin. I try to understand how to use receiver type with lambdas specified as functional SAM interfaces.

Let the code speak for itself.

fun interface Delegator <T> {
    fun delegate(receiver: T)
}

fun <T> invokeWithDynamicReceiver(receiver: T, fn: T.() -> Unit) = receiver.fn()
fun <T> invokeWithSamInterface(receiver: T, fn: Delegator<T>) = fn.delegate(receiver)

fun dynamicReceiver() {
    invokeWithDynamicReceiver("Foo") { length } // Dynamic receiver
    invokeWithSamInterface("Foo") { it.length } // Can't bind receiver as "this"
}

How do I need to change the code to use the Delegator lambda with dynamic receiver?

CodePudding user response：

You can define the functions inside the Delegator as extension function, this way the receiver is passed as this to the lambda.

fun interface ExtensionDelegator <T, R> {
    fun T.delegate(): R
}

fun <T, R> invokeWithExtensionSamInterface(receiver: T, fn: ExtensionDelegator<T, R>): R =
    with(fn) { receiver.delegate() }

Alternatively, you can simply define the dynamic receiver with a typealias to achieve the same result.

typealias AliasDelegator<T, R> = T.() -> R

fun <T, R> invokeWithAliasSamInterface(receiver: T, fn: AliasDelegator<T, R>): R = fn.invoke(receiver)

On the use site, both approaches look the same.

fun main() {
    val result = invokeWithExtensionSamInterface("Foo") { length }
    println(result)

    val otherResult = invokeWithAliasSamInterface("Fizz") { length }
    println(otherResult)
}