Kotlin: Use a lambda in place of a functional interface?

Question

In Java we can do this Events.handler(Handshake.class, hs -> out.println(hs));

In Kotlin however I am trying to replicate the behavior to replace this:

Events.handler(Handshake::class, object : EventHandler<Handshake> {
    override fun handle(event: Handshake) {
        println(event.sent)
    }
})

With a more convenient:

Events.handler(Handshake::class, EventHandler<Handshake> { println(it.sent) })

For some reason in reference to EventHandler:

More preferably however I'd like to use something even shorter like this: Events.handler(Handshake::class, { println(it.sent) })

Or use the advertised feature to use the method like this: Events.handler(Handshake::class) { println(it.sent) }

This is my Events object:

import java.util.*
import kotlin.reflect.KClass

object Events {

    private val map = HashMap<Class<*>, Set<EventHandler<*>>>()

    fun <T : Any> handler(eventType: KClass<T>, handler: EventHandler<T>) {
        handler(eventType.java, handler)
    }

    fun <T> handler(eventType: Class<T>, handler: EventHandler<T>) = handlers(eventType).add(handler)

    fun post(event: Any) = handlers(event.javaClass).forEach { it.handle(event) }

    operator fun plus(event: Any) = post(event)

    private fun <T> handlers(eventType: Class<T>): HashSet<EventHandler<T>> {
        var set = map[eventType]
        if (set == null) {
            set = HashSet<EventHandler<*>>()
            map.put(eventType, set)
        }
        return set as HashSet<EventHandler<T>>
    }

}

And my EventHandler interface:

@FunctionalInterface
interface EventHandler<T> {

    fun handle(event: T)

}

Did you try Handshake::class.java for the first argument? In any case, try Ctrl + P to view the possible parameters of the function. — Kirill Rakhman
Events.handler(Handshake::class) { println(it.sent) } — this syntax has nothing to do with inline functions — Andrey Breslav
@cypressious Yes I did try that. I updated the question with the method signatures for handler. @AndreyBreslav @Jonathan-Beaudoin Adjusted the initial question. — Jire
@AndreyBreslav I noticed you answered a similar question here devnet.jetbrains.com/thread/461516 Are SAM conversion to Kotlin lambdas a project in consideration? Is there anything I can do in my current situation besides use Kotlin lambdas? — Jire

Andrey Breslav Andrey Breslav · Accepted Answer · 2015-11-09T13:49:22

Assuming below that you really need EventHandler as a separate interface (e.g. for Java interop). If you don't, you can simply use a type alias (since Kotlin 1.1):

typealias EventHandler<T> = (T) -> Unit

In this case a simple lambda will work right away.

But if you don't want to use a type alias, the issue still stands. It is that Kotlin only does SAM-conversion for functions defined in Java. Since Events.handler is defined in Kotlin, SAM-conversions do not apply to it.

To support this syntax:

Events.handler(Handshake::class, EventHandler<Handshake> { println(it.sent) })

You can define a function named EventHandler:

fun <T> EventHandler(handler: (T) -> Unit): EventHandler<T> 
    = object : EventHandler<T> { 
        override fun handle(event: T) = handler(event) 
    }

To support this syntax:

Events.handler(Handshake::class, { println(it.sent) })

or this:

Events.handler(Handshake::class) { println(it.sent) }

You need to overload the handler function to take a function instead of EventHandler:

fun <T> Events.handler(eventType: Class<T>, handler: (T) -> Unit) = EventHandler(handler)

Kotlin: Use a lambda in place of a functional interface?

2 Answers