Passing lambda instead of interface

I have created an interface:

interface ProgressListener {
    fun transferred(bytesUploaded: Long)
}

but can use it only as an anonymous class, not lambda

dataManager.createAndSubmitSendIt(title, message,
object : ProgressListener {
    override fun transferred(bytesUploaded: Long) {
        System.out.println(bytesUploaded.toString())
    }
})

I think it should be a possibility to replace it by lambda:

dataManager.createAndSubmitSendIt(title, message, {System.out.println(it.toString())})

But I am getting error: Type mismatch; required - ProgressListener, found - () -> Unit?

What am I doing wrong?

Solution 1:

As @zsmb13 said, SAM conversions are only supported for Java interfaces.

You could create an extension function to make it work though:

// Assuming the type of dataManager is DataManager.
fun DataManager.createAndSubmitSendIt(title: String, 
                                      message: String, 
                                      progressListener: (Long) -> Unit) {
    createAndSubmitSendIt(title, message,
        object : ProgressListener {
            override fun transferred(bytesUploaded: Long) {
                progressListener(bytesUploaded)
            }
        })
}

Edit:

Kotlin 1.4 will bring function interfaces that enables SAM conversions for interfaces defined in Kotlin. This means that you can call your function with a lambda if you define your interface with the fun keyword. Like this:

fun interface ProgressListener {
    fun transferred(bytesUploaded: Long)
}

Solution 2:

Kotlin only supports SAM conversions for Java interfaces.

... note that this feature works only for Java interop; since Kotlin has proper function types, automatic conversion of functions into implementations of Kotlin interfaces is unnecessary and therefore unsupported.

-- Official documentation

If you want to use a lambda in the parameter, make your function take a function parameter instead of an interface. (For now at least. Supporting SAM conversions for Kotlin interfaces is an ongoing discussion, it was one of the possible future features at the Kotlin 1.1 live stream.)

Solution 3:

UPDATED: September 7th, 2020

As the Kotlin documentation for the Kotlin 1.4 release points out:

Before Kotlin 1.4.0, you could apply SAM (Single Abstract Method) conversions only when working with Java methods and Java interfaces from Kotlin. From now on, you can use SAM conversions for Kotlin interfaces as well. To do so, mark a Kotlin interface explicitly as functional with the fun modifier.

fun interface Operation1 {
    operator fun invoke(x: String): String
}

fun interface Operation2 {
    fun doSomething(x: Int): String
}

val operation1 = Operation1 { "$it world!" }
val operation2 = Operation2 { "$it world!" }

fun main() {
    // Usage: First sample.
    println(operation1("Hello"))
    println(operation2.doSomething(0))
    // Usage: Second sample.
    println(Operation1 { "$it world!" }("Hello"))
    println(Operation2 { "$it!" }.doSomething(0))
}

You can read more about functional interfaces here.

Previous solution:

Declare a typealias, inject it somewhere and invoke it later on:

internal typealias WhateverListener = (String) -> Unit

and then we inject that typealias to our class:

class Gallery constructor(private val whateverListener: WhateverListener) {
    
    ...
    
    galleryItemClickListener.invoke("hello")

    ...
}

so we have our lambda:

val gallery = Gallery { appNavigator.openVideoPlayer(it) }

Credits to my colleague Joel Pedraza, who showed me the trick while trying to find a solution <3.

NOTE: Check out the Kotlin documentation in case you want to know when to use either functional interfaces (lambdas) or type aliases.

Solution 4:

A little late to the party: instead of making an interface, you let the compile create one by taking a function directly instead of an interface in your datamanager, like this:

fun createAndSubmitSendIt(title: String, message: String, transferred: (Long) -> Unit) {
    val answer = TODO("whatever you need to do")
    transferred(answer)
}

and then you just use it like how you want it! If I remember correctly, what the kotlin/jvm compiler do is the same as making an interface.

Hope it helps!