kotlin - Passing lambda instead of interface

Question

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?

Answer 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)
}