Using lambdas does not compile when trying to pass in a method expecting a SAM interface

Question

I am trying to understand lambdas and Kotlin. I created this trivial example

interface OnClickListener {
    fun onClick(s: String)
}

class Button {
    var cli         


        

Answer 1

As Taseer Ahmad points out, SAM conversion only works for Java interfaces since Kotlin already has proper function types. Of course, an easy way around this is to simply define a second setOnClickListener method that takes a function type

class Button {
    var clickListener: OnClickListener? = null

    fun setOnClickListener(listener: OnClickListener?) {
        clickListener = listener
    }

    inline fun setOnClickListener(crossinline listener: (String) -> Unit) {
        setOnClickListener(object : OnClickListener { 
            override fun onClick(s: String) = listener(s)
        })
    }

    fun click() {
        clickListener?.onClick("hello")
    }
}

This then allows you to write b.setOnClickListener { println(it) }. I always inline methods like this as a habit, but it's not really required, so you can remove the inline and crossinline if you want.