Getting an interface reference inside a lambda function

Rafael :

Consider the following code:

val hwnd = Handler()
hwnd.postDelayed(object : Runnable {
        override fun run()
            hwnd.postDelayed(this, 5000)
        }
}, 5000)

This way, I can post the same Runnable to the Handler by using this (which refers to the Runnable) in the run() method. But how could I do the same using only a lambda expression?

val hwnd = Handler()
hwnd.postDelayed({
    //How to get "this" here?
}, 5000)

Is that even possible?

Pawel :

Since default lambda conversion gives you lambda with signature of () -> Unit, that means underlying Runnable is completely hidden.

You have to either deal with manual object creation, or write a wrapper extension function that will consume lambda with another signature:

// custom extension function for handler
inline fun Handler.postDelayed(delayMilis: Long, crossinline runnable: (Runnable) -> Unit) = postDelayed(object : Runnable{
    override fun run() {
        runnable(this)
    }
}, delayMilis)

Then at calling side you will be provided with Runnable object as lambda parameter (only parameter: it):

hwnd.postDelayed(5000){ 
    // it : Runnable
    hwnd.postDelayed(it, 5000)
}

Or if you want to get really fancy, change extension parameter to Handler.(Runnable) -> Unit, then you will be able to call:

hwnd.postDelayed(5000){ 
    // this : Handler, it : Runnable
    postDelayed(it, 5000)
}