Problem Passing a function reference in Kotlin

Question

The following line of code is throwing a compile error related to Renderer::onScoreChange. The reported error is: Type mismatch. Required:(Int, Int) → Unit Found: KFunction3<Renderer, @ParameterName Int, @ParameterName Int, Unit>

var score by EngineObserver<Int>(20, Renderer::onScoreChange)

I'm trying to pass a function reference to a custom Delegate extending from the ObservableProperty class but not sure why its not recognizing the passed member function as satisfying the requirements of the function argument. Thoughts from anyone?

class EngineObserver<T>(
   initialValue: T, 
   val notify : (oldVal : T, newVal : T) -> Unit
) : ObservableProperty<T>(initialValue) {
   override fun afterChange(property: KProperty<*>, 
                            oldValue: T, 
                            newValue: T) {
        super.afterChange(property, oldValue, newValue)
        notify(oldValue, newValue)
   }
}

class Renderer {
   fun onScoreChange(oldVal : Int, newVal : Int){
      println("Score changed from $oldVal to $newVal")
   }
}

Miha_x64 · Accepted Answer · 2019-05-31 15:18:44Z

1

Renderer::onScoreChange is a member function which has Renderer as its first parameter. You probably need a function reference applied to a certain instance: renderer::onScoreChange

answered May 31, 2019 at 15:18

Miha_x64

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1 Comment

Erik Medina Over a year ago

Passing an instance, not the class itself, solved the issue.

Dirk Bolte · Accepted Answer · 2019-06-01 11:39:41Z

0

Miha_x64 is right. To make your code work, you have to do the following:

class Renderer {
    companion object {
        fun onScoreChange(oldVal: Int, newVal: Int) {
            println("Score changed from $oldVal to $newVal")
        }
    }
}

var score by EngineObserver<Int>(20, Renderer.Companion::onScoreChange)

Alternatively, if you have a Renderer instance you want to call that method on, you can do:

val renderer = Renderer()
var score by EngineObserver<Int>(20, renderer::onScoreChange)

answered Jun 1, 2019 at 11:39

Dirk Bolte

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