Today, I saw something that is new for me in kotlin's syntax. In https://github.com/zach-klippenstein/compose-fractal-nav, there is an interface

FractalParent

that is declared as follows:

internal interface FractalParent : FractalNavScope {
    // ...
    val zoomAnimationSpecFactory: () -> AnimationSpec<Float>
  // ...
}

and the class that implements the interface is:

internal class FractalNavStateImpl : FractalParent {
    // ...
    override lateinit var zoomAnimationSpecFactory: () -> AnimationSpec<Float>

my question is how he was able to override

zoomAnimationSpecFactory

and change its modifier from

val

to

lateinit var

without getting a compilation error?

Think of it in terms of what a property in an interface actually means. Interfaces cannot actually have backing fields which hold data, so all the compiler knows is that

val zoomAnimationSpecFactory

is effectively no different from

fun getZoomAnimationSpecFactory()

. So when

FractalNavStateImpl

implements the interface,

override lateinit var

does still include that “get function” from the property, which means it is implementing everything required from the interface. It just also adds

fun setZoomAnimationSpecFactory(value)

, effectively

this is explicitly mentioned in the docs, https://kotlinlang.org/docs/inheritance.html#overriding-properties

thanks for the heads-up @ephemient 👏