CloseableRefCallback.kt

interface Cleanup {
    fun registerForCleanup(onCleanup: () -> Unit): Closeable
}

class CloseableRefCallback<T : Cleanup>(
    private val callback: T.() -> Unit
) : Closeable {

    private val resourceRef = AtomicReference<Pair<T, Closeable>?>(null)

    @Suppress("MemberVisibilityCanBePrivate")
    var resource: T?
        get() = resourceRef.get()?.first
        set(newResource) {
            val new = newResource?.let { it to it.registerForCleanup { this.close() } }
            val old = resourceRef.getAndSet(new)
            old?.second?.close()
        }

    override fun close() {
        resource = null
    }

    operator fun invoke() {
        resource?.callback()
    }
}