RefTest.kt

import java.util.concurrent.atomic.AtomicBoolean
import kotlin.concurrent.thread
import kotlin.properties.ReadWriteProperty
import kotlin.reflect.KProperty

fun atomicBoolean(default: Boolean = false) =
    object : ReadWriteProperty<Any?, Boolean> {
        val storage = AtomicBoolean(default)
        override fun getValue(thisRef: Any?, property: KProperty<*>) =
            storage.get()

        override fun setValue(thisRef: Any?, property: KProperty<*>, value: Boolean) =
            storage.set(value)
    }

fun main() {
    fun threadSafeButUgly() {
        println("threadSafeButUgly()")
        val run = AtomicBoolean(false)
        val thread = thread(start = true, block = { Thread.sleep(100); run.set(true) })
        while (!run.get());
        thread.join()
        check(run.get()) { "run was set to $run" }
        println("done")
    }

    fun threadSafeAndElegant() {
        println("threadSafeAndElegant()")
        var run by atomicBoolean()
        val thread = thread(start = true, block = { Thread.sleep(100); run = true })
        while (!run);
        thread.join()
        check(run) { "run was set to $run" }
        println("done")
    }

    fun notThreadSafeButElegant() {
        println("notThreadSafeButElegant()")
        var run = false // implemented by Ref.BooleanRef
        val thread = thread(start = true, block = { Thread.sleep(100);run = true })
        while (!run);
        thread.join()
        check(run) { "run was set to $run" }
        println("done")
    }

    threadSafeButUgly()
    threadSafeAndElegant()
    notThreadSafeButElegant() // does not return
}

Playing around with references of local variables, thread safe and not.