Dead man's switch in Kotlin - kotlin

Question

I want to implement a Dead man's switch in Kotlin. What this does is fire a notification TIME_INTERVAL seconds after the last MyEvent was received. When a new MyEvent is received, it restarts the timer.
private val stopWatch = object : () -> Unit {
var timer = System.currentTimeMillis()
var isRunning = false
override fun invoke() {
timer = System.currentTimeMillis()
if (isRunning) return
synchronized(this) {
isRunning = true
while (System.currentTimeMillis() - timer <= TIME_INTERVAL) {}
fireNotification()
isRunning = false
}
}
}
override fun onSomeEvent(e: MyEvent?) {
runAsync(stopWatch)
}
Is there a simpler or easier way to get this functionality by using either the kotlin.concurrent or Java standard libraries?

Answer 1

If I understand correctly, your code consists in looping doing nothing until the time interval has elapsed. This is a bad idea, since it consumes a whole lot of CPU doing nothing, instead of just waiting.
I would use a ScheduledExecutr to schedule the firing of the notification. And I would cancel the returned future when an event comes in before the notification is fired:
import java.time.Instant.now
import java.util.concurrent.Executors
import java.util.concurrent.ScheduledFuture
import java.util.concurrent.TimeUnit
class StopWatch {
private var future: ScheduledFuture<Void>? = null;
private val executor = Executors.newSingleThreadScheduledExecutor()
fun onSomeEvent() {
synchronized(this) {
future.let {
future?.cancel(false)
future = null
}
val command = {
synchronized(this#StopWatch) {
future = null
}
fireNotification()
null
}
future = executor.schedule(command, 2, TimeUnit.SECONDS)
}
println("${now()} - event")
}
private fun fireNotification() {
println("${now()} - notification")
}
fun shutdown() {
executor.shutdown()
}
}
fun main(args: Array<String>) {
val stopWatch = StopWatch()
stopWatch.onSomeEvent()
Thread.sleep(1000)
stopWatch.onSomeEvent()
Thread.sleep(1000)
stopWatch.onSomeEvent()
Thread.sleep(1000)
stopWatch.onSomeEvent()
Thread.sleep(3000)
stopWatch.onSomeEvent()
stopWatch.shutdown()
}
Which prints:
2017-05-07T12:45:55.647Z - event
2017-05-07T12:45:56.741Z - event
2017-05-07T12:45:57.743Z - event
2017-05-07T12:45:58.745Z - event
2017-05-07T12:46:00.747Z - notification
2017-05-07T12:46:01.750Z - event
2017-05-07T12:46:03.753Z - notification