Exercise: Correct EventListenerRegistry
What is the potential memory leak in EventListenerRegistry? Fix it.
class EventListenerRegistry<E> {
private val listeners = ConcurrentHashMap
.newKeySet<EventListener<E>>()
fun addEventListener(
event: E,
handler: () -> Unit
): EventListener<E> {
val listener = EventListener(event, handler)
listeners += listener
return listener
}
fun invokeListeners(event: E) {
listeners
.filter { it.event == event && it.isActive }
.forEach { it.handleEvent() }
}
}
class EventListener<E>(
val event: E,
val handler: () -> Unit,
) {
var isActive: Boolean = true
private set
fun handleEvent() {
handler()
}
fun cancel() {
isActive = false
}
}
This problem can either be solved in the below playground or you can clone kotlin-exercises project and solve it locally. In the project, you can find code template for this exercise in effective/efficient/EventListenerRegistry.kt. You can find there starting code and unit tests.
Once you are done with the exercise, you can check your solution here.
Playground
import org.junit.Test
import java.util.concurrent.ConcurrentHashMap
import kotlin.test.assertEquals
class EventListenerRegistry<E> {
private val listeners = ConcurrentHashMap
.newKeySet<EventListener<E>>()
fun addEventListener(
event: E,
handler: () -> Unit
): EventListener<E> {
val listener = EventListener(event, handler)
listeners += listener
return listener
}
fun invokeListeners(event: E) {
listeners
.filter { it.event == event && it.isActive }
.forEach { it.handleEvent() }
}
}
class EventListener<E>(
val event: E,
val handler: () -> Unit,
) {
var isActive: Boolean = true
private set
fun handleEvent() {
handler()
}
fun cancel() {
isActive = false
}
}
enum class Event { A, B, C }
class EventListenerRegistryTest {
@Test
fun `should invoke proper handlers`() {
val eventListenerRepository = EventListenerRegistry<Event>()
var a = 0
var b = 0
var c = 0
eventListenerRepository.addEventListener(Event.A) { a++ }
eventListenerRepository.addEventListener(Event.B) { b++ }
eventListenerRepository.addEventListener(Event.C) { c++ }
assertEquals(0, a)
assertEquals(0, b)
assertEquals(0, c)
eventListenerRepository.invokeListeners(Event.A)
assertEquals(1, a)
assertEquals(0, b)
assertEquals(0, c)
eventListenerRepository.invokeListeners(Event.B)
eventListenerRepository.invokeListeners(Event.B)
assertEquals(1, a)
assertEquals(2, b)
assertEquals(0, c)
eventListenerRepository.invokeListeners(Event.C)
eventListenerRepository.invokeListeners(Event.C)
eventListenerRepository.invokeListeners(Event.C)
assertEquals(1, a)
assertEquals(2, b)
assertEquals(3, c)
}
@Test
fun `should allow setting more than one handler for an event`() {
val eventListenerRepository = EventListenerRegistry<Event>()
var a = 0
var b = 0
var c = 0
eventListenerRepository.addEventListener(Event.A) { a++ }
eventListenerRepository.addEventListener(Event.A) { b++ }
eventListenerRepository.addEventListener(Event.A) { c++ }
eventListenerRepository.invokeListeners(Event.A)
assertEquals(1, a)
assertEquals(1, b)
assertEquals(1, c)
}
@Test
fun `should allow listener cancelation`() {
val eventListenerRepository = EventListenerRegistry<Event>()
var a = 0
val listener = eventListenerRepository.addEventListener(Event.A) { a++ }
listener.cancel()
eventListenerRepository.invokeListeners(Event.A)
assertEquals(0, a)
}
}
