Exercise: CancellingRefresher
You want to make sure that if a new refresh is started, the previous one is cancelled. You have the following code:
class CancellingRefresher(
private val scope: CoroutineScope,
private val refreshData: suspend () -> Unit,
) {
private var refreshJob: Job? = null
fun refresh() {
refreshJob?.cancel()
refreshJob = scope.launch {
refreshData()
}
}
}
The problem is that this implementation is not correct, because if started concurrently, refresh function might not cancel some coroutines. Your task is to fix it.
This is a popular pattern on Android, but there in most cases there is no need for synchronization, because UI handlers are always called on the main thread.
Make sure that the refresh function is thread-safe using the following techniques:
- Using
synchronized block. - Using
Mutex. - Using a dispatcher limited to a single thread.
Compare how much time your unit tests take for each solution.
Check if you can solve this problem using a concurrent set of jobs, just remember to remove jobs that are cancelled already.
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/safe/CancellingRefresher.kt. You can find there starting code and unit tests.
Once you are done with the exercise, you can check your solution here.
import kotlinx.coroutines.CoroutineScope
import kotlinx.coroutines.Dispatchers
import kotlinx.coroutines.Job
import kotlinx.coroutines.cancel
import kotlinx.coroutines.coroutineScope
import kotlinx.coroutines.delay
import kotlinx.coroutines.job
import kotlinx.coroutines.launch
import kotlinx.coroutines.runBlocking
import kotlinx.coroutines.test.currentTime
import kotlinx.coroutines.test.runTest
import kotlinx.coroutines.withContext
import org.junit.Test
import kotlin.test.assertEquals
class CancellingRefresher(
private val scope: CoroutineScope,
private val refreshData: suspend () -> Unit,
) {
private var refreshJob: Job? = null
fun refresh() {
refreshJob?.cancel()
refreshJob = scope.launch {
refreshData()
}
}
}
class CancellingRefresherTest {
@Test
fun `should cancel previous refresh when starting new one`(): Unit = runTest {
val userRefresher = CancellingRefresher(
scope = backgroundScope,
refreshData = {
delay(1000)
}
)
coroutineScope {
repeat(1000) {
launch { userRefresher.refresh() }
}
delay(1000)
repeat(1000) {
launch { userRefresher.refresh() }
}
delay(1000)
repeat(1000) {
launch { userRefresher.refresh() }
}
}
assertEquals(2000, currentTime) // Delays
val children = backgroundScope.coroutineContext[Job]!!.children
assertEquals(1, children.count { it.isActive })
children.forEach { it.join() }
assertEquals(3000, currentTime)
}
@Test
fun `should cancel all previous jobs`(): Unit = runTest {
val userRefresher = CancellingRefresher(
scope = backgroundScope,
refreshData = { delay(Long.MAX_VALUE) }
)
coroutineScope {
repeat(50_000) {
launch {
userRefresher.refresh()
}
}
}
delay(1000)
assertEquals(1, backgroundScope.coroutineContext.job.children.count { it.isActive })
}
@Test
fun `should cancel all previous jobs (real time)`(): Unit = runBlocking(Dispatchers.Default) {
val backgroundScope = CoroutineScope(Job() + Dispatchers.Default)
val userRefresher = CancellingRefresher(
scope = backgroundScope,
refreshData = { delay(Long.MAX_VALUE) }
)
coroutineScope {
repeat(50_000) {
launch {
userRefresher.refresh()
}
}
}
delay(1000)
assertEquals(1, backgroundScope.coroutineContext.job.children.count { it.isActive })
backgroundScope.cancel()
}
}
Marcin Moskala is a highly experienced developer and Kotlin instructor as the founder of Kt. Academy, an official JetBrains partner specializing in Kotlin training, Google Developers Expert, known for his significant contributions to the Kotlin community. Moskala is the author of several widely recognized books, including "Effective Kotlin," "Kotlin Coroutines," "Functional Kotlin," "Advanced Kotlin," "Kotlin Essentials," and "Android Development with Kotlin."
Beyond his literary achievements, Moskala is the author of the largest Medium publication dedicated to Kotlin. As a respected speaker, he has been invited to share his insights at numerous programming conferences, including events such as Droidcon and the prestigious Kotlin Conf, the premier conference dedicated to the Kotlin programming language.
