Exercise: CounterContext
Implement a CounterContext class that is a mutable counter and can always be asked for a new value by using the next function. It should be possible to create multiple instances of CounterContext, each of which should have its own counter.
fun main(): Unit = runBlocking(CounterContext()) {
println(coroutineContext[CounterContext]?.next()) // 0
println(coroutineContext[CounterContext]?.next()) // 1
launch {
println(coroutineContext[CounterContext]?.next())// 2
println(coroutineContext[CounterContext]?.next())// 3
}
launch(CounterContext()) {
println(coroutineContext[CounterContext]?.next())// 0
println(coroutineContext[CounterContext]?.next())// 1
}
}
The simplest way to create a custom context is by creating a class that extends the AbstractCoroutineContextElement class. This class implements the CoroutineContext.Element interface, and it is a good starting point for creating custom contexts. It requires the value that is used as a key for this context. It is recommended to use a companion object for this purpose. Here is an example of a simple context:
class MyContext : AbstractCoroutineContextElement(MyContext){
companion object : CoroutineContext.Key<MyContext>
}
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 coroutines/context/CounterContext.kt. You can find there example usage and unit tests.
Once you are done with the exercise, you can check your solution here.
import kotlinx.coroutines.*
import org.junit.Test
import java.util.concurrent.atomic.AtomicInteger
import kotlin.coroutines.*
import kotlin.test.assertEquals
// TODO
fun main(): Unit = runBlocking(CounterContext()) {
println(coroutineContext[CounterContext]?.next()) // 0
println(coroutineContext[CounterContext]?.next()) // 1
launch {
println(coroutineContext[CounterContext]?.next())// 2
println(coroutineContext[CounterContext]?.next())// 3
}
launch(CounterContext()) {
println(coroutineContext[CounterContext]?.next())// 0
println(coroutineContext[CounterContext]?.next())// 1
}
}
class CounterContextTests {
@Test
fun `should return next numbers in the same coroutine`() = runBlocking<Unit>(CounterContext()) {
assertEquals(0, coroutineContext[CounterContext]?.next())
assertEquals(1, coroutineContext[CounterContext]?.next())
assertEquals(2, coroutineContext[CounterContext]?.next())
assertEquals(3, coroutineContext[CounterContext]?.next())
assertEquals(4, coroutineContext[CounterContext]?.next())
}
@Test
fun `should have independent counter for each instance`() = runBlocking<Unit> {
launch(CounterContext()) {
assertEquals(0, coroutineContext[CounterContext]?.next())
assertEquals(1, coroutineContext[CounterContext]?.next())
assertEquals(2, coroutineContext[CounterContext]?.next())
}
launch(CounterContext()) {
assertEquals(0, coroutineContext[CounterContext]?.next())
assertEquals(1, coroutineContext[CounterContext]?.next())
assertEquals(2, coroutineContext[CounterContext]?.next())
}
}
@Test
fun `should propagate to the child`() = runBlocking<Unit>(CounterContext()) {
assertEquals(0, coroutineContext[CounterContext]?.next())
launch {
assertEquals(1, coroutineContext[CounterContext]?.next())
launch {
assertEquals(2, coroutineContext[CounterContext]?.next())
}
launch(CounterContext()) {
assertEquals(0, coroutineContext[CounterContext]?.next())
assertEquals(1, coroutineContext[CounterContext]?.next())
launch {
assertEquals(2, coroutineContext[CounterContext]?.next())
}
}
}
}
}
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.
