Exercise: Flow with history
As an exercise in my Kotlin Coroutines workshop, I ask participants to implement a withHistory function that returns a flow of all the values that have been emitted. As a result, I often see the following implementation:
fun <T> Flow<T>.withHistory(): Flow<List<T>> = flow {
val history = mutableListOf<T>()
emit(history)
collect {
history += it
emit(history)
}
}
This implementation is not correct, as is illustrated by the usage example below. Your task is to fix it.
suspend fun main() {
flowOf(1, 2, 3)
.withHistory()
.toList()
.let(::println)
// [[1, 2, 3], [1, 2, 3], [1, 2, 3], [1, 2, 3]]
}
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/FlowHistory.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 kotlinx.coroutines.delay
import kotlinx.coroutines.flow.*
import kotlinx.coroutines.test.*
import org.junit.Test
import kotlin.test.assertEquals
fun <T> Flow<T>.withHistory(): Flow<List<T>> = flow {
val history = mutableListOf<T>()
emit(history)
collect {
history += it
emit(history)
}
}
suspend fun main() {
flowOf(1, 2, 3)
.withHistory()
.toList()
.let(::println)
// [[1, 2, 3], [1, 2, 3], [1, 2, 3], [1, 2, 3]]
}
class FlowHistoryTests {
@Test
fun `should emit empty for empty`() = runTest {
assertEquals(listOf(listOf()), flowOf<String>().withHistory().toList())
}
@Test
fun `should emit history`() = runTest {
val flow = flowOf(1, 2, 3).withHistory()
assertEquals(listOf(listOf(), listOf(1), listOf(1, 2), listOf(1, 2, 3)), flow.toList())
}
@Test
fun `should emit elements as they appear`() = runTest {
val flow = flow {
delay(100)
emit(1)
delay(1000)
emit(2)
delay(10000)
emit(3)
}.withHistory()
.withVirtualTime(this)
assertEquals(
listOf(
ValueAndTime(listOf(), 0),
ValueAndTime(listOf(1), 100),
ValueAndTime(listOf(1, 2), 1100),
ValueAndTime(listOf(1, 2, 3), 11100),
), flow.toList()
)
}
}
fun <T> Flow<T>.withVirtualTime(testScope: TestScope): Flow<ValueAndTime<T>> =
map { ValueAndTime(it, testScope.currentTime) }
data class ValueAndTime<T>(val value: T, val timeMillis: Long)
