Solution: Flow with history
The problem is that the withHistory function always returns the same list reference; so, when its values are changed, all the previously emitted lists are also changed. The simplest solution is to use a read-only list instead of a mutable list.
fun <T> Flow<T>.withHistory(): Flow<List<T>> = flow {
var history = listOf<T>()
emit(history)
collect {
history += it
emit(history)
}
}
This function can also be implemented using scan:
fun <T> Flow<T>.withHistory(): Flow<List<T>> =
scan(emptyList()) { history, value ->
history + value
}
Example solution in 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 {
var history = listOf<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)
