Flow operators review: docs tweaks and code style fixes in tests

kotlinx-coroutines-core/common/src/flow/operators/Delay.kt

@@ -20,9 +20,7 @@ import kotlinx.coroutines.flow.unsafeFlow as flow
 @FlowPreview
 public fun <T> Flow<T>.delayFlow(timeMillis: Long): Flow<T> = flow {
  delay(timeMillis)
- collect { value ->
- emit(value)
- }
+ collect(this@flow)
 }
 
 /**
@@ -38,26 +36,30 @@ public fun <T> Flow<T>.delayEach(timeMillis: Long): Flow<T> = flow {
 
 /**
  * Returns a flow that mirrors the original flow, but filters out values
- * that are followed by the newer values within the given [timeout][timeoutMs].
+ * that are followed by the newer values within the given [timeout][timeoutMillis].
  * The latest value is always emitted.
+ *
  * Example:
  * ```
  * flow {
- * emit(1)
- * delay(99)
- * emit(2)
- * delay(99)
- * emit(3)
- * delay(1001)
- * emit(4)
- * delay(1001)
- * emit(5)
+ *  emit(1)
+ *  delay(99)
+ *  emit(2)
+ *  delay(99)
+ *  emit(3)
+ *  delay(1001)
+ *  emit(4)
+ *  delay(1001)
+ *  emit(5)
  * }.debounce(1000)
  * ```
- * will produce `3, 4, 5`.
+ * produces `3, 4, 5`.
+ *
+ * Note that the resulting flow does not emit anything as long as the original flow emits
+ * items faster than every [timeoutMillis] milliseconds.
  */
-public fun <T> Flow<T>.debounce(timeoutMs: Long): Flow<T> {
- require(timeoutMs > 0) { "Debounce timeout should be positive" }
+public fun <T> Flow<T>.debounce(timeoutMillis: Long): Flow<T> {
+ require(timeoutMillis > 0) { "Debounce timeout should be positive" }
  return flow {
  coroutineScope {
  val values = Channel<Any?>(Channel.CONFLATED) // Actually Any, KT-30796
@@ -79,10 +81,11 @@ public fun <T> Flow<T>.debounce(timeoutMs: Long): Flow<T> {
  lastValue = it
  }
 
- onTimeout(timeoutMs) {
- val value = lastValue ?: return@onTimeout
- lastValue = null // Consume the value
- emit(NullSurrogate.unbox(value))
+ lastValue?.let { value -> // set timeout when lastValue != null
+ onTimeout(timeoutMillis) {
+ lastValue = null // Consume the value
+ emit(NullSurrogate.unbox(value))
+ }
  }
 
  // Close with value 'idiom'
@@ -97,21 +100,23 @@ public fun <T> Flow<T>.debounce(timeoutMs: Long): Flow<T> {
 }
 
 /**
- * Returns a flow that emits only the latest value emitted by the original flow during the given sampling [period][periodMs].
+ * Returns a flow that emits only the latest value emitted by the original flow during the given sampling [period][periodMillis].
+ *
  * Example:
  * ```
  * flow {
- * repeat(10) {
- * emit(it)
- * delay(50)
- * }
+ *  repeat(10) {
+ *  emit(it)
+ *  delay(50)
+ *  }
  * }.sample(100)
  * ```
- * will produce `1, 3, 5, 7, 9`.
+ * produces `1, 3, 5, 7, 9`.
+ * 
  * Note that the latest element is not emitted if it does not fit into the sampling window.
  */
-public fun <T> Flow<T>.sample(periodMs: Long): Flow<T> {
- require(periodMs > 0) { "Sample period should be positive" }
+public fun <T> Flow<T>.sample(periodMillis: Long): Flow<T> {
+ require(periodMillis > 0) { "Sample period should be positive" }
  return flow {
  coroutineScope {
  val values = produce<Any?>(capacity = Channel.CONFLATED) { // Actually Any, KT-30796
@@ -120,7 +125,7 @@ public fun <T> Flow<T>.sample(periodMs: Long): Flow<T> {
 
  var isDone = false
  var lastValue: Any? = null
- val ticker = fixedPeriodTicker(periodMs, periodMs)
+ val ticker = fixedPeriodTicker(periodMillis)
  while (!isDone) {
  select<Unit> {
  values.onReceiveOrNull {
@@ -132,6 +137,7 @@ public fun <T> Flow<T>.sample(periodMs: Long): Flow<T> {
  }
  }
 
+ // todo: shall be start sampling only when an element arrives or sample aways as here?
  ticker.onReceive {
  val value = lastValue ?: return@onReceive
  lastValue = null // Consume the value

kotlinx-coroutines-core/common/src/flow/operators/Merge.kt

@@ -90,8 +90,8 @@ public fun <T> Flow<Flow<T>>.flattenConcat(): Flow<T> = flow {
 public fun <T> Flow<Flow<T>>.flattenMerge(concurrency: Int = 16, bufferSize: Int = 16): Flow<T> = flatMapMerge(concurrency, bufferSize) { it }
 
 /**
- * Returns a flow that switches to the new flow produced by [transform] function every time the original flow emits a value.
- * When switch on the newer flow is performed, the previous one is cancelled.
+ * Returns a flow that switches to a new flow produced by [transform] function every time the original flow emits a value.
+ * When switch on the a flow is performed, the previous one is cancelled.
  *
  * For example, the following flow:
  * ```
@@ -107,7 +107,7 @@ public fun <T> Flow<Flow<T>>.flattenMerge(concurrency: Int = 16, bufferSize: Int
  * }
  * }
  * ```
- * will produce `aa bb b_last`
+ * produces `aa bb b_last`
  */
 @FlowPreview
 public fun <T, R> Flow<T>.switchMap(transform: suspend (value: T) -> Flow<R>): Flow<R> = flow {

kotlinx-coroutines-core/common/test/flow/VirtualTime.kt

@@ -10,7 +10,7 @@ import kotlin.jvm.*
 private class VirtualTimeDispatcher(enclosingScope: CoroutineScope) : CoroutineDispatcher(), Delay {
 
  private val originalDispatcher = enclosingScope.coroutineContext[ContinuationInterceptor] as CoroutineDispatcher
- private val heap = ArrayList<TimedTask>() // TODO use MPP heap/ordered set implementation
+ private val heap = ArrayList<TimedTask>() // TODO use MPP heap/ordered set implementation (commonize ThreadSafeHeap)
  private var currentTime = 0L
 
  init {
@@ -21,10 +21,10 @@ private class VirtualTimeDispatcher(enclosingScope: CoroutineScope) : CoroutineD
  */
  enclosingScope.launch {
  while (true) {
- val secret = ThreadLocalEventLoop.currentOrNull()?.processNextEvent()
+ val delayNanos = ThreadLocalEventLoop.currentOrNull()?.processNextEvent()
  ?: error("Event loop is missing, virtual time source works only as part of event loop")
- if (secret <= 0) continue
- if (secret > 0 && secret != Long.MAX_VALUE) error("Unexpected external delay: $secret")
+ if (delayNanos <= 0) continue
+ if (delayNanos > 0 && delayNanos != Long.MAX_VALUE) error("Unexpected external delay: $delayNanos")
  val nextTask = heap.minBy { it.deadline } ?: return@launch
  heap.remove(nextTask)
  currentTime = nextTask.deadline

kotlinx-coroutines-core/common/test/flow/operators/CombineLatestTest.kt

@@ -16,44 +16,44 @@ class CombineLatestTest : TestBase() {
  fun testCombineLatest() = runTest {
  val flow = flowOf("a", "b", "c")
  val flow2 = flowOf(1, 2, 3)
- val list = flow.combineLatest(flow2, { i, j -> i + j }).toList()
+ val list = flow.combineLatest(flow2) { i, j -> i + j }.toList()
  assertEquals(listOf("a1", "b1", "b2", "c2", "c3"), list)
  }
 
  @Test
  fun testNulls() = runTest {
  val flow = flowOf("a", null, null)
  val flow2 = flowOf(1, 2, 3)
- val list = flow.combineLatest(flow2,  { i, j -> i + j }).toList()
+ val list = flow.combineLatest(flow2, { i, j -> i + j }).toList()
  assertEquals(listOf("a1", "null1", "null2", "null2", "null3"), list)
  }
 
  @Test
  fun testNullsOther() = runTest {
  val flow = flowOf("a", "b", "c")
  val flow2 = flowOf(null, 2, null)
- val list = flow.combineLatest(flow2,  { i, j -> i + j }).toList()
+ val list = flow.combineLatest(flow2, { i, j -> i + j }).toList()
  assertEquals(listOf("anull", "bnull", "b2", "c2", "cnull"), list)
  }
 
  @Test
  fun testEmptyFlow() = runTest {
- val flow = emptyFlow<String>().combineLatest(emptyFlow<Int>(),  { i, j -> i + j })
+ val flow = emptyFlow<String>().combineLatest(emptyFlow<Int>(), { i, j -> i + j })
  assertNull(flow.singleOrNull())
  }
 
  @Test
  fun testFirstIsEmpty() = runTest {
  val f1 = emptyFlow<String>()
  val f2 = flowOf(1)
- assertEquals(emptyList(), f1.combineLatest(f2, { i, j -> i + j }).toList())
+ assertEquals(emptyList(), f1.combineLatest(f2) { i, j -> i + j }.toList())
  }
 
  @Test
  fun testSecondIsEmpty() = runTest {
  val f1 = flowOf("a")
  val f2 = emptyFlow<Int>()
- assertEquals(emptyList(), f1.combineLatest(f2, { i, j -> i + j }).toList())
+ assertEquals(emptyList(), f1.combineLatest(f2) { i, j -> i + j }.toList())
  }
 
  @Test
@@ -80,7 +80,7 @@ class CombineLatestTest : TestBase() {
  emit(3)
  }
 
- val result = f1.combineLatest(f2, { i, j -> i + j }).toList()
+ val result = f1.combineLatest(f2) { i, j -> i + j }.toList()
  assertEquals(listOf("a1", "b1", "c1", "c2", "c3"), result)
  finish(8)
  }
@@ -137,7 +137,7 @@ class CombineLatestTest : TestBase() {
  }
  }
 
- val value =  withContext(NamedDispatchers("main")) {
+ val value = withContext(NamedDispatchers("main")) {
  f1.combineLatest(f2) { i, j ->
  assertEquals("main", NamedDispatchers.name())
  expect(5)
@@ -170,8 +170,8 @@ class CombineLatestTest : TestBase() {
  expect(1)
  i + j
  }.flowOn(NamedDispatchers("combine")).onEach {
-  throw TestException()
-  }
+ throw TestException()
+ }
 
  assertFailsWith<TestException>(flow)
  finish(4)

kotlinx-coroutines-core/common/test/flow/operators/SampleTest.kt

@@ -129,6 +129,22 @@ class SampleTest : TestBase() {
  assertNull(flow.singleOrNull())
  }
 
+ @Test
+ // note that this test depends on the sampling strategy -- when sampling time starts on a quiescent flow that suddenly emits
+ fun testLongWait() = runTest {
+ val flow = flow {
+ emit("A")
+ delay(3500) // long delay -- multiple sampling intervals
+ emit("B")
+ delay(900) // crosses time = 4000 barrier
+ emit("C")
+ delay(3000) // long wait again
+
+ }
+ val result = flow.sample(1000).toList()
+ assertEquals(listOf("A", "B", "C"), result)
+ }
+
  @Test
  fun testPace() = withVirtualTime {
  val flow = flow {

kotlinx-coroutines-core/common/test/flow/operators/SwitchMapTest.kt

@@ -42,11 +42,11 @@ class SwitchMapTest : TestBase() {
  emit(2)
  emit(4)
  }.flowOn(NamedDispatchers("source")).switchMap { value ->
-  flow {
-  assertEquals("switch$value", NamedDispatchers.name())
-  emit(value)
-  expect(value)
-  }.flowOn(NamedDispatchers("switch$value"))
+ flow {
+ assertEquals("switch$value", NamedDispatchers.name())
+ emit(value)
+ expect(value)
+ }.flowOn(NamedDispatchers("switch$value"))
  }.onEach {
  expect(it + 1)
  assertEquals("main", NamedDispatchers.nameOr("main"))