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Coroutines support libraries for Kotlin
/*
* Copyright 2016-2021 JetBrains s.r.o. Use of this source code is governed by the Apache 2.0 license.
*/
@file:JvmMultifileClass
@file:JvmName("FlowKt")
package kotlinx.coroutines.flow
import kotlinx.coroutines.*
import kotlinx.coroutines.channels.*
import kotlinx.coroutines.channels.Channel.Factory.BUFFERED
import kotlinx.coroutines.flow.internal.*
import kotlin.coroutines.*
import kotlin.jvm.*
import kotlinx.coroutines.flow.internal.unsafeFlow as flow
/**
* Creates a _cold_ flow from the given suspendable [block].
* The flow being _cold_ means that the [block] is called every time a terminal operator is applied to the resulting flow.
*
* Example of usage:
*
* ```
* fun fibonacci(): Flow = flow {
* var x = BigInteger.ZERO
* var y = BigInteger.ONE
* while (true) {
* emit(x)
* x = y.also {
* y += x
* }
* }
* }
*
* fibonacci().take(100).collect { println(it) }
* ```
*
* Emissions from [flow] builder are [cancellable] by default — each call to [emit][FlowCollector.emit]
* also calls [ensureActive][CoroutineContext.ensureActive].
*
* `emit` should happen strictly in the dispatchers of the [block] in order to preserve the flow context.
* For example, the following code will result in an [IllegalStateException]:
*
* ```
* flow {
* emit(1) // Ok
* withContext(Dispatcher.IO) {
* emit(2) // Will fail with ISE
* }
* }
* ```
*
* If you want to switch the context of execution of a flow, use the [flowOn] operator.
*/
public fun flow(@BuilderInference block: suspend FlowCollector.() -> Unit): Flow = SafeFlow(block)
// Named anonymous object
private class SafeFlow(private val block: suspend FlowCollector.() -> Unit) : AbstractFlow() {
override suspend fun collectSafely(collector: FlowCollector) {
collector.block()
}
}
/**
* Creates a _cold_ flow that produces a single value from the given functional type.
*/
@FlowPreview
public fun (() -> T).asFlow(): Flow = flow {
emit(invoke())
}
/**
* Creates a _cold_ flow that produces a single value from the given functional type.
*
* Example of usage:
*
* ```
* suspend fun remoteCall(): R = ...
* fun remoteCallFlow(): Flow = ::remoteCall.asFlow()
* ```
*/
@FlowPreview
public fun (suspend () -> T).asFlow(): Flow = flow {
emit(invoke())
}
/**
* Creates a _cold_ flow that produces values from the given iterable.
*/
public fun Iterable.asFlow(): Flow = flow {
forEach { value ->
emit(value)
}
}
/**
* Creates a _cold_ flow that produces values from the given iterator.
*/
public fun Iterator.asFlow(): Flow = flow {
forEach { value ->
emit(value)
}
}
/**
* Creates a _cold_ flow that produces values from the given sequence.
*/
public fun Sequence.asFlow(): Flow = flow {
forEach { value ->
emit(value)
}
}
/**
* Creates a flow that produces values from the specified `vararg`-arguments.
*
* Example of usage:
*
* ```
* flowOf(1, 2, 3)
* ```
*/
public fun flowOf(vararg elements: T): Flow = flow {
for (element in elements) {
emit(element)
}
}
/**
* Creates a flow that produces the given [value].
*/
public fun flowOf(value: T): Flow = flow {
/*
* Implementation note: this is just an "optimized" overload of flowOf(vararg)
* which significantly reduces the footprint of widespread single-value flows.
*/
emit(value)
}
/**
* Returns an empty flow.
*/
public fun emptyFlow(): Flow = EmptyFlow
private object EmptyFlow : Flow {
override suspend fun collect(collector: FlowCollector) = Unit
}
/**
* Creates a _cold_ flow that produces values from the given array.
* The flow being _cold_ means that the array components are read every time a terminal operator is applied
* to the resulting flow.
*/
public fun Array.asFlow(): Flow = flow {
forEach { value ->
emit(value)
}
}
/**
* Creates a _cold_ flow that produces values from the array.
* The flow being _cold_ means that the array components are read every time a terminal operator is applied
* to the resulting flow.
*/
public fun IntArray.asFlow(): Flow = flow {
forEach { value ->
emit(value)
}
}
/**
* Creates a _cold_ flow that produces values from the given array.
* The flow being _cold_ means that the array components are read every time a terminal operator is applied
* to the resulting flow.
*/
public fun LongArray.asFlow(): Flow = flow {
forEach { value ->
emit(value)
}
}
/**
* Creates a flow that produces values from the range.
*/
public fun IntRange.asFlow(): Flow = flow {
forEach { value ->
emit(value)
}
}
/**
* Creates a flow that produces values from the range.
*/
public fun LongRange.asFlow(): Flow = flow {
forEach { value ->
emit(value)
}
}
/**
* Creates an instance of a _cold_ [Flow] with elements that are sent to a [SendChannel]
* provided to the builder's [block] of code via [ProducerScope]. It allows elements to be
* produced by code that is running in a different context or concurrently.
* The resulting flow is _cold_, which means that [block] is called every time a terminal operator
* is applied to the resulting flow.
*
* This builder ensures thread-safety and context preservation, thus the provided [ProducerScope] can be used
* concurrently from different contexts.
* The resulting flow completes as soon as the code in the [block] and all its children completes.
* Use [awaitClose] as the last statement to keep it running.
* A more detailed example is provided in the documentation of [callbackFlow].
*
* A channel with the [default][Channel.BUFFERED] buffer size is used. Use the [buffer] operator on the
* resulting flow to specify a user-defined value and to control what happens when data is produced faster
* than consumed, i.e. to control the back-pressure behavior.
*
* Adjacent applications of [channelFlow], [flowOn], [buffer], and [produceIn] are
* always fused so that only one properly configured channel is used for execution.
*
* Examples of usage:
*
* ```
* fun Flow.merge(other: Flow): Flow = channelFlow {
* // collect from one coroutine and send it
* launch {
* collect { send(it) }
* }
* // collect and send from this coroutine, too, concurrently
* other.collect { send(it) }
* }
*
* fun contextualFlow(): Flow = channelFlow {
* // send from one coroutine
* launch(Dispatchers.IO) {
* send(computeIoValue())
* }
* // send from another coroutine, concurrently
* launch(Dispatchers.Default) {
* send(computeCpuValue())
* }
* }
* ```
*/
public fun channelFlow(@BuilderInference block: suspend ProducerScope.() -> Unit): Flow =
ChannelFlowBuilder(block)
/**
* Creates an instance of a _cold_ [Flow] with elements that are sent to a [SendChannel]
* provided to the builder's [block] of code via [ProducerScope]. It allows elements to be
* produced by code that is running in a different context or concurrently.
*
* The resulting flow is _cold_, which means that [block] is called every time a terminal operator
* is applied to the resulting flow.
*
* This builder ensures thread-safety and context preservation, thus the provided [ProducerScope] can be used
* from any context, e.g. from a callback-based API.
* The resulting flow completes as soon as the code in the [block] completes.
* [awaitClose] should be used to keep the flow running, otherwise the channel will be closed immediately
* when block completes.
* [awaitClose] argument is called either when a flow consumer cancels the flow collection
* or when a callback-based API invokes [SendChannel.close] manually and is typically used
* to cleanup the resources after the completion, e.g. unregister a callback.
* Using [awaitClose] is mandatory in order to prevent memory leaks when the flow collection is cancelled,
* otherwise the callback may keep running even when the flow collector is already completed.
* To avoid such leaks, this method throws [IllegalStateException] if block returns, but the channel
* is not closed yet.
*
* A channel with the [default][Channel.BUFFERED] buffer size is used. Use the [buffer] operator on the
* resulting flow to specify a user-defined value and to control what happens when data is produced faster
* than consumed, i.e. to control the back-pressure behavior.
*
* Adjacent applications of [callbackFlow], [flowOn], [buffer], and [produceIn] are
* always fused so that only one properly configured channel is used for execution.
*
* Example of usage that converts a multi-shot callback API to a flow.
* For single-shot callbacks use [suspendCancellableCoroutine].
*
* ```
* fun flowFrom(api: CallbackBasedApi): Flow = callbackFlow {
* val callback = object : Callback { // Implementation of some callback interface
* override fun onNextValue(value: T) {
* // To avoid blocking you can configure channel capacity using
* // either buffer(Channel.CONFLATED) or buffer(Channel.UNLIMITED) to avoid overfill
* trySendBlocking(value)
* .onFailure { throwable ->
* // Downstream has been cancelled or failed, can log here
* }
* }
* override fun onApiError(cause: Throwable) {
* cancel(CancellationException("API Error", cause))
* }
* override fun onCompleted() = channel.close()
* }
* api.register(callback)
* /*
* * Suspends until either 'onCompleted'/'onApiError' from the callback is invoked
* * or flow collector is cancelled (e.g. by 'take(1)' or because a collector's coroutine was cancelled).
* * In both cases, callback will be properly unregistered.
* */
* awaitClose { api.unregister(callback) }
* }
* ```
*
* > The callback `register`/`unregister` methods provided by an external API must be thread-safe, because
* > `awaitClose` block can be called at any time due to asynchronous nature of cancellation, even
* > concurrently with the call of the callback.
*/
public fun callbackFlow(@BuilderInference block: suspend ProducerScope.() -> Unit): Flow = CallbackFlowBuilder(block)
// ChannelFlow implementation that is the first in the chain of flow operations and introduces (builds) a flow
private open class ChannelFlowBuilder(
private val block: suspend ProducerScope.() -> Unit,
context: CoroutineContext = EmptyCoroutineContext,
capacity: Int = BUFFERED,
onBufferOverflow: BufferOverflow = BufferOverflow.SUSPEND
) : ChannelFlow(context, capacity, onBufferOverflow) {
override fun create(context: CoroutineContext, capacity: Int, onBufferOverflow: BufferOverflow): ChannelFlow =
ChannelFlowBuilder(block, context, capacity, onBufferOverflow)
override suspend fun collectTo(scope: ProducerScope) =
block(scope)
override fun toString(): String =
"block[$block] -> ${super.toString()}"
}
private class CallbackFlowBuilder(
private val block: suspend ProducerScope.() -> Unit,
context: CoroutineContext = EmptyCoroutineContext,
capacity: Int = BUFFERED,
onBufferOverflow: BufferOverflow = BufferOverflow.SUSPEND
) : ChannelFlowBuilder(block, context, capacity, onBufferOverflow) {
override suspend fun collectTo(scope: ProducerScope) {
super.collectTo(scope)
/*
* We expect user either call `awaitClose` from within a block (then the channel is closed at this moment)
* or being closed/cancelled externally/manually. Otherwise "user forgot to call
* awaitClose and receives unhelpful ClosedSendChannelException exceptions" situation is detected.
*/
if (!scope.isClosedForSend) {
throw IllegalStateException(
"""
'awaitClose { yourCallbackOrListener.cancel() }' should be used in the end of callbackFlow block.
Otherwise, a callback/listener may leak in case of external cancellation.
See callbackFlow API documentation for the details.
""".trimIndent()
)
}
}
override fun create(context: CoroutineContext, capacity: Int, onBufferOverflow: BufferOverflow): ChannelFlow =
CallbackFlowBuilder(block, context, capacity, onBufferOverflow)
}