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FlowExt | Kotlinx Coroutines Flow Extensions | Kotlinx Coroutines Flow Extensions. Extensions to the Kotlin Flow library.

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FlowExt

Maven Central codecov Build Validate Gradle Wrapper License: MIT Kotlin version KotlinX Coroutines version badge badge badge badge badge badge badge badge badge Hits

  • FlowExt is a Kotlin Multiplatform library, that provides many operators and extensions to Kotlin Coroutines Flow.
  • FlowExt provides a collection of operators, Flows and utilities for Flow, that are not provided by Kotlinx Coroutine themselves, but are common in other Reactive Frameworks (rxjs, RxJava, RxSwift, rxdart, ...) and standards.

Kotlinx Coroutines Flow Extensions. Extensions to the Kotlin Flow library. Kotlin Flow extensions. Multiplatform Kotlinx Coroutines Flow Extensions. Multiplatform Extensions to the Kotlin Flow library. Multiplatform Kotlin Flow extensions. RxJS Kotlin Coroutines Flow. RxSwift Kotlin Coroutines Flow. RxJava Kotlin Coroutines Flow. RxJS Kotlin Flow. RxSwift Kotlin Flow. RxJava Kotlin Flow. RxJS Coroutines Flow. RxSwift Coroutines Flow. RxJava Coroutines Flow. Kotlin Flow operators. Coroutines Flow operators.

Author: Petrus Nguyễn Thái Học

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Supported targets

  • android.
  • jvm.
  • js (IR).
  • iosArm64, iosX64, iosSimulatorArm64.
  • watchosArm32, watchosArm64, watchosX64, watchosSimulatorArm64, watchosDeviceArm64.
  • tvosX64, tvosSimulatorArm64, tvosArm64.
  • macosX64, macosArm64.
  • mingwX64
  • linuxX64, linuxArm64.
  • androidNativeArm32, androidNativeArm64, androidNativeX86, androidNativeX64.

API

Note: This is still a relatively early version of FlowExt, with much more to be desired. I gladly accept PRs, ideas, opinions, or improvements. Thank you! :)

0.x release docs: https://hoc081098.github.io/FlowExt/docs/0.x

Snapshot docs: https://hoc081098.github.io/FlowExt/docs/latest

Installation

allprojects {
  repositories {
    ...
    mavenCentral()
  }
}

Multiplatform

implementation("io.github.hoc081098:FlowExt:0.7.3")

Snapshot

Snapshots of the development version are available in Sonatype's snapshots repository.
  • Kotlin
allprojects {
  repositories {
    ...
    maven(url = "https://s01.oss.sonatype.org/content/repositories/snapshots/")
  }
}

dependencies {
  implementation("io.github.hoc081098:FlowExt:0.7.4-SNAPSHOT")
}
  • Groovy
allprojects {
  repositories {
    ...
    maven { url "https://s01.oss.sonatype.org/content/repositories/snapshots/" }
  }
}

dependencies {
  implementation("io.github.hoc081098:FlowExt:0.7.4-SNAPSHOT")
}

Table of contents

bufferCount / chunked

Buffers the source Flow values until the size hits the maximum bufferSize given.

Note, chunked is an alias to bufferCount.

range(start = 0, count = 10)
  .bufferCount(bufferSize = 3)
  .collect { println("bufferCount: $it") }

println("---")

range(start = 0, count = 10)
  .bufferCount(bufferSize = 3, startBufferEvery = 2)
  .collect { println("bufferCount: $it") }

Output:

bufferCount: [0, 1, 2]
bufferCount: [3, 4, 5]
bufferCount: [6, 7, 8]
bufferCount: [9]
---
bufferCount: [0, 1, 2]
bufferCount: [2, 3, 4]
bufferCount: [4, 5, 6]
bufferCount: [6, 7, 8]
bufferCount: [8, 9]

concat

Creates an output Flow which sequentially emits all values from the first given Flow and then moves on to the next.

concat(
  flow1 = flowOf(1, 2, 3),
  flow2 = flowOf(4, 5, 6)
).collect { println("concat: $it") }

Output:

concat: 1
concat: 2
concat: 3
concat: 4
concat: 5
concat: 6

defer

Creates a Flow that, on collection, calls a Flow factory to make a Flow for each new FlowCollector. In some circumstances, waiting until the last minute (that is, until collection time) to generate the Flow can ensure that collectors receive the freshest data.

var count = 0L
val flow = defer {
  delay(count)
  flowOf(count++)
}

flow.collect { println("defer: $it") }
println("---")
flow.collect { println("defer: $it") }
println("---")
flow.collect { println("defer: $it") }

Output:

defer: 0
---
defer: 1
---
defer: 2

flowFromNonSuspend

Creates a cold flow that produces a single value from the given function. It calls the function for each new FlowCollector.

See also flowFromSuspend for the suspend version.

var count = 0L
val flow = flowFromNonSuspend { count++ }

flow.collect { println("flowFromNonSuspend: $it") }
println("---")
flow.collect { println("flowFromNonSuspend: $it") }
println("---")
flow.collect { println("flowFromNonSuspend: $it") }

Output:

flowFromNonSuspend: 0
---
flowFromNonSuspend: 1
---
flowFromNonSuspend: 2

flowFromSuspend

Creates a cold flow that produces a single value from the given function. It calls the function for each new FlowCollector.

See also flowFromNonSuspend for the non-suspend version.

var count = 0L
val flow = flowFromSuspend {
  delay(count)
  count++
}

flow.collect { println("flowFromSuspend: $it") }
println("---")
flow.collect { println("flowFromSuspend: $it") }
println("---")
flow.collect { println("flowFromSuspend: $it") }

Output:

flowFromSuspend: 0
---
flowFromSuspend: 1
---
flowFromSuspend: 2

interval

Returns a Flow that emits a 0L after the initialDelay and ever-increasing numbers after each period of time thereafter.

interval(initialDelay = 100.milliseconds, period = 1.seconds)
  .take(5)
  .collect { println("interval: $it") }

Output:

interval: 0
interval: 1
interval: 2
interval: 3
interval: 4

neverFlow

Returns a NeverFlow that never emits any values to the FlowCollector and never completes.

neverFlow()
  .startWith(7)
  .collect { println("neverFlow: $it") }

println("Completed!")

Output:

neverFlow: 7
// Never prints "Completed!"

race / amb

Mirrors the one Flow in an Iterable of several Flows that first either emits a value or sends a termination event (error or complete event).

When you pass a number of source Flows to race, it will pass through the emissions and events of exactly one of these Flows: the first one that sends an event to race, either by emitting a value or sending an error or complete event. race will cancel the emissions and events of all of the other source Flows.

race(
  flow {
    delay(100)
    emit(1)
    emit(2)
    emit(3)
  },
  flow {
    delay(200)
    emit(2)
    emit(3)
    emit(4)
  }
).collect { println("race: $it") }

Output:

race: 1
race: 2
race: 3

range

Creates a Flow that emits a sequence of numbers within a specified range.

range(start = 0, count = 5)
  .collect { println("range: $it") }

Output:

range: 1
range: 2
range: 3
range: 4

timer

Creates a Flow that will wait for a given duration, before emitting the value.

timer(value = Unit, duration = 1.seconds)
  .collect { println("timer: $it") }

Output:

// After 1 second
timer: kotlin.Unit

combine

cast / castNotNull / castNullable / safeCast

cast

Adapt this Flow to be a Flow<R>.

This Flow is wrapped as a Flow<R> which checks at run-time that each value event emitted by this Flow is also an instance of R.

At the collection time, if this Flow has any value that is not an instance of R, a ClassCastException will be thrown.

flowOf<Any?>(1, 2, 3)
  .cast<Int>()
  .collect { v: Int -> println("cast: $v") }

Output:

cast: 1
cast: 2
cast: 3
castNotNull

Adapt this Flow<T?> to be a Flow<T>.

At the collection time, if this Flow has any null value, a NullPointerException will be thrown.

flowOf<Int?>(1, 2, 3)
  .castNotNull()
  .collect { v: Int -> println("castNotNull: $v") }

Output:

castNotNull: 1
castNotNull: 2
castNotNull: 3
safeCast

Adapt this Flow<*> to be a Flow<R?>.

At the collection time, if this Flow has any value that is not an instance of R, null will be emitted.

flowOf<Any?>(1, 2, 3, "Kotlin", null)
  .safeCast<Int?>()
  .collect { v: Int? -> println("safeCast: $v") }

Output:

safeCast: 1
safeCast: 2
safeCast: 3
safeCast: null
safeCast: null

concatWith / plus

Returns a Flow that emits the items emitted from the current Flow, then the next, one after the other, without interleaving them.

Note, plus is an alias to concatWith.

flowOf(1, 2, 3)
  .concatWith(flowOf(4, 5, 6))
  .collect { println("concatWith: $it") }

println("---")

val flow1 = flowOf(1, 2, 3)
val flow2 = flowOf(4, 5, 6)

(flow1 + flow2).collect { println("plus: $it") }

Output:

concatWith: 1
concatWith: 2
concatWith: 3
concatWith: 4
concatWith: 5
concatWith: 6
---
plus: 1
plus: 2
plus: 3
plus: 4
plus: 5
plus: 6

startWith

Returns a Flow that emits a specified item (or many items) before it begins to emit items emitted by the current Flow.

flowOf(1, 2, 3)
  .startWith(0)
  .collect { println("startWith: $i") }

Output:

startWith: 0
startWith: 1
startWith: 2
startWith: 3

flatMapFirst / exhaustMap

Projects each source value to a Flow which is merged in the output Flow only if the previous projected Flow has completed. If value is received while there is some projected Flow sequence being merged, it will simply be ignored.

This method is a shortcut for map(transform).flattenFirst().

range(1, 5)
  .onEach { delay(100) }
  .flatMapFirst { timer(it, 130) }
  .collect { println("flatMapFirst: $it") }

Output:

flatMapFirst: 1
flatMapFirst: 3
flatMapFirst: 5

flattenFirst / exhaustAll

Converts a higher-order Flow into a first-order Flow by dropping inner Flow while the previous inner Flow has not yet completed.

range(1, 5)
  .onEach { delay(100) }
  .map { timer(it, 130) }
  .flattenFirst()
  .collect { println("flattenFirst: $it") }

Output:

flattenFirst: 1
flattenFirst: 3
flattenFirst: 5

groupBy

Groups the items emitted by the current Flow according to a specified criterion, and emits these grouped items as GroupedFlows.

range(1, 10)
  .groupBy { it % 2 }
  .flatMapMerge { groupedFlow ->
    groupedFlow
      .map { groupedFlow.key to it }
  }
  .collect { println("groupBy: $it") }

Output:

groupBy: (1, 1)
groupBy: (0, 2)
groupBy: (1, 3)
groupBy: (0, 4)
groupBy: (1, 5)
groupBy: (0, 6)
groupBy: (1, 7)
groupBy: (0, 8)
groupBy: (1, 9)
groupBy: (0, 10)

ignoreElements

Ignores all elements emitted by the source Flow, only passes calls of complete or error.

flowOf("you", "talking", "to", "me")
  .ignoreElements()
  .materialize()
  .collect { println("ignoreElements: $it") }

Output:

ignoreElements: Event.Complete

flatMapConcatEager

Transforms elements emitted by the original Flow by applying transform, that returns another flow, and then merging and flattening these flows.

This operator calls transform sequentially and then concatenates the resulting flows with a concurrency limit on the number of concurrently collected flows. It is a shortcut for map(transform).flattenConcatEager(concurrency).

range(1, 5)
  .onEach { delay(100) }
  .flatMapConcatEager(concurrency = 2) { v ->
    timer(v, 130)
      .onStart { println("flatMapConcatEager: onStart $v") }
      .onCompletion { println("flatMapConcatEager: onCompletion $v") }
  }
  .collect { println("flatMapConcatEager: $it") }

Output:

flatMapConcatEager: onStart 1
flatMapConcatEager: onStart 2
flatMapConcatEager: 1
flatMapConcatEager: onCompletion 1
flatMapConcatEager: onStart 3
flatMapConcatEager: 2
flatMapConcatEager: onCompletion 2
flatMapConcatEager: onStart 4
flatMapConcatEager: 3
flatMapConcatEager: onCompletion 3
flatMapConcatEager: onStart 5
flatMapConcatEager: 4
flatMapConcatEager: onCompletion 4
flatMapConcatEager: 5
flatMapConcatEager: onCompletion 5

mapIndexed

Returns a flow containing the results of applying the given transform function to each value and its index in the original flow.

range(1, 3)
  .mapIndexed { index, value -> index to value }
  .collect { println("mapIndexed: $it") }

Output:

mapIndexed: (0, 1)
mapIndexed: (1, 2)
mapIndexed: (2, 3)

mapTo

Emits the given constant value on the output Flow every time the source Flow emits a value.

range(1, 3)
  .mapTo("Value")
  .collect { println("mapTo: $it") }

Output:

mapTo: Value
mapTo: Value
mapTo: Value

mapToUnit

Emits kotlin.Unit value on the output Flow every time the source Flow emits a value.

range(1, 3)
  .mapToUnit()
  .collect { println("mapToUnit: $it") }

Output:

mapToUnit: kotlin.Unit
mapToUnit: kotlin.Unit
mapToUnit: kotlin.Unit

materialize

Represents all of the notifications from the source Flow as value emissions marked with their original types within Event objects.

flowOf(1, 2, 3)
  .materialize()
  .collect { println("materialize: $it") }

Output:

materialize: Event.Value(1)
materialize: Event.Value(2)
materialize: Event.Value(3)
materialize: Event.Complete

dematerialize

Converts a Flow of Event objects into the emissions that they represent.

flowOf(Event.Value(1), Event.Value(2), Event.Value(3))
  .dematerialize()
  .collect { println("dematerialize: $it") }

Output:

dematerialize: 1
dematerialize: 2
dematerialize: 3

raceWith / ambWith

Mirrors the current Flow or the other Flows provided of which the first either emits a value or sends a termination event (error or complete event).

flow {
  delay(100)
  emit(1)
  emit(2)
  emit(3)
}.raceWith(
  flow {
    delay(200)
    emit(2)
    emit(3)
    emit(4)
  }
).collect { println("raceWith: $it") }

Output:

raceWith: 1
raceWith: 2
raceWith: 3

pairwise / zipWithNext

Groups pairs of consecutive emissions together and emits them as a pair. Emits the (n)th and (n-1)th events as a pair. The first value won't be emitted until the second one arrives.

Note, zipWithNext is an alias to pairwise.

range(0, 4)
  .pairwise()
  .collect { println("pairwise: $it") }

println("---")

range(0, 4)
  .zipWithNext { a, b -> "$a -> $b" }
  .collect { println("zipWithNext: $it") }

Output:

pairwise: (0, 1)
pairwise: (1, 2)
pairwise: (2, 3)
---
zipWithNext: 0 -> 1
zipWithNext: 1 -> 2
zipWithNext: 2 -> 3

repeat

Returns a Flow that will recollect to the source stream when the source stream completes.

flowFromSuspend {
  println("Start collecting...")

  Random
    .nextInt(0..3)
    .also { println("Emit: $it") }
}
  .repeat(
    delay = 1.seconds,
    count = 10
  )
  .filter { it == 2 }
  .take(1)
  .collect { println("repeat: $it") }

Output:

Start collecting...
Emit: 1
Start collecting...
Emit: 3
Start collecting...
Emit: 1
Start collecting...
Emit: 0
Start collecting...
Emit: 1
Start collecting...
Emit: 3
Start collecting...
Emit: 2
repeat: 2

retryWhenWithDelayStrategy

Retries collection of the given flow when an exception occurs in the upstream flow and the predicate returns true. The predicate also receives an attempt number as parameter, starting from zero on the initial call. When predicate returns true, the next retries will be delayed after a duration computed by DelayStrategy.nextDelay.

var count = -1

flowFromSuspend {
  ++count
  println("Call count=$count")

  when (count) {
    0 -> throw MyException(message = "Will retry...", cause = null)
    1 -> "Result: count=$count"
    else -> error("Unexpected: count=$count")
  }
}
  .retryWhenWithDelayStrategy(
    strategy = DelayStrategy.FixedTimeDelayStrategy(duration = 200.milliseconds),
    predicate = { cause, attempt -> cause is MyException && attempt < 1 }
  )
  .collect { println("retryWhenWithDelayStrategy: $it") }

Output:

Call count=0
Call count=1
retryWhenWithDelayStrategy: Result: count=1

retryWhenWithExponentialBackoff

Retries collection of the given flow with exponential backoff delay strategy when an exception occurs in the upstream flow and the predicate returns true. When predicate returns true, the next retries will be delayed after a duration computed by DelayStrategy.ExponentialBackoffDelayStrategy.

var count = -1

flowFromSuspend {
  ++count
  println("Call count=$count")

  when (count) {
    0 -> throw MyException(message = "Will retry...", cause = null)
    1 -> "Result: count=$count"
    else -> error("Unexpected: count=$count")
  }
}
  .retryWhenWithExponentialBackoff(
    initialDelay = 500.milliseconds,
    factor = 2.0,
  ) { cause, attempt -> cause is MyException && attempt < 1 }
  .collect { println("retryWhenWithExponentialBackoff: $it") }

Output:

Call count=0
Call count=1
retryWhenWithExponentialBackoff: Result: count=1

retryWithExponentialBackoff

Retries collection of the given flow with exponential backoff delay strategy when an exception occurs in the upstream flow and the predicate returns true. When predicate returns true, the next retries will be delayed after a duration computed by DelayStrategy.ExponentialBackoffDelayStrategy.

var count = -1

flowFromSuspend {
  ++count
  println("Call count=$count")

  when (count) {
    0 -> throw MyException(message = "Will retry...", cause = null)
    1 -> "Result: count=$count"
    else -> error("Unexpected: count=$count")
  }
}
  .retryWithExponentialBackoff(
    maxAttempt = 2,
    initialDelay = 500.milliseconds,
    factor = 2.0,
  ) { it is MyException }
  .collect { println("retryWithExponentialBackoff: $it") }

Output:

Call count=0
Call count=1
retryWithExponentialBackoff: Result: count=1

scanWith

Folds the given flow with [operation], emitting every intermediate result, including the initial value supplied by [initialSupplier] at the collection time.

This is a variant of scan that the initial value is lazily supplied, which is useful when the initial value is expensive to create or depends on a logic that should be executed at the collection time (lazy semantics).

var count = 0
val mutex = Mutex()

suspend fun calculateInitialValue(): Int {
  println("calculateInitialValue")
  delay(1000)
  return mutex.withLock { count++ }
}

flowOf(1, 2, 3)
  .scanWith(::calculateInitialValue) { acc, e -> acc + e }
  .collect { println("scanWith[1]: $it") }

flowOf(1, 2, 3)
  .scanWith(::calculateInitialValue) { acc, e -> acc + e }
  .collect { println("scanWith[2]: $it") }

Output:

calculateInitialValue
scanWith[1]: 0
scanWith[1]: 1
scanWith[1]: 3
scanWith[1]: 6
calculateInitialValue
scanWith[2]: 1
scanWith[2]: 2
scanWith[2]: 4
scanWith[2]: 7

select

  • Inspirited by NgRx memoized selector.

  • Selectors are pure functions used for obtaining slices of a Flow of state. FlowExt provides a few helper functions for optimizing this selection.

    • Selectors can compute derived data, to store the minimal possible state.
    • Selectors are efficient. A selector is not recomputed unless one of its arguments changes.
    • When using the [select] functions, it will keep track of the latest arguments in which your selector function was invoked. Because selectors are pure functions, the last result can be returned when the arguments match without re-invoking your selector function. This can provide performance benefits, particularly with selectors that perform expensive computation. This practice is known as memoization.
data class UiState(
  val items: List<String> = emptyList(),
  val term: String? = null,
  val isLoading: Boolean = false,
  val error: Throwable? = null
)

flow {
  println("select: emit 1")
  emit(UiState())

  println("select: emit 2")
  emit(
    UiState(
      items = listOf("a", "b", "c"),
      term = "a",
      isLoading = true,
      error = Throwable("error")
    )
  )

  println("select: emit 3")
  emit(
    UiState(
      items = listOf("a", "b", "c"),
      term = "a",
      isLoading = false,
      error = Throwable("error")
    )
  )

  println("select: emit 4")
  emit(
    UiState(
      items = listOf("a", "b", "c"),
      term = "b",
      isLoading = false,
      error = Throwable("error")
    )
  )
}
  .select(
    selector1 = { it.items },
    selector2 = { it.term },
    projector = { items, term ->
      term?.let { v ->
        items.filter { it.contains(v, ignoreCase = true) }
      }
    }
  )
  .collect { println("select: $it") }

Output:

select: emit 1
select: null
select: emit 2
select: [a]
select: emit 3
select: emit 4
select: [b]

skipUntil / dropUntil

Returns a Flow that skips items emitted by the source Flow until a second Flow emits a value or completes.

flowOf(1, 2, 3)
  .onEach { delay(100) }
  .skipUntil(timer(Unit, 150))
  .collect { println("skipUntil: $it") }

Output:

skipUntil: 2
skipUntil: 3

takeUntil

Emits the values emitted by the source Flow until a notifier Flow emits a value or completes.

range(0, 5)
  .onEach { delay(100) }
  .takeUntil(timer(Unit, 270.milliseconds))
  .collect { println("takeUntil: $it") }

Output:

takeUntil: 0
takeUntil: 1

throttleTime

Returns a Flow that emits a value from the source Flow, then ignores subsequent source values for a duration determined by durationSelector, then repeats this process for the next source value.

(1..10)
  .asFlow()
  .onEach { delay(200) }
  .throttleTime(500)
  .collect { println("throttleTime: $it") }

Output:

throttleTime: 1
throttleTime: 4
throttleTime: 7
throttleTime: 10

withLatestFrom

Merges two Flows into one Flow by combining each value from self with the latest value from the second Flow, if any. Values emitted by self before the second Flow has emitted any values will be omitted.

range(0, 5)
  .onEach { delay(100) }
  .withLatestFrom(
    range(0, 10)
      .onEach { delay(70) }
  )
  .collect { println("withLatestFrom: $it") }

Output:

withLatestFrom: (0, 0)
withLatestFrom: (1, 1)
withLatestFrom: (2, 3)
withLatestFrom: (3, 4)
withLatestFrom: (4, 6)

... and more, please check out Docs 0.x/Docs snapshot.

License

MIT License

Copyright (c) 2021-2023 Petrus Nguyễn Thái Học

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