coroutines

Coroutines

General purpose extensions to kotlinx.coroutines.

Supported platforms: Linux (x64), MingW (x64), macOS (x64), iOS (arm32, arm64 & x64), JS, JVM (including Android).

Setup

If you want to use this dependency without using one of the fun packs, you can use Splitties.coroutines, provided you have refreshVersions added to the project.

For reference, the maven coordinates of this module are com.louiscad.splitties:splitties-coroutines.

Offering a value to a SendChannel (including inside callbackFlow) safely

Calling the offer function in SendChannel as it can crash your program if the channel is closed, as can be seen in this issue: Kotlin/kotlinx.coroutines#974.

To be safe from this pitfall, Splitties provides an extension named offerCatching(…) that returns false if offer throws, making you safe in the edge cases where a call is made just after the channel gets closed, as can happen when using callbackFlow { … } in multi-threaded environments.

Cancellable infinite loops

The repeatWhileActive { … } top level function is like a while (true) { … } loop, except that:

• it evaluates to Nothing, allowing you to use it in any lambda, regardless of its return type.
• it will check for cancellation before each execution of the passed lambda, protecting from an infinite loop if the code in the lambda doesn't check for cancellation but eventually ends running.

It also has an overload that takes a Boolean parameter named ignoreInnerCancellations. This one is made for the cases where you're throwing a CancellationException to signal the cancellation of this loop run/iteration, but want to continue looping again. Can be useful if you are using withTimeout { … } or want to easily handle user requested cancellation. It will still check that the scope has not been cancelled before each iteration, so it's perfectly safe to use among code that must be cancellable.

Example:

suspend fun doStuffUntilCancelled(ui: SomeUi, someObject: SomeType): Nothing {
    repeatWhileActive {
        val command = ui.awaitSomeUserAction()
        someObject.doSomething(command.whatever)
        ui.awaitAnotherUserAction()
        val result = someObject.doSomethingElse()
        ui.showOutcome(result)
    }
}

Racing coroutines

Number of racers fixed at compile time

The raceOf(…) function takes a variable number of coroutines, that also have a CoroutineScope as a receiver. The first coroutine to finish (includes that any child coroutines that might have been launched are completed or cancelled too) will cancel the others, and its value will be returned by raceOf.

Note that calling raceOf with no racers is forbidden, and any attempt to do so will throw an IllegalArgumentException. There's a deprecated overload of raceOf() taking no parameters to prevent you from making this mistake. For variable number of racing coroutines, use race + launchRacer documented below.

Simple example (unit test):

suspend fun testCoroutinesRacing() {
    val result = raceOf({
        delay(3)
        "slow"
    }, {
        delay(0)
        "fast"
    })
    assertEquals(expected = "fast", actual = result)
}

UI code example

suspend fun awaitUserChoice(ui: SomeUi, choices: List<Stuff>): Stuff? = raceOf({
    ui.awaitSomeUserAction(choices)
}, {
    ui.awaitDismissal()
    null
}, {
    ui.showSomethingInRealtimeUntilCancelled() // Returns Nothing, will run, but never "win".
})

Dynamic racers and late racers

The raceOf(…) function might not suit your use case if the racing coroutines need to be launched based on some conditions, that might also take some time to be evaluated.

These more advanced use-cases are satisfied by the race { … } function, where you can call launchRacer { … } in its lambda.

If no racing coroutines are ever launched, the race { … } function will suspend until it is cancelled.

Example

suspend fun awaitSomeActionTrigger(
    config: SomeConfig,
    deviceCapabilities: HardwareSupportInfo,
    defaultInput: SomeInput,
    awaitSpecialInput: suspend () -> Unit
) {
    race {
        launchRacer { defaultInput.awaitSomeAction() }

        if (deviceCapabilities.supportsSpecialInput) {
            launchRacer { awaitSpecialInput() }
            delay(config.delayBeforeHintMillis) // The race lambda can suspend
            showHintForSpecialInput()
        }
    }
}

Suspending version of lazy

These 3 functions provide a SuspendLazy instance:

• CoroutineScope.suspendLazy { … }
• suspendBlockingLazy { … } (uses Dispatchers.Default by default)
• suspendBlockingLazyIO { … } (JVM-only for now, uses Dispatchers.IO)

They can be used almost like Kotlin's lazy, but you need parentheses to get the value.

Note that if an Exception or any Throwable is thrown in their initializing lambda, the SuspendLazy instance will not be recoverable, so if retry logic is needed, it must include instantiating a new SuspendLazy. If you want it to allow retries on the same instance, please open an issue and tell about your use case.

Example: Initializing a database only once

For an app-wide remote database:

val db: SomeDatabase = GlobalScope.suspendLazy {
    createDatabase().also { it.connect() } // Both functions suspend
}

fun doStuff(someData: SomeData) {
    db().runSomeTransaction(someData)
}

For an application-wide local database (e.g. in an Android or desktop JVM app):

val db: AppDatabase = suspendBlockingLazyIO { buildDatabase() }

fun doStuff(someData: SomeData) {
    db().runSomeTransaction(someData)
}

Passing a SuspendLazy type can be handy:

suspend fun doStuff(getDb: SuspendLazy<AppDatabase>, someData: SomeData) {
    val db = getDb()
    db.runSomeTransaction(someData)
}

Example: Offloading heavy object instantiation or expensive computation

Objects that need a significant amount of memory (example bitmap/images, large arrays/lists, or large/deep data structures such as possibly non trivial trees) will block the current thread while being instantiated, waiting for the CPU to find enough free memory (RAM), which might include moving lots of stuff to have the contiguous space that can be required.

To avoid blocking the user interface and altering the perceived performance of the application, you need to not run such code on the main/UI thread, but offload it to another thread such as one from Dispatchers.Default. The suspendBlockingLazy { … } makes it very easy to do it right:

val expensiveThing = suspendBlockingLazy { DoHeavyInstantiation() }

fun doStuff(ui: SomeUi) { // Can run on main/UI thread.
    val thing = expensiveThing() // Will suspend until DoHeavyInstantiation() is done.
    ui.displaySomeDataNicely(thing)
}