Calling kotlin suspend functions from Swift

SuspendFunctionsFromSwift.md

Works

Suspend function can be called from swift on Main thread. Examples:

// Sample.kt
suspend fun add(a: Int, b: Int): Int {
    return a + b;
}

// Sample.swift
// Option - 1
SampleKt.add(a: 1, b: 2) { total, error in
    debugPrint("completionHandler: total \(total) error: \(error)")
}

// Option - 2
Task.init {
    do {
        let total = try await SampleKt.add(a: 1, b: 2)
            debugPrint("await: total \(total)")
        } catch {
            print("error \(error)")
        }
    }

Doesn't Work

Running suspend functions on background thread from swift doesn't work. Examples:

• Backgroud thread using DispatchQueue

typealias Dispatch = DispatchQueue

extension Dispatch {
    static func background(_ task: @escaping () -> Void) {
        Dispatch.global(qos: .background).async {
            task()
        }
    }
    static func main(_ task: @escaping () -> Void) {
        Dispatch.main.async {
            task()
        }
    }
}

Dispatch.background {
    SampleKt.add(a: 1, b: 2) { total, error in
        debugPrint("Dispatch: total \(total) error: \(error)")
    }
}

Exception NSException * "Calling Kotlin suspend functions from Swift/Objective-C is currently supported only on main thread"

• Running suspend function block from swift using coroutine from kotlin file.

// Sample.kt
fun <T> runWithCoroutine(dispatcher: CoroutineDispatcher, block: () -> T): Closeable {
    val job = Job()
    CoroutineScope(dispatcher + job).launch {
        block()
    }
    return object : Closeable {
        override fun close() {
            job.cancel()
        }
    }
}

// Sample.swift
SampleKt.runWithCoroutine(dispatcher: AppModule.dispatchers.io) {
    SampleKt.add(a: 1, b: 2) { total, error in
        debugPrint("runWithCoroutine: total \(total) error: \(error)")
    }
}

Exception NSException * "Calling Kotlin suspend functions from Swift/Objective-C is currently supported only on main thread"

Starting with kotlin 1.7.20 its now possible to lift the restriction of calling suspend function only from the main thread.
Simply upgrade your project to use kotlinVersion 1.7.20 and add the following to your gradle.properties file:

kotlin.native.binary.objcExportSuspendFunctionLaunchThreadRestriction=none

Source: https://youtrack.jetbrains.com/issue/KT-51297/Native-allow-calling-Kotlin-suspend-functions-on-non-main-thread-from-Swift#focus=Comments-27-6106760.0-0

Thanks @hiasel, will check it out!