FixedLaunchBuilderDemo.kt

import java.lang.Exception
import kotlin.concurrent.thread
import java.util.concurrent.Executors
import kotlin.coroutines.Continuation
import kotlin.coroutines.CoroutineContext

import kotlin.coroutines.intrinsics.createCoroutineUnintercepted
import kotlin.coroutines.intrinsics.suspendCoroutineUninterceptedOrReturn
import kotlin.coroutines.intrinsics.COROUTINE_SUSPENDED

fun main() {
    launch(Dispatcher.Main) {
        println("Download started - $threadName")
        download()
        println("Download complete - $threadName")
    }

}

val threadName get() = Thread.currentThread().name
val dummyMainThread = Executors.newSingleThreadExecutor { Thread(it, "main") }

suspend fun download() {
    suspendCoroutineUninterceptedOrReturn<Unit> { cont ->
        thread {
            println("Download in progress - $threadName")
            when (cont.context as Dispatcher) {
                Dispatcher.Main -> dummyMainThread.execute {
                    cont.resumeWith(Result.success(Unit))
                }

                Dispatcher.Background -> thread {
                    cont.resumeWith(Result.success(Unit))
                }
            }
        }
        COROUTINE_SUSPENDED
    }
}

sealed class Dispatcher : CoroutineContext.Element {
    object Main : Dispatcher()
    object Background : Dispatcher()

    override val key = DispatcherKey
}

object DispatcherKey : CoroutineContext.Key<Dispatcher> {}

fun launch(dispatcher: Dispatcher, block: suspend () -> Unit) {
    val callback = object : Continuation<Unit> {
        // Passing dispatcher as CoroutineContext
        override val context: CoroutineContext = dispatcher
        override fun resumeWith(result: Result<Unit>) {}
    }
    val coroutine = block.createCoroutineUnintercepted(callback)
    when (dispatcher) {
        Dispatcher.Main -> dummyMainThread.execute {
            coroutine.resumeWith(Result.success(Unit))
        }

        Dispatcher.Background -> thread {
            coroutine.resumeWith(Result.success(Unit))
        }
    }
}