GenericWithContextBuilder.kt

suspend fun <R> withContext(dispatcher: Dispatcher, block: suspend () -> R): R {
    return suspendCoroutineUninterceptedOrReturn<R> { cont ->
        val callback = object : Continuation<R> {
            override val context = dispatcher

            override fun resumeWith(result: Result<R>) {
                when (cont.context as Dispatcher) {
                    Dispatcher.Main -> Handler(Looper.getMainLooper()).post { 
                        cont.resumeWith(result) 
                    }

                    Dispatcher.Background -> thread { 
                        cont.resumeWith(result) 
                    }
                }
            }
        }
        val coroutine = block.createCoroutineUnintercepted(callback)
        when (dispatcher) {
            Dispatcher.Main -> Handler(Looper.getMainLooper()).post {
                coroutine.resumeWith(Result.success(Unit))
            }

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