The actor function contains a receiver parameter of the ActorScope type. The source code of the actor function looks as follows:

public fun <E> CoroutineScope.actor(
    context: CoroutineContext = EmptyCoroutineContext,
    capacity: Int = 0,
    start: CoroutineStart = CoroutineStart.DEFAULT,
    onCompletion: CompletionHandler? = null,
    block: suspend ActorScope<E>.() -> Unit
): SendChannel<E> {
    val newContext = newCoroutineContext(context)
    val channel = Channel<E>(capacity)
    val coroutine = if (start.isLazy)
        LazyActorCoroutine(newContext, channel, block) else
        ActorCoroutine(newContext, channel, active = true)
    if (onCompletion != null) coroutine.invokeOnCompletion(handler = onCompletion)
    coroutine.start(start, coroutine, block)
    return coroutine
}

The ActorScope interface looks similar to the ProducerScope interface, but implements the ReceiveChannel interface:

public interface ActorScope<E> : CoroutineScope, ReceiveChannel<E> {
    val channel: Channel<E>
}

As you probably know, it is not a good idea to access mutable data from different coroutines. To deal with this, we can use channels and the actor function, in the following way:

suspend fun numberConsumer() = GlobalScope.actor<Int> {
    var counter = 0
    for (value in channel) {
        counter += value
        println(counter)
    }
}

The preceding snippet contains a mutable variable named counter. We change the value of the counter variable when a channel receives a new value. Since a channel suspends the caller until a consumer finishes processing the current value, we can be sure that the counter variable will be modified in the right way.

The numbersCounter function can be used as follows:

@Test
fun actorExample() = runBlocking<Unit> {
    val actor = numberConsumer()
    (0..10).forEach {
        launch {
            actor.send(it)
        }
    }
}

The preceding snippet launches ten coroutines that send a value to an actor in parallel.

The output looks as follows:

 0
 1
 3
 6
 10
 15
 21
 28
 36
 45
 55

The output shows that the counter variable is modified in the right way.