The Hello World Program

This section contains a simple, working Hello World program. We won’t get into too much details - you can find deeper information in the subsequent sections. We will define a reactor that waits for one incoming event, prints a message to the standard output, and then terminates.

import io.reactors._

object HelloWorld {
  def main(args: Array[String]) {
    val welcomeReactor = Reactor[String] {
      self => onEvent { name =>
          println(s"Welcome, $name!")
    val system = ReactorSystem.default("test-system")
    val ch = system.spawn(welcomeReactor)
    ch ! "Alan"

The program above declares an anonymous reactor called welcomeReactor, which waits for a name to arrive on its main event stream, prints that name, and then seals its main channel, therefore terminating itself. The main program then creates a new reactor system, uses the reactor template to start a new running instance of the reactor, and sends an event to it.

Above, we note the following:

  • A reactor is defined using the Reactor[T] constructor.
  • A reactor reacts to incoming events as specified in the callback function passed to the onEvent method.
  • Calling main.seal() terminates the reactor.
  • A reactor with a specific definition is started with the spawn method, which returns the reactor’s default channel.
  • Events are sent to the reactor by calling the ! operator on its channels.

The subsequent sections will explain these features in depth.