=== Writing Verticles

There are two alternatives to create verticles in Scala:

 * a Scala class extending the `ScalaVerticle` class
 * a Scala class implementing the `Verticle` interface or extending the `AbstractVerticle` class

 We will not cover the third case in this manual, as it is a plain Java class and instead you should read the
 Vert.x manual for Java.

Alternatively, you can extend the `ScalaVerticle` class and implement the `start` and
`stop` methods:

[source, scala]
----
import io.vertx.lang.scala.ScalaVerticle;

class HelloWorldHttpVerticle extends ScalaVerticle {

 override def start(): Unit = {
   println("Starting")
 }

 override def stop(): Unit = {
   println("Stopping")
 }
----

As in Java, vert.x creates a new instance of the class automatically.  When Vert.x deploys the verticle it will call
the `start` method, and when the method has completed the verticle will be considered started.

You can also optionally override the `stop` method. This will be called by Vert.x when the verticle is undeployed
and when the method has completed the verticle will be considered stopped.

=== Accessing the vertx instance from a verticle

Regardless the way you use to implement your verticle, you access the vert.x instance using the `vertx` field.

Access to the vert.x instance in a ScalaVerticle class
[source, scala]
----
import io.vertx.lang.scala.ScalaVerticle;

public class HelloWorldVerticle extends ScalaVerticle {

   override def start(): Unit = {
     vertx.deployVerticle("another_verticle.js")
   }
}
----

To deploy a Scala based verticle you have to prefix the classname with `scala:`
[source, scala]
----
val vertx = Vertx.vertx
vertx.deployVerticle(s"scala:${classOf[HelloWorldVerticle].getName}")
----

There is also a little helper available.
----
val vertx = Vertx.vertx
vertx.deployVerticle(ScalaVerticle.nameForVerticle[HelloWorldVerticle])
----

=== Asynchronous Verticle start and stop

Sometimes you want to do something in your verticle start-up which takes some time and you don't want the verticle to
be considered deployed until that happens. For example you might want to deploy other verticles in the start method.

You can't block waiting for the other verticles to deploy in your start method as that would break the <<golden_rule, Golden Rule>>.

So how can you do this?

The way to do it is to implement the *asynchronous* start method. This version of the method takes a Promise as a parameter.When the method returns the verticle will *not* be considered deployed.

Some time later, after you've done everything you need to do (e.g. start other verticles), you can call success/failure
on the Promise to signal that you're done. Similarly, there is an asynchronous version of the stop method too.
You use this if you want to do some verticle cleanup that takes some time.

If your verticle extends `ScalaVerticle`, you override the
`start` and
`stop` methods:

[source, scala]
----
import io.vertx.lang.scala.ScalaVerticle
import scala.concurrent.{Future, Promise}

public class HelloWorldVerticle extends ScalaVerticle {
 public void start(): Future[Unit] = {
  println "starting"
  val promise = Promise[Unit]()
  vertx.deployVerticleFuture("v.rb").onComplete{
   case Success(deploymentId) => promise.success(())
   case Failure(throwable) => promise.failure(throwable)
  }
  promise.future
 }

 public void stop(): Future[Unit] = {
  println("stopping")
  Future.successful(())
 }
}
----

NOTE: You don't need to manually undeploy child verticles started by a verticle, in the verticle's stop method.
Vert.x will automatically undeploy any child verticles when the parent is undeployed.