2.3 The ChatServer
The ChatServer code is:
ChatServer.scala
package code
package comet
import net.liftweb._
import http._
import actor._
/**
* A singleton that provides chat features to all clients.
* It's an Actor so it's thread-safe because only one
* message will be processed at once.
*/
object ChatServer extends LiftActor with ListenerManager {
private var msgs = Vector("Welcome") // private state
/**
* When we update the listeners, what message do we send?
* We send the msgs, which is an immutable data structure,
* so it can be shared with lots of threads without any
* danger or locking.
*/
def createUpdate = msgs
/**
* process messages that are sent to the Actor. In
* this case, we're looking for Strings that are sent
* to the ChatServer. We append them to our Vector of
* messages, and then update all the listeners.
*/
override def lowPriority = {
case s: String => msgs :+= s; updateListeners()
}
}
The ChatServer is defined as an object rather than a class. This makes it a singleton which can be referenced by the name ChatServer anywhere in the application. Scala’s singletons differ from Java’s static in that the singleton is an instance of an object and that instance can be passed around like any other instance. This is why we can return the ChatServer instance from the registerWith method in that Chat component.
The ChatServer has private state, a Vector[String] representing the list of chat messages. Note that Scala’s type inferencer infers the type of msgs so you do not have to explicitly define it.
The createUpdate method generates an update to send to listeners. This update is sent when a listener registers with the ChatServer or when the updateListeners() method is invoked.
Finally, the lowPriority method defines the messages that this component can handle. If the ChatServer receives a String as a message, it appends the String to the Vector of messages and updates listeners.
(C) 2012 David Pollak