GWTEventService Hello World

Yesterday I’ve discovered GWTEventService. Today i started to play around with it, after some research in the example-svn and reading the docs, I was able to write my own “Hello World”.

This Hello World is a small chatting application. You are only able to send messages but not set a nickname or something similar.

Here you can see what it will look like:

In this short overview I’ll just show you the parts that are associated with the GWTEventSerice. The whole regular GWT UI/RPC stuff is left out. If you want to see the complete example, you can download it here.

First of all you need an Event:

public class MyEvent implements Event {
	public String message;

	public MyEvent(){}

	public MyEvent(String message) {
		this.setMessage(message);
	}

	public String getMessage() {
		return message;
	}

	public void setMessage(String message) {
		this.message = message;
	}

}

This Event gets fired when the server gets a new message from a client.

The next thing you need is a EventListener that gets notified when an Event occures:

public class MyListener implements RemoteEventListener {

	/**
	 * This function gets called by EventService
	 */
	public void apply(Event anEvent) {
		/**
		 * Check if the incoming Event is from the type MyEvent and if so, call the corresponding function
		 */
		if(anEvent instanceof MyEvent)
			onMyEvent((MyEvent)anEvent);
	}

	/**
	 * This function gets called when the incomming Event is from the Type MyEvent
	 *
	 * @param event
	 */
	public void onMyEvent(MyEvent event){}

}

The Listener receives the Events from the server an processes them.

Now on the client-side you need to register your Listener to the server:

	/**
	 * The domain to which we want to register our listener to
	 */
	private static final Domain DOMAIN = DomainFactory.getDomain("my_domain");

	public void onModuleLoad() {

		/**
		 * Create an EventService
		 */
		RemoteEventServiceFactory theEventServiceFactory = RemoteEventServiceFactory.getInstance();
		RemoteEventService theEventService = theEventServiceFactory.getRemoteEventService();

		/**
		 * Register our listener to the domain
		 */
		theEventService.addListener(DOMAIN, new MyListener(){
			public void onMyEvent(MyEvent event){
				/**
				 * Get the message from the event and append it to the allready received messages
				 */
				getIncommingMessages().setText( getIncommingMessages().getText() + event.getMessage() + "n" );
			}
		});

                [.....]
	}

On the client-side we just register our Listener, and tell him what to do when an Event occurs.

The last thing that we need is the Server:

public class HelloWorldServiceImpl extends RemoteEventServiceServlet implements HelloWorldService {

	/**
	 * The Domain that this Service sends Events to.
	 */
	private static final Domain DOMAIN = DomainFactory.getDomain("my_domain");

	/**
	 * A normal function that gets called when a client sends a message to the server.
	 */
	public void sendMessage(String message) {
		/**
		 * We dont save the incomming messages on the server. We just send an MyEvent to all
		 * registered Listeners on the Domain.
		 */
		this.addEvent(DOMAIN, new MyEvent(message));
	}

}

Instead of the default RemoteServiceServlet we extend this one with RemoteEventServiceServlet from the GWTEventService lib. This gives you the ability to send Events to the clients.