This section covers the core messaging concepts of the ErraiBus messaging framework.

ErraiBus forms the backbone of the Errai framework's approach to application design. Most importantly, it provides a straight-forward approach to a complex problem space. Providing common APIs across the client and server, developers will have no trouble working with complex messaging scenarios from building instant messaging clients, stock tickers, to monitoring instruments. There's no more messing with RPC APIs, or unweildy AJAX or COMET frameworks. We've built it all in to one, consice messaging framework. It's single-paradigm, and it's fun to work with.

public class HelloWorld implements EntryPoint {

// Get an instance of the RequestDispatcher
  private RequestDispatcher dispatcher = ErraiBus.getDispatcher();

  public void onModuleLoad() {
    Button button = new Button("Send message");

    button.addClickHandler(new ClickHandler() {
      public void onClick(ClickEvent event) {
        // Send a message to the 'HelloWorldService'.
        MessageBuilder.createMessage()
          .toSubject("HelloWorldService") // (1)
          .signalling() // (2)
          .noErrorHandling() // (3)
          .sendNowWith(dispatcher); // (4)
        });

        [...]
      }
   }
}

@Service
  public class HelloWorldService implements MessageCallback {
    public void callback(Message message) {
      System.out.println("Hello, World!");
    }
  }

@Service
public class HelloWorldService implements MessageCallback {

  private RequestDispatcher dispatcher;

  @Inject
  public HelloWorldService(RequestDispatcher disaptcher) {
    dispatcher = dispatcher;
  }

  public void callback(CommandMessage message) {
    // Send a message to the 'HelloWorldClient'.
    MessageBuilder.createMessage()
      .toSubject("HelloWorldClient") // (1)
      .signalling()                  // (2)
      .with("text", "Hi There")      // (3)
      .noErrorHandling()             // (4)
      .sendNowWith(dispatcher);      // (5)
    });
  }
}

public class HelloWorld implements EntryPoint {

  private MessageBus bus = ErraiBus.get();

  public void onModuleLoad() {
     [...]

     /**
      * Declare a local service to receive messages on the subject
      * "BroadcastReceiver".
      */
     bus.subscribe("BroadcastReceiver", new MessageCallback() {
       public void callback(CommandMessage message) {
         /**
          * When a message arrives, extract the "text" field and
          * do something with it
          */
          String messageText = message.get(String.class, "text");
        }
     });

     [...]
  }
}

@Service
public class HelloWorldService implements MessageCallback {
  public void callback(CommandMessage message) {
    // Send a message to the 'HelloWorldClient' on the client that sent us the
    // the message.
    MessageBuilder.createConversation(message)
      .toSubject("HelloWorldClient")
      .signalling()
      .with("text", "Hi There! We're having a reply!")
      .noErrorHandling().reply();
    });
  }
}

MessageBuilder.createMessage()
    .toSubject("HelloWorldService")
    .signalling()
    .with("msg", "Hi there!")
    .errorsHandledBy(new ErrorCallback() {
      public boolean error(Message message, Throwable throwable) {
        throwable.printStackTrace();
          return true;
      }
    })
    .sendNowWith(dispatcher);

ErrorCallback error = new ErrorCallback() {
  public boolean error(Message message, Throwable throwable) {
    throwable.printStackTrace();
    return true;
  }
}

MessageBuilder.createMessage()
  .toSubject("HelloWorldService")
  .signalling()
  .with("msg", "Hi there!")
  .errorsHandledBy(error)
  .sendNowWith(dispatcher);

bus.subscribe(DefaultErrorCallback.CLIENT_ERROR_SUBJECT, new MessageCallback() {
  @Override
  public void callback(Message message) {
    try {
      caught = message.get(Throwable.class, MessageParts.Throwable);
      throw caught;
    }
    catch(TransportIOException e) {
      // thrown in case the server can't be reached or an unexpected status code was returned
    }
    catch (Throwable throwable) {
      // handle system errors (e.g response marshalling errors) - that of course should never happen :)
    }
  }
});

MessageBuilder.createMessage()
  .toSubject("ConversationalService").signalling()
  .with("SomeField", someValue)
  .noErrorHandling()
  .repliesTo(new MessageCallback() {
    public void callback(Message message) {
      System.out.println("I received a response");
    }
   })

MessageBuilder.createMessage().
    .toSubject("MessageListener")
    .with("Text", "Hello, from your overlords in the cloud")
    .noErrorHandling().sendGlobalWith(dispatcher);

MessageBuilder.createConversation(msg)
    .toSubject("FunSubject")
    .signalling()
    .noErrorHandling()
    .replyDelayed(TimeUnit.SECONDS, 5); // sends the message after 5 seconds.

MessageBuilder.createMessage()
     .toSubject("FunSubject")
     .signalling()
     .noErrorHandling()
     .sendDelayed(requestDispatcher, TimeUnit.SECONDS, 5); // sends the message after 5 seconds.

MessageBuilder.createMessage()
    .toSubject("FunSubject")
    .signalling()
    .withProvided("time", new ResourceProvider<String>() {
       SimpleDateFormat fmt = new SimpleDateFormat("hh:mm:ss");
     
       public String get() {
         return fmt.format(new Date(System.currentTimeMillis());
       }
     }
     .noErrorHandling()
     .sendRepeatingWith(requestDispatcher, TimeUnit.SECONDS, 1); //sends a message every 1 second

AsyncTask task = MessageBuilder.createConversation(message)
  .toSubject("TimeChannel").signalling()
  .withProvided(TimeServerParts.TimeString, new ResourceProvider<String>() {
     public String get() {
       return String.valueOf(System.currentTimeMillis());
     }
   }).defaultErrorHandling().replyRepeating(TimeUnit.MILLISECONDS, 100);

    ...

  // cancel the task and interrupt it's thread if necessary.
  task.cancel(true);

MessageBuilder.createMessage()
    .toSubject("ObjectService").signalling()
    .with(MessageParts.ReplyTo, "ClientEndpoint")
    .noErrorHandling().sendNowWith(dispatcher);

MessageBuilder.createConversation(message)
    .subjectProvided().signalling()
    .with("Records", records)
    .noErrorHandling().reply();

...
  public void callback(Message message) {
    String sessionId = ServerBusUtils.getSessionId(message);

    // Record this sessionId somewhere.
    ...
  }

MessageBuilder.createMessage()
    .toSubject("ClientMessageListener")
    .signalling()
    .with(MessageParts.SessionID, sessionId)
    .with("Message", "We're relaying a message!")
    .noErrorHandling().sendNowWith(dispatcher);

public class TestService implements MessageCallback {
  public void callback(final Message message) {
    // obtain a reference to the session context by referencing the incoming message.
    SessionContext injectionContext = SessionContext.get(message);

    // set an attribute.
    injectionContext.setAttribute("MyAttribute", "Foo");
  }
}

public class TestService implements MessageCallback {
  public void callback(final Message message) {
    // obtain a reference to the local context by referencing the incoming message.
    LocalContext injectionContext = LocalContext.get(message);

   // set an attribute.
    injectionContext.setAttribute("MyAttribute", "Foo");
  }
}