There is no special configuration required to use GWT in a Seam application, however the Seam resource servlet must be installed. See Chapter 25, Configuring Seam and packaging Seam applications for details.

The first step in preparing a Seam component to be called via GWT, is to create both synchronous and asynchronous service interfaces for the methods you wish to call. Both of these interfaces should extend the GWT interface com.google.gwt.user.client.rpc.RemoteService:

  public interface MyService extends RemoteService
  {
    public String askIt(String question);      
  }

The asynchronous interface should be identical, except that it also contains an additional AsyncCallback parameter for each of the methods it declares:

  public interface MyServiceAsync extends RemoteService 
  {
    public void askIt(String question, AsyncCallback callback);
  }

The asynchronous interface, in this example MyServiceAsync, will be implemented by GWT and should never be implemented directly.

The next step, is to create a Seam component that implements the synchronous interface:

  @Name("org.jboss.seam.example.remoting.gwt.client.MyService")
  public class ServiceImpl implements MyService
  {
    @WebRemote
    public String askIt(String question)
    {
      if (!validate(question)) 
      {
        throw new IllegalStateException("Hey, this shouldn't happen, I checked on the client, " +
               "but its always good to double check.");
      }
      return "42. Its the real question that you seek now.";
    }
   
    public boolean validate(String q) 
    {
      ValidationUtility util = new ValidationUtility();
      return util.isValid(q);
    }
  }

The methods that should be made accessible via GWT need to be annotated with the @WebRemote annotation, which is required for all web-remoteable methods.

The next step, is to write a method that returns the asynchronous interface to the component. This method can be located inside the widget class, and will be used by the widget to obtain a reference to the asynchronous client stub:

   private MyServiceAsync getService() 
   {       
      String endpointURL = GWT.getModuleBaseURL() + "seam/resource/gwt";      
      
      MyServiceAsync svc = (MyServiceAsync) GWT.create(MyService.class);
      ((ServiceDefTarget) svc).setServiceEntryPoint(endpointURL);
      return svc;     
   }

The final step is to write the widget code that invokes the method on the client stub. The following example creates a simple user interface with a label, text input and a button:

public class AskQuestionWidget extends Composite
{
   private AbsolutePanel panel = new AbsolutePanel();
   
   public AskQuestionWidget() 
   {      
      Label lbl = new Label("OK, what do you want to know?");
      panel.add(lbl);
      final TextBox box = new TextBox();
      box.setText("What is the meaning of life?");
      panel.add(box);
      Button ok = new Button("Ask");
      ok.addClickListener(new ClickListener() 
      {
         public void onClick(Widget w)
         {
            ValidationUtility valid = new ValidationUtility();
            if (!valid.isValid(box.getText())) 
            {
               Window.alert("A question has to end with a '?'");
            } 
            else 
            {
               askServer(box.getText());
            } 
         }
      });
      panel.add(ok);
      
      initWidget(panel);
   }

   private void askServer(String text)
   {
      getService().askIt(text, new AsyncCallback() 
      {
         public void onFailure(Throwable t)
         {
            Window.alert(t.getMessage());
         }

         public void onSuccess(Object data)
         {
            Window.alert((String) data);
         }         
      });      
   }
   
   ...    
    

When clicked, the button invokes the askServer() method passing the contents of the input text (in this example, validation is also performed to ensure that the input is a valid question). The askServer() method acquires a reference to the asynchronous client stub (returned by the getService() method) and invokes the askIt() method. The result (or error message if the call fails) is shown in an alert window.

The complete code for this example can be found in the Seam distribution in the examples/remoting/gwt directory.