How to use the GWT EventBus

Question

I wonder how to use the EventBus or whether there are some better solutions to send an Event through the project.

Widget1 has a Button. Widget2 has a Label, that should change when I press the button. These widgets are in a DockLayout:

RootLayoutPanel rootLayoutPanel = RootLayoutPanel.get();
DockLayoutPanel dock = new DockLayoutPanel(Unit.EM);

dock.addWest(new Widget1(), 10);
dock.add(new Widget2());

rootLayoutPanel.add(dock);

I have declared an handleClickAlert in Widget1:

@UiHandler("button")
void handleClickAlert(ClickEvent e) {
    //fireEvent(e); 
}

Solution

When you divide the project into logical parts (for example with MVP), then the different parts sometimes need to communicate. Typical this communication is done by sending status changes, e.g.:

• user logged-in / logged-out.
• user navigated directly via URL to a page, so the menu needs to be updated.

Using the event bus is quite logical in those cases.

To use it you instantiate one EventBus per app which is then used by all other classes. To achieve this use a static field, factory or dependency injection (GIN in case of GWT).

Example with your own event types:

public class AppUtils{

    public static EventBus EVENT_BUS = GWT.create(SimpleEventBus.class);
}

Normally you'd also create your own event types and handlers:

public class AuthenticationEvent extends GwtEvent<AuthenticationEventHandler> {

public static Type<AuthenticationEventHandler> TYPE = new Type<AuthenticationEventHandler>();

  @Override
public Type<AuthenticationEventHandler> getAssociatedType() {
    return TYPE;
}

@Override
protected void dispatch(AuthenticationEventHandler handler) {
    handler.onAuthenticationChanged(this);
}
}

and the handler:

public interface AuthenticationEventHandler extends EventHandler {
    void onAuthenticationChanged(AuthenticationEvent authenticationEvent);
}

Then you use it like this:

AppUtils.EVENT_BUS.addHandler(AuthenticationEvent.TYPE, new AuthenticationEventHandler()     {
        @Override
        public void onAuthenticationChanged(AuthenticationEvent authenticationEvent) {
            // authentication changed - do something
        }
    });

and fire the event:

AppUtils.EVENT_BUS.fireEvent(new AuthenticationEvent());

Comments

[elvispt]

When firing the event, how could I send an object? In my case I would need to send an class that contains my users information.

[Peter Knego]

You can have a field in your custom event and set it via constructor, i.e. 'new AuthenticationEvent(someObject)'

[Igor Klimer]

Protip: use gwteventbinder to minimize the code needed to define a new event. For the AuthenticationEvent above, your event would just have to extend GenericEvent.

[baris]

Better to use 'ResettableEventBus' not to have a global event bus which won't be killed after the lifecycle of 'Presenter' and might result multiple identical event handlers for one event.