best way to combine guava eventbus and AWT Event thread handling

Question

When you have a asynchronous event bus, and fire events, lets say within the model which get catched in the UI you have probably the following problem:

The registered handler gets executed in a worker thread, but all UI swing changes need to be executed within the AWT event thread. This means you need to envelope all your handler clode in EventQueue.invokeLater(...) .

This looks like a lot of boiler plate code. I wonder if there is a smarter solution for that problem.

What about an extension to the guava event bus that marks a handler for execution within a special thread? This could be marked with a annotion eg @ExecuteWithinEDT :

class EventBusChangeRecorder {
  @Subscribe @ExecuteWithinEDT void recordCustomerChange(ChangeEvent e) {
    recordChange(e.getChange());
  }
}

Answer 1

The handlers registered with an async event bus are executed on whatever thread the provided Executor chooses to run them on, not necessarily a worker thread.

What I've done is created an implementation of Executor that runs stuff on the event queue thread. It's pretty simple:

public class EventQueueExecutor implements Executor {
  @Override public void execute(Runnable command) {
    EventQueue.invokeLater(command);
  }
}

You can then just create your EventBus with that:

EventBus eventBus = new AsyncEventBus(new EventQueueExecutor());

Then all handlers will be executed on the event queue thread.

Edit:

An example of forwarding events:

public class EventForwarder {
  private final EventBus uiEventBus;

  public EventForwarder(EventBus uiEventBus) {
    this.uiEventBus = uiEventBus;
  }

  // forward all events
  @Subscribe
  public void forwardEvent(Object event) {
    uiEventBus.post(event);
  }

  // or if you only want a specific type of event forwarded
  @Subscribe
  public void forwardEvent(UiEvent event) {
    uiEventBus.post(event);
  }
}

Just subscribe that to your main event bus and post all events to the main event bus, but subscribe all UI components to the UI event bus.

Answer 2

You can create an EventBus that dispatches only on the AWT thread:

EventBus mybus = new AsyncEventBus("awt",
    new Executor() {
        public void execute (Runnable cmd) {
            if (EventQueue.isDispatchThread()) {
                cmd.run();
            } else {
                EventQueue.invokeLater(cmd);
            }
        }
    });