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    <p>I created my own widget by extending GWT's Composite class. I wanted to create my own custom event in this class. I wanted the events to be accessible to GWT's WindowBuilder Editor.</p> <p>I learned a lot of from the answers on this page, but I had to make some changes.</p> <p>I wanted to start from the Hilbrand Bouwkamp answer, because it was newer. But I ran into a couple of problems. 1) That answer made reference to the event bus. The even bus is a global variable owned by the main program. It's not clear how a widget library could get access to that. 2) I wasn't starting from scratch. I was was extending GWT library code. In order to make that work, I had to start from the GwtEvent class, rather than the Event class.</p> <p>Piotr's answer is essentially correct, but it was very long. My class (indirectly) extends GWT's Widget class. Widget takes care of many details, such as creating a HandlerManager object. (I looked through the source code, and that's exactly how standard Widgets work, not by using an EventBus.)</p> <p>I only had to add two things to my widget class to add a custom event handler. Those are shown here:</p> <pre><code>public class TrackBar extends Composite { public HandlerRegistration addValueChangedHandler(TrackBarEvent.Handler handler) { return addHandler(handler, TrackBarEvent.TYPE); } private void fireValueChangedEvent() { final TrackBarEvent e = new TrackBarEvent(value); fireEvent(e); } </code></pre> <p>My new event is almost exactly the same as Piotr's event class, shown above. One thing is worth noting. I started with getValue(), based on that example. Later I added getTrackBar() to give a lot more information. If I was starting from scratch I'd focus on the latter, not the former. The complete event class is shown below.</p> <pre><code>import com.google.gwt.event.shared.EventHandler; import com.google.gwt.event.shared.GwtEvent; public class TrackBarEvent extends GwtEvent&lt; TrackBarEvent.Handler &gt; { public interface Handler extends EventHandler { void onTrackBarValueChanged(TrackBarEvent event); } static final Type&lt;TrackBarEvent.Handler&gt; TYPE = new Type&lt;TrackBarEvent.Handler&gt;(); private final int value; public TrackBarEvent(int value) { this.value = value; } @Override public Type&lt;TrackBarEvent.Handler&gt; getAssociatedType() { return TYPE; } public int getValue() { return value; } public TrackBar getTrackBar() { return (TrackBar)getSource(); } @Override protected void dispatch(Handler handler) { handler.onTrackBarValueChanged(this); } } </code></pre>
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