GWT Widget.addHandler - gwt

Question

I am trying to utilize Widget.addHandler(). However, the handler never gets called. Below is my sample code. What do I need to change to fix this?
My Handler Implementation:
public class CustomMouseMoveHandler
extends GwtEvent.Type<MouseMoveHandler>
implements MouseMoveHandler
{
#Override
public void onMouseMove(MouseMoveEvent event) {
System.out.println("----> onMouseMove.");
}
}
My EntryPoint.OnModuleLoad():
ContentPanel cp = new ContentPanel();
cp.setHeaderVisible(false);
cp.setHeight(com.google.gwt.user.client.Window.getClientHeight());
CustomMouseMoveHandler handler = new CustomMouseMoveHandler();
cp.addHandler(handler, handler);
RootPanel.get().add(cp);
/////
Added on 7/1/2011.
The following complete GWT simple code does not work either (with Jason's hint applied). Please help me out. Thanks
package tut.client;
import com.google.gwt.core.client.EntryPoint;
import com.google.gwt.event.dom.client.MouseMoveEvent;
import com.google.gwt.event.dom.client.MouseMoveHandler;
import com.google.gwt.user.client.ui.RootPanel;
import com.google.gwt.user.client.ui.TextArea;
/**
* Entry point classes define <code>onModuleLoad()</code>.
*/
public class GwtHandler implements EntryPoint, MouseMoveHandler {
/**
* This is the entry point method.
*/
public void onModuleLoad() {
TextArea comp = new TextArea();
comp.setSize("200px", "200px");
comp.setText("Testing Text");
comp.addHandler(this, MouseMoveEvent.getType());
RootPanel.get().add(comp);
}
#Override
public void onMouseMove(MouseMoveEvent event) {
com.google.gwt.user.client.Window.alert("onMouseMove");
}
}

Answer 1

GwtEvent.Type is used to dispatch events based on an event specific and unique object (object equality - == - is used to match event types). Passing your CustomMouseMoveHandler as the Type to addHandler indicates an event type other than that used for MouseMoveEvents (Indeed in this case every CustomMouseMoveHandler would be assigned to a different event Type since each object is different).
Instead of extending GwtEvent.Type<MouseMoveHandler> in your handler you need to get the event Type from MouseMoveEvent itself (using the static getType() method).
Don't extend GwtEvent.Type in your CustomMouseMoveHandler:
public class CustomMouseMoveHandler
implements MouseMoveHandler
{
...
}
And to add the handler:
cp.addDomHandler(handler, MouseMoveEvent.getType());

Answer 2

DomEvents have to be registered using addDomHandler, or you have to sinkEvents for their event type. addDomHandler is a shortcut for sinkEvents+addHandler.

Answer 3

Here's how I solved my problem. I wanted to add handlers to a NumberLabel. This is what worked:
final NumberLabel<Long> label = new NumberLabel<Long>();
label.setValue(2000l);
label.setHorizontalAlignment(HasHorizontalAlignment.ALIGN_RIGHT);
MouseOverHandler handler = new MouseOverHandler() {
public void onMouseOver(MouseOverEvent event) {
System.out.println("mouse over");
}
};
Widget widget = label.asWidget();
widget.addDomHandler(handler, MouseOverEvent.getType());
Treating is as a Widget did the trick.
By the way, System.out.println worked.