Events and Listeners

Events in GWT use the "listener interface" model similar to other user interface frameworks. A listener interface defines one or more methods that the widget calls to announce an event. A class wishing to receive events of a particular type implements the associated listener interface and then passes a reference to itself to the widget to "subscribe" to a set of events.

The Button class, for example, publishes click events. The associated listener interface is ClickListener. 

public void anonClickListenerExample() {
  Button b = new Button("Click Me");
  b.addClickListener(new ClickListener() {
    public void onClick(Widget sender) {
      // handle the click event
    }
  });
}
Using anonymous inner classes as in the above example can be inefficient for a large number of widgets, since it could result in the creation of many listener objects. Widgets supply their this pointer as the sender parameter when they invoke a listener method, allowing a single listener to distinguish between multiple event publishers. This makes better use of memory but requires slightly more code, as shown in the following example: 
public class ListenerExample extends Composite implements ClickListener {
  private FlowPanel fp = new FlowPanel();
  private Button b1 = new Button("Button 1");
  private Button b2 = new Button("Button 2");

  public ListenerExample() {
    initWidget(fp);
    fp.add(b1);
    fp.add(b2);
    b1.addClickListener(this);
    b2.addClickListener(this);
  }

  public void onClick(Widget sender) {
    if (sender == b1) {
      // handle b1 being clicked
    } else if (sender == b2) {
      // handle b2 being clicked
    }
  }
}
Some event interfaces specify more than one event. If you are only interested in a subset of these events, subclass one of the event "adapters". Adapters are simply empty concrete implementations of a particular event interface, from which you can derive a listener class without having to implement every method. 
public void adapterExample() {
  TextBox t = new TextBox();
  t.addKeyboardListener(new KeyboardListenerAdapter() {
    public void onKeyPress(Widget sender, char keyCode, int modifiers) {
      // handle only this one event
    }
  });
}