Inside the Delegation Event Model

JDK 1.1's version of AWT supports a new delegation model for handling events. The old model dealt with events in a hierarchical fashion. The AWT delivered an event to the target component (e.g., an ACTION_EVENT was delivered to the button that was clicked), which either handled the event or repeatedly passed it up the line to the containing objects. Ultimately, either a containing object handled (i.e., consumed) the event, or the event bubbled out the top of the containment hierarchy.

Programmers working in this model built applets with one of two event-handling architectures. In the first, they subclassed each potential target object (button, checkbox, and so forth) and overr ode the object's event-catching methods with code that responded appropriately to events. This architecture generated lots of subclasses as objects were individualized to their specific function. In the second architecture, programmers overrode the outermost containing object's handleEvent method and dealt with events through an often large and complex switch statement.

The new delegation model identifies those classes that can produce events (e.g., buttons, checkboxes, and text fields) as sources. Listener objects -- those that contain code to respond to events -- register themselves with source objects.

Furthermore, listener objects can register themselves according to the kinds of events they can handle. For example, a listener object might register itself to receive only action events (e.g., a button click) and would consequently not be bothered by adjustment events (e.g., changing the maximum value on a scroll bar). When a source object trig gers an event, that event is sent to all listener objects that are registered with that source to receive that kind of event.

Because code that handles an event is more directly connected to the event's source, the new model should result in more efficient programs. Additionally, new-model programs should be easier to understand; large, confusing switch statements are unnecessary.