Building Reusable Java Widgets

Event multicasters handle asynchronous broadcasting of events to listeners. You will be relieved to know that you do not have to write your own multicaster from scratch. Instead you will need to create a subclass of the existing AWTEventMulticaster and write a few methods so that it can handle your new events. When you create your multicaster follow these steps:

Take a look at Listing 4 to see how I created the multicaster for this example. Don't let the multicaster scare you. The code is very basic, cookie-cutter-style. You will be relying on the superclass to do all of the hard multicasting work. All that your multicaster must do is be aware of your new events and listeners.

We have now created all of the necessary components,, and what remains is to connect it up properly. The first order of business is to complete the EmailWidget class. As we left it, it only had a constructor. Next you must give it the ability to add listeners. Here is the addEmailEntryEventListener method:

public void addEmailEntryListener(
        EmailEntryListener e ) {
_emailEntryListener = MyMulticaster.add(
        _emailEntryListener, e ) ;
}

Notice that the widget has an instance variable that maintains its listener. If you look closely, you will see that this variable is actually an instance of your multicaster class. Any widget can potentially have many listeners. Your multicaster will maintain the list of listeners and ensure that they get their appropriate events. Finally, you must handle the internal events (from the buttons) and generate your new event.

public void actionPerformed( ActionEvent e ) {
        EmailEntryEvent newEvent ;
        if ( _emailEntryListener == null ) return ;
        if ( e.getSource() == _doneButton ) {
                newEvent = new EmailEntryEvent(
                                getEmailAddress()) ;
        _emailEntryListener.done( newEvent );
    }
        else if ( e.getSource() == _cancelButton ) {
                newEvent = new EmailEntryEvent() ;
                _emailEntryListener.cancel( newEvent );
    }

Figure 7. The EmailEntryWidget in Use as an Applet

One important note: the e-mail entry class generates a done event. How is this different from the action event generated by the done button? The difference is subtle, but important. If you rely upon detecting the events of a specific button, other classes need to know the details of the inner workings of your class. If this changes, every class that uses it has to change as well.

Here are a few ideas for expanding the EmailEntry widget that you might enjoy trying: