Previous | Next | Trail Map | JavaBeans Tutorial | Writing a Simple Bean

Manipulating Events in the BeanBox

You'll need a good understanding of the JDK 1.1 event mechanism before reading this section. Here's the list of recommended readings:

Beans use the new event mechanism implemented in JDK 1.1, so implementing Bean events is the same as implementing events in any JDK 1.1 component. This section describes how this event mechanism is used by Beans and the BeanBox.

How the BeanBox Discovers a Beans Event Capabilities

The BeanBox uses either design pattern introspection or a BeanInfo class to discover what events a Bean can fire.

Using Introspection to Discover the Events A Bean Fires

JavaBeans provides event-oriented design patterns to give introspecting tools the ability to discover what events a Bean can fire. For a Bean to be the source of an event, it must implement methods that add and remove listener objects for that type of event. The design patterns for these methods are 

public void add<EventListenerType>(<EventListenerType> a)
public void remove<EventListenerType>(<EventListenerType> a)
These methods let a source Bean know where to fire events. The source Bean then fires events at those listener Beans using the methods for those particular interfaces. For example, if a source Bean registers ActionListener objects, it will fire events at those objects by calling the actionPerformed() method on those listeners.

To see events discovered using design patterns, drop an instance of OurButton into the BeanBox and pull down the Edit|Events menu. This displays a list of event interfaces to which OurButton can fire events. Note that OurButton itself only adds and removes two of these interfaces; the rest are inherited from the base class.

Using BeanInfo to Define the Events a Bean Fires

You can explicitly "publish" the events a Bean fires by using a class that implements the BeanInfo interface. The ExplicitButton demo Bean subclasses OurButton, and provides an associated ExplicitButtonBeanInfo class. ExplicitButtonBeanInfo implements the following method to explicitly define interfaces to which ExplicitButton fires events. 

public EventSetDescriptor[] getEventSetDescriptors() {
    try {
        EventSetDescriptor push = new EventSetDescriptor(beanClass,
                    "actionPerformed",
                    java.awt.event.ActionListener.class,
                    "actionPerformed");
 
        EventSetDescriptor changed = new EventSetDescriptor(beanClass,
                    "propertyChange",
                    java.beans.PropertyChangeListener.class,
                    "propertyChange");
 
        push.setDisplayName("button push");
        changed.setDisplayName("bound property change");
     
        EventSetDescriptor[] rv = { push, changed};
        return rv;
    } catch (IntrospectionException e) {
        throw new Error(e.toString());
    }
}
Drop an ExplicitButton instance in the BeanBox, and pull down the Edit|Events menu. Notice that only those interfaces explicitly exposed in the ExplicitButtonBeanInfo class are listed. No inherited capabilities are exposed. See the Using BeanInfo section for more information on the BeanInfo interface.

Viewing a Bean's Events in the BeanBox

If you select an OurButton Bean in the BeanBox, then pull down the Edit|Events menu, you will see a list of interfaces that OurButton can fire events at. Each interface item will, when selected, display the methods that fire different events at those interfaces. These correspond to all the events that OurButton can fire.

Hooking Up Events in the BeanBox

In this example you will use two OurButton bean instances to stop and start an instance of the animated Juggler bean. You will label the buttons "start" and "stop"; make the start button, when pressed, invoke the Juggler bean's startJuggling method; and make the stop button, when pressed, invoke the Juggler bean's stopJuggling method.

  1. Start the BeanBox.
  2. Drop a Juggler bean and two OurButton bean instances into the BeanBox.
  3. Select an OurButton instance. In the Properties sheet, change the label property to "start". Select the second OurButton instance and change its label to "stop".
  4. Select the start button. Choose the Edit|Events|action|actionPerformed menu item. This causes a rubber band line to track between the start button and the cursor. Click on the Juggler instance. This brings up the EventTargetDialog:

    This list contains Juggler methods that take no arguments, or arguments of type actionPerformed.

  5. Select the startJuggling method and press OK. You will see a message that the BeanBox is generating adapter classes.
  6. Do the above two steps on the stop button, except choose the stopJuggling method in the EventTargetDialog.

Clicking on the start and stop buttons will now start and stop the Juggler. Here is a general description of what happened:

You will use this juggler example later when you learn how to generate applets, so use the File|Save menu item to save this example to a file of your choice.

Event Adapter Classes

The BeanBox generates an adapter class the interposes between the source and the target. The adapter class implements the appropriate event listener interface (and so is the actual listener, not the target Juggler Bean), catches the event fired by the button, and then calls the selected target method. Here is the BeanBox-generated adapter class that interposes between the start button and the JugglerBean: 

// Automatically generated event hookup file.

package tmp.sunw.beanbox;
import sunw.demo.juggler.Juggler;
import java.awt.event.ActionListener;
import java.awt.event.ActionEvent;

public class ___Hookup_1474c0159e implements
           java.awt.event.ActionListener, java.io.Serializable {

    public void setTarget(sunw.demo.juggler.Juggler t) {
        target = t;
    }

    public void actionPerformed(java.awt.event.ActionEvent arg0) {
        target.startJuggling(arg0);
    }

    private sunw.demo.juggler.Juggler target;
}

The adapter implements the ActionListener interface that you selected in the BeanBox's Edit|Events menu. ActionListener declares one method, actionPerformed(), which is implemented by the adapter to call the target Bean method that you selected. The setTarget() method is called by the BeanBox to set the target Bean.

The EventMonitor Demo Bean

The EventMonitor Bean (beans/demo/sunw/demo/encapsulatedEvents) prints out source Bean event reports, as they occur, in a scrolling listbox. To see how this works, take the following steps:

  1. Drop OurButton and EventMonitor instances in the BeanBox. You might want to resize the EventMonitor (and the BeanBox) to accommodate viewing the event reports.
  2. Select the OurButton instance, and choose any event method in the Edit|Events menu.
  3. Connect the rubber band line to the EventMonitor, and choose its initiateEventSourcMonitoring in the EventTargetDialog.
  4. Select the OurButton Bean. You will begin seeing event reports in the EventMonitor
When the first event is delivered, EventMonitor analyzes the source Bean to discover all the events it fires, creates and registers an event listener for each event type, and then reports whenever any event is fired. This is useful for debugging. Try connecting other demo Beans to EventMonitor to observer their events.

Events for Bound and Constrained Properties

The bound and constrained properties sections describe two specific event listener interfaces.

Previous | Next | Trail Map | JavaBeans Tutorial | Writing a Simple Bean